path: root/open_issues/performance/io_system/read-ahead.mdwn

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[[!tag open_issue_gnumach open_issue_hurd]]

[[!toc]]


# [[community/gsoc/project_ideas/disk_io_performance]]


# [[gnumach_page_cache_policy]]


# 2011-02

[[Etenil]] has been working in this area.


## IRC, freenode, #hurd, 2011-02-13

    <etenil> youpi: Would libdiskfs/diskfs.h be in the right place to make
      readahead functions?
    <youpi> etenil: no, it'd rather be at the memory management layer,
      i.e. mach, unfortunately
    <youpi> because that's where you see the page faults
    <etenil> youpi: Linux also provides a readahead() function for higher level
      applications. I'll probably have to add the same thing in a place that's
      higher level than mach
    <youpi> well, that should just be hooked to the same common implementation
    <etenil> the man page for readahead() also states that portable
      applications should avoid it, but it could be benefic to have it for
      portability
    <youpi> it's not in posix indeed


## IRC, freenode, #hurd, 2011-02-14

    <etenil> youpi: I've investigated prefetching (readahead) techniques. One
      called DiskSeen seems really efficient. I can't tell yet if it's patented
      etc. but I'll keep you informed
    <youpi> don't bother with complicated techniques, even the most simple ones
      will be plenty :)
    <etenil> it's not complicated really
    <youpi> the matter is more about how to plug it into mach
    <etenil> ok
    <youpi> then don't bother with potential pattents
    <antrik> etenil: please take a look at the work KAM did for last year's
      GSoC
    <youpi> just use a trivial technique :)
    <etenil> ok, i'll just go the easy way then

    <braunr> antrik: what was etenil referring to when talking about
      prefetching ?
    <braunr> oh, madvise() stuff
    <braunr> i could help him with that


## IRC, freenode, #hurd, 2011-02-15

    <etenil> oh, I'm looking into prefetching/readahead to improve I/O
      performance
    <braunr> etenil: ok
    <braunr> etenil: that's actually a VM improvement, like samuel told you
    <etenil> yes
    <braunr> a true I/O improvement would be I/O scheduling
    <braunr> and how to implement it in a hurdish way
    <braunr> (or if it makes sense to have it in the kernel)
    <etenil> that's what I've been wondering too lately
    <braunr> concerning the VM, you should look at madvise()
    <etenil> my understanding is that Mach considers devices without really
      knowing what they are
    <braunr> that's roughly the interface used both at the syscall() and the
      kernel levels in BSD, which made it in many other unix systems
    <etenil> whereas I/O optimisations are often hard disk drives specific
    <braunr> that's true for almost any kernel
    <braunr> the device knowledge is at the driver level
    <etenil> yes
    <braunr> (here, I separate kernels from their drivers ofc)
    <etenil> but Mach also contains some drivers, so I'm going through the code
      to find the apropriate place for these improvements
    <braunr> you shouldn't tough the drivers at all
    <braunr> touch
    <etenil> true, but I need to understand how it works before fiddling around
    <braunr> hm
    <braunr> not at all
    <braunr> the VM improvement is about pagein clustering
    <braunr> you don't need to know how pages are fetched
    <braunr> well, not at the device level
    <braunr> you need to know about the protocol between the kernel and
      external pagers
    <etenil> ok
    <braunr> you could also implement pageout clustering
    <etenil> if I understand you well, you say that what I'd need to do is a
      queuing system for the paging in the VM?
    <braunr> no
    <braunr> i'm saying that, when a page fault occurs, the kernel should
      (depending on what was configured through madvise()) transfer pages in
      multiple blocks rather than one at a time
    <braunr> communication with external pagers is already async, made through
      regular ports
    <braunr> which already implement message queuing
    <braunr> you would just need to make the mapped regions larger
    <braunr> and maybe change the interface so that this size is passed
    <etenil> mmh
    <braunr> (also don't forget that page clustering can include pages *before*
      the page which caused the fault, so you may have to pass the start of
      that region too)
    <etenil> I'm not sure I understand the page fault thing
    <etenil> is it like a segmentation error?
    <etenil> I can't find a clear definition in Mach's manual
    <braunr> ah
    <braunr> it's a fundamental operating system concept
    <braunr> http://en.wikipedia.org/wiki/Page_fault
    <etenil> ah ok
    <etenil> I understand now
    <etenil> so what's currently happening is that when a page fault occurs,
      Mach is transfering pages one at a time and wastes time 
    <braunr> sometimes, transferring just one page is what you want
    <braunr> it depends on the application, which is why there is madvise()
    <braunr> our rootfs, on the other hand, would benefit much from such an
      improvement
    <braunr> in UVM, this optimization is account for around 10% global
      performance improvement
    <braunr> accounted*
    <etenil> not bad
    <braunr> well, with an improved page cache, I'm sure I/O would matter less
      on systems with more RAM
    <braunr> (and another improvement would make mach support more RAM in the
      first place !)
    <braunr> an I/O scheduler outside the kernel would be a very good project
      IMO
    <braunr> in e.g. libstore/storeio
    <etenil> yes
    <braunr> but as i stated in my thesis, a resource scheduler should be as
      close to its resource as it can
    <braunr> and since mach can host several operating systems, I/O schedulers
      should reside near device drivers
    <braunr> and since current drivers are in the kernel, it makes sens to have
      it in the kernel too
    <braunr> so there must be some discussion about this
    <etenil> doesn't this mean that we'll have to get some optimizations in
      Mach and have the same outside of Mach for translators that access the
      hardware directly?
    <braunr> etenil: why ?
    <etenil> well as you said Mach contains some drivers, but in principle, it
      shouldn't, translators should do disk access etc, yes?
    <braunr> etenil: ok
    <braunr> etenil: so ?
    <etenil> well, let's say if one were to introduce SATA support in Hurd,
      nothing would stop him/her to do so with a translator rather than in Mach
    <braunr> you should avoid the term translator here
    <braunr> it's really hurd specific
    <braunr> let's just say a user space task would be responsible for that
      job, maybe multiple instances of it, yes
    <etenil> ok, so in this case, let's say we have some I/O optimization
      techniques like readahead and I/O scheduling within Mach, would these
      also apply to the user-space task, or would they need to be
      reimplemented?
    <braunr> if you have user space drivers, there is no point having I/O
      scheduling in the kernel
    <etenil> but we also have drivers within the kernel
    <braunr> what you call readahead, and I call pagein/out clustering, is
      really tied to the VM, so it must be in Mach in any case
    <braunr> well
    <braunr> you either have one or the other
    <braunr> currently we have them in the kernel
    <braunr> if we switch to DDE, we should have all of them outside
    <braunr> that's why such things must be discussed
    <etenil> ok so if I follow you, then future I/O device drivers will need to
      be implemented for Mach
    <braunr> currently, yes
    <braunr> but preferrably, someone should continue the work that has been
      done on DDe so that drivers are outside the kernel
    <etenil> so for the time being, I will try and improve I/O in Mach, and if
      drivers ever get out, then some of the I/O optimizations will need to be
      moved out of Mach
    <braunr> let me remind you one of the things i said
    <braunr> i said I/O scheduling should be close to their resource, because
      we can host several operating systems
    <braunr> now, the Hurd is the only system running on top of Mach
    <braunr> so we could just have I/O scheduling outside too
    <braunr> then you should consider neighbor hurds
    <braunr> which can use different partitions, but on the same device
    <braunr> currently, partitions are managed in the kernel, so file systems
      (and storeio) can't make good scheduling decisions if it remains that way
    <braunr> but that can change too
    <braunr> a single storeio representing a whole disk could be shared by
      several hurd instances, just as if it were a high level driver
    <braunr> then you could implement I/O scheduling in storeio, which would be
      an improvement for the current implementation, and reusable for future
      work
    <etenil> yes, that was my first instinct
    <braunr> and you would be mostly free of the kernel internals that make it
      a nightmare
    <etenil> but youpi said that it would be better to modify Mach instead
    <braunr> he mentioned the page clustering thing
    <braunr> not I/O scheduling
    <braunr> theseare really two different things
    <etenil> ok
    <braunr> you *can't* implement page clustering outside Mach because Mach
      implements virtual memory
    <braunr> both policies and mechanisms
    <etenil> well, I'd rather think of one thing at a time if that's alright
    <etenil> so what I'm busy with right now is setting up clustered page-in
    <etenil> which need to be done within Mach
    <braunr> keep clustered page-outs in mind too
    <braunr> although there are more constraints on those
    <etenil> yes
    <etenil> I've looked up madvise(). There's a lot of documentation about it
      in Linux but I couldn't find references to it in Mach (nor Hurd), does it
      exist?
    <braunr> well, if it did, you wouldn't be caring about clustered page
      transfers, would you ?
    <braunr> be careful about linux specific stuff
    <etenil> I suppose not
    <braunr> you should implement at least posix options, and if there are
      more, consider the bsd variants
    <braunr> (the Mach VM is the ancestor of all modern BSD VMs)
    <etenil> madvise() seems to be posix
    <braunr> there are system specific extensions
    <braunr> be careful
    <braunr> CONFORMING TO POSIX.1b.   POSIX.1-2001 describes posix_madvise(3)
      with constants POSIX_MADV_NORMAL, etc., with a behav‐ ior close to that
      described here.  There is a similar posix_fadvise(2) for file access.
    <braunr> MADV_REMOVE, MADV_DONTFORK, MADV_DOFORK, MADV_HWPOISON,
      MADV_MERGEABLE, and MADV_UNMERGEABLE  are  Linux- specific.
    <etenil> I was about to post these
    <etenil> ok, so basically madvise() allows tasks etc. to specify a usage
      type for a chunk of memory, then I could apply the relevant I/O
      optimization based on this
    <braunr> that's it
    <etenil> cool, then I don't need to worry about knowing what the I/O is
      operating on, I just need to apply the optimizations as advised
    <etenil> that's convenient
    <etenil> ok I'll start working on this tonight
    <etenil> making a basic readahead shouldn't be too hard
    <braunr> readahead is a misleading name
    <etenil> is pagein better?
    <braunr> applies to too many things, doesn't include the case where
      previous elements could be prefetched
    <braunr> clustered page transfers is what i would use
    <braunr> page prefetching maybe
    <etenil> ok
    <braunr> you should stick to something that's already used in the
      literature since you're not inventing something new
    <etenil> yes I've read a paper about prefetching
    <etenil> ok
    <etenil> thanks for your help braunr
    <braunr> sure
    <braunr> you're welcome
    <antrik> braunr: madvise() is really the least important part of the
      picture...
    <antrik> very few applications actually use it. but pretty much all
      applications will profit from clustered paging
    <antrik> I would consider madvise() an optional goody, not an integral part
      of the implementation
    <antrik> etenil: you can find some stuff about KAM's work on
      http://www.gnu.org/software/hurd/user/kam.html
    <antrik> not much specific though
    <etenil> thanks
    <antrik> I don't remember exactly, but I guess there is also some
      information on the mailing list. check the archives for last summer
    <antrik> look for Karim Allah Ahmed
    <etenil> antrik: I disagree, madvise gives me a good starting point, even
      if eventually the optimisations should run even without it
    <antrik> the code he wrote should be available from Google's summer of code
      page somewhere...
    <braunr> antrik: right, i was mentioning madvise() because the kernel (VM)
      interface is pretty similar to the syscall
    <braunr> but even a default policy would be nice
    <antrik> etenil: I fear that many bits were discussed only on IRC... so
      you'd better look through the IRC logs from last April onwards...
    <etenil> ok

    <etenil> at the beginning I thought I could put that into libstore
    <etenil> which would have been fine

    <antrik> BTW, I remembered now that KAM's GSoC application should have a
      pretty good description of the necessary changes... unfortunately, these
      are not publicly visible IIRC :-(


## IRC, freenode, #hurd, 2011-02-16

    <etenil> braunr: I've looked in the kernel to see where prefetching would
      fit best. We talked of the VM yesterday, but I'm not sure about it. It
      seems to me that the device part of the kernel makes more sense since
      it's logically what manages devices, am I wrong?
    <braunr> etenil: you are
    <braunr> etenil: well
    <braunr> etenil: drivers should already support clustered sector
      read/writes
    <etenil> ah
    <braunr> but yes, there must be support in the drivers too
    <braunr> what would really benefit the Hurd mostly concerns page faults, so
      the right place is the VM subsystem

[[clustered_page_faults]]


# 2012-03


## IRC, freenode, #hurd, 2012-03-21

    <mcsim> I thought that readahead should have some heuristics, like
      accounting size of object and last access time, but i didn't find any in
      kam's patch. Are heuristics needed or it will be overhead for
      microkernel? 
    <youpi> size  of object and last access time are not necessarily useful to
      take into account
    <youpi> what would usually typically be kept is the amount of contiguous
      data that has been read lately
    <youpi> to know whether it's random or sequential, and how much is read
    <youpi> (the whole size of the object does not necessarily give any
      indication of how much of it will be read)
    <mcsim> if big object is accessed often, performance could be increased if
      frame that will be read ahead will be increased too.
    <youpi> yes, but the size of the object really does not matter
    <youpi> you can just observe how much data is read and realize that it's
      read a lot
    <youpi> all the more so with userland fs translators
    <youpi> it's not because you mount a CD image that you need to read it all
    <mcsim> youpi: indeed. this will be better. But on other hand there is
      principle about policy and mechanism. And kernel should implement
      mechanism, but heuristics seems to be policy. Or in this case moving
      readahead policy to user level would be overhead?
    <antrik> mcsim: paging policy is all in kernel anyways; so it makes perfect
      sense to put the readahead policy there as well
    <antrik> (of course it can be argued -- probably rightly -- that all of
      this should go into userspace instead...)
    <mcsim> antrik: probably defpager partly could do that. AFAIR, it is
      possible for defpager to return more memory than was asked.
    <mcsim> antrik: I want to outline what should be done during gsoc. First,
      kernel should support simple readahead for specified number of pages
      (regarding direction of access) + simple heuristic for changing frame
      size. Also default pager could make some analysis, for instance if it has
      many data located consequentially it could return more data then was
      asked. For other pagers I won't do anything. Is it suitable?
    <antrik> mcsim: I think we actually had the same discussion already with
      KAM ;-)
    <antrik> for clustered pageout, the kernel *has* to make the decision. I'm
      really not convinced it makes sense to leave the decision for clustered
      pagein to the individual pagers
    <antrik> especially as this will actually complicate matters because a) it
      will require work in *every* pager, and b) it will probably make handling
      of MADVISE & friends more complex
    <antrik> implementing readahead only for the default pager would actually
      be rather unrewarding. I'm pretty sure it's the one giving the *least*
      benefit
    <antrik> it's much, much more important for ext2
    <youpi> mcsim: maybe try to dig in the irc logs, we discussed about it with
      neal. the current natural place would be the kernel, because it's the
      piece that gets the traps and thus knows what happens with each
      projection, while the backend just provides the pages without knowing
      which projection wants it. Moving to userland would not only be overhead,
      but quite difficult
    <mcsim> antrik: OK, but I'm not sure that I could do it for ext2. 
    <mcsim> OK, I'll dig.


## IRC, freenode, #hurd, 2012-04-01

    <mcsim> as part of implementing of readahead project I have to add
      interface for setting appropriate behaviour for memory range.  This
      interface than should be compatible with madvise call, that has a lot of
      possible advises, but most part of them are specific for Linux (according
      to man page). Should mach also support these Linux-specific values?
    <mcsim> p.s. these Linux-specific values shouldn't affect readahead
      algorithm.
    <youpi> the interface shouldn't prevent from adding them some day
    <youpi> so that we don't have to add them yet
    <mcsim> ok. And what behaviour with value MADV_NORMAL should be look like?
      Seems that it should be synonym to MADV_SEQUENTIAL, isn't it?
    <youpi> no, it just means "no idea what it is"
    <youpi> in the linux implementation, that means some given readahead value
    <youpi> while SEQUENTIAL means twice as much
    <youpi> and RANDOM means zero
    <mcsim> youpi: thank you.
    <mcsim> youpi: Than, it seems to be better that kernel interface for
      setting behaviour will accept readahead value, without hiding it behind
      such constants, like VM_BEHAVIOR_DEFAULT (like it was in kam's
      patch). And than implementation of madvise will call vm_behaviour_set
      with appropriate frame size. Is that right?
    <youpi> question of taste, better ask on the list
    <mcsim> ok


## IRC, freenode, #hurd, 2012-06-09

    <mcsim> hello. What fictitious pages in gnumach are needed for?
    <mcsim> I mean why real page couldn't be grabbed straight, but in sometimes
      fictitious page is grabbed first and than converted to real?
    <braunr> mcsim: iirc, fictitious pages are needed by device pagers which
      must comply with the vm pager interface
    <braunr> mcsim: specifically, they must return a vm_page structure, but
      this vm_page describes device memory
    <braunr> mcsim: and then, it must not be treated like normal vm_page, which
      can be added to page queues (e.g. page cache)


## IRC, freenode, #hurd, 2012-06-22

    <mcsim> braunr: Ah. Patch for large storages introduced new callback
      pager_notify_evict. User had to define this callback on his own as
      pager_dropweak, for instance. But neal's patch change this. Now all
      callbacks could have any name, but user defines structure with pager ops
      and supplies it in pager_create.
    <mcsim> So, I just changed notify_evict to confirm it to new style.
    <mcsim> braunr: I want to changed interface of mo_change_attributes and
      test my changes with real partitions. For both these I have to update
      ext2fs translator, but both partitions I have are bigger than 2Gb, that's
      why I need apply this patch.z
    <mcsim> But what to do with mo_change_attributes? I need somehow inform
      kernel about page fault policy.
    <mcsim> When I change mo_ interface in kernel I have to update all programs
      that use this interface and ext2fs is one of them.

    <mcsim> braunr: Who do you think better to inform kernel about fault
      policy? At the moment I've added fault_strategy parameter that accepts
      following strategies: randow, sequential with single page cluster,
      sequential with double page cluster and sequential with quad page
      cluster. OSF/mach has completely another interface of
      mo_change_attributes. In OSF/mach mo_change_attributes accepts structure
      of parameter. This structure could have different formats depending o
    <mcsim> This rpc could be useful because it is not very handy to update
      mo_change_attributes for kernel, for hurd libs and for glibc. Instead of
      this kernel will accept just one more structure format.
    <braunr> well, like i wrote on the mailing list several weeks ago, i don't
      think the policy selection is of concern currently
    <braunr> you should focus on the implementation of page clustering and
      readahead
    <braunr> concerning the interface, i don't think it's very important
    <braunr> also, i really don't like the fact that the policy is per object
    <braunr> it should be per map entry
    <braunr> i think it mentioned that in my mail too
    <braunr> i really think you're wasting time on this
    <braunr> http://lists.gnu.org/archive/html/bug-hurd/2012-04/msg00064.html
    <braunr> http://lists.gnu.org/archive/html/bug-hurd/2012-04/msg00029.html
    <braunr> mcsim: any reason you completely ignored those ?
    <mcsim> braunr: Ok. I'll do clustering for map entries.
    <braunr> no it's not about that either :/
    <braunr> clustering is grouping several pages in the same transfer between
      kernel and pager
    <braunr> the *policy* is held in map entries
    <antrik> mcsim: I'm not sure I properly understand your question about the
      policy interface... but if I do, it's IMHO usually better to expose
      individual parameters as RPC arguments explicitly, rather than hiding
      them in an opaque structure...
    <antrik> (there was quite some discussion about that with libburn guy)
    <mcsim> antrik: Following will be ok? kern_return_t vm_advice(map, address,
      length, advice, cluster_size)
    <mcsim> Where advice will be either random or sequential
    <antrik> looks fine to me... but then, I'm not an expert on this stuff :-)
    <antrik> perhaps "policy" would be clearer than "advice"?
    <mcsim> madvise has following prototype: int madvise(void *addr, size_t
      len, int advice);
    <mcsim> hmm... looks like I made a typo. Or advi_c_e is ok too?
    <antrik> advise is a verb; advice a noun... there is a reason why both
      forms show up in the madvise prototype :-)
    <mcsim> so final variant should be kern_return_t vm_advise(map, address,
      length, policy, cluster_size)?
    <antrik> mcsim: nah, you are probably right that its better to keep
      consistency with madvise, even if the name of the "advice" parameter
      there might not be ideal...
    <antrik> BTW, where does cluster_size come from? from the filesystem?
    <antrik> I see merits both to naming the parameter "policy" (clearer) or
      "advice" (more consistent) -- you decide :-)
    <mcsim> antrik: also there is variant strategy, like with inheritance :)
      I'll choose advice for now.
    <mcsim> What do you mean under "where does cluster_size come from"?
    <antrik> well, madvise doesn't have this parameter; so the value must come
      from a different source?
    <mcsim> in madvise implementation it could fixed value or somehow
      calculated basing on size of memory range. In OSF/mach cluster size is
      supplied too (via mo_change_attributes).
    <antrik> ah, so you don't really know either :-)
    <antrik> well, my guess is that it is derived from the cluster size used by
      the filesystem in question
    <antrik> so for us it would always be 4k for now
    <antrik> (and thus you can probably leave it out alltogether...)
    <antrik> well, fatfs can use larger clusters
    <antrik> I would say, implement it only if it's very easy to do... if it's
      extra effort, it's probably not worth it
    <mcsim> There is sense to make cluster size bigger for ext2 too, since most
      likely consecutive clusters will be within same group.
    <mcsim> But anyway I'll handle this later.
    <antrik> well, I don't know what cluster_size does exactly; but by the
      sound of it, I'd guess it makes an assumption that it's *always* better
      to read in this cluster size, even for random access -- which would be
      simply wrong for 4k filesystem clusters...
    <antrik> BTW, I agree with braunr that madvice() is optional -- it is way
      way more important to get readahead working as a default policy first


## IRC, freenode, #hurd, 2012-07-01

    <mcsim> youpi: Do you think you could review my code?
    <youpi> sure, just post it to the list
    <youpi> make sure to break it down into logical pieces
    <mcsim> youpi: I pushed it my branch at gnumach repository
    <mcsim> youpi: or it is still better to post changes to list?
    <youpi> posting to the list would permit feedback from other people too
    <youpi> mcsim: posix distinguishes normal, sequential and random
    <youpi> we should probably too
    <youpi> the system call should probably be named "vm_advise", to be a verb
      like allocate etc.
    <mcsim> youpi: ok. A have a talk with antrik regarding naming, I'll change
      this later because compiling of glibc take a lot of time.
    <youpi> mcsim: I find it odd that vm_for_every_page allocates non-existing
      pages
    <youpi> there should probably be at least a flag to request it or not
    <mcsim> youpi: normal policy is synonym to default. And this could be
      treated as either random or sequential, isn't it?
    <braunr> mcsim: normally, no
    <youpi> yes, the normal policy would be the default
    <youpi> it doesn't mean random or sequential
    <youpi> it's just to be a compromise between both
    <youpi> random is meant to make no read-ahead, since that'd be spurious
      anyway
    <youpi> while by default we should make readahead
    <braunr> and sequential makes even more aggressive readahead, which usually
      implies a greater number of pages to fetch
    <braunr> that's all
    <youpi> yes
    <youpi> well, that part is handled by the cluster_size parameter actually
    <braunr> what about reading pages preceding the faulted paged ?
    <mcsim> Shouldn't sequential clean some pages (if they, for example, are
      not precious) that are placed before fault page?
    <braunr> ?
    <youpi> that could make sense, yes
    <braunr> you lost me
    <youpi> and something that you wouldn't to with the normal policy
    <youpi> braunr: clear what has been read previously
    <braunr> ?
    <youpi> since the access is supposed to be sequential
    <braunr> oh
    <youpi> the application will proabably not re-read what was already read
    <braunr> you mean to avoid caching it ?
    <youpi> yes
    <braunr> inactive memory is there for that
    <youpi> while with the normal policy you'd assume that the application
      might want to go back etc.
    <youpi> yes, but you can help it
    <braunr> yes
    <youpi> instead of making other pages compete with it
    <braunr> but then, it's for precious pages
    <youpi> I have to say I don't know what a precious page it
    <youpi> s
    <youpi> does it mean dirty pages?
    <braunr> no
    <braunr> precious means cached pages
    <braunr> "If precious is FALSE, the kernel treats the data as a temporary
      and may throw it away if it hasn't been changed. If the precious value is
      TRUE, the kernel treats its copy as a data repository and promises to
      return it to the manager; the manager may tell the kernel to throw it
      away instead by flushing and not cleaning the data"
    <braunr> hm no
    <braunr> precious means the kernel must keep it
    <mcsim> youpi: According to vm_for_every_page. What kind of flag do you
      suppose? If object is internal, I suppose not to cross the bound of
      object, setting in_end appropriately in vm_calculate_clusters.
    <mcsim> If object is external we don't know its actual size, so we should
      make mo request first. And for this we should create fictitious  pages.
    <braunr> mcsim: but how would you implement this "cleaning" with sequential
      ?
    <youpi> mcsim: ah, ok, I thought you were allocating memory, but it's just
      fictitious pages
    <youpi> comment "Allocate a new page" should be fixed :)
    <mcsim> braunr: I don't now how I will implement this specifically (haven't
      tried yet), but I don't think that this is impossible
    <youpi> braunr: anyway it's useful as an example where normal and
      sequential would be different
    <braunr> if it can be done simply
    <braunr> because i can see more trouble than gains in there :)
    <mcsim> braunr: ok :)
    <braunr> mcsim: hm also, why fictitious pages ?
    <braunr> fictitious pages should normally be used only when dealing with
      memory mapped physically which is not real physical memory, e.g. device
      memory
    <mcsim> but vm_fault could occur when object represent some device memory.
    <braunr> that's exactly why there are fictitious pages
    <mcsim> at the moment of allocating of fictitious page it is not know what
      backing store of object is.
    <braunr> really ?
    <braunr> damn, i've got used to UVM too much :/
    <mcsim> braunr: I said something wrong?
    <braunr> no no
    <braunr> it's just that sometimes, i'm confusing details about the various
      BSD implementations i've studied
    <braunr> out-of-gsoc-topic question: besides network drivers, do you think
      we'll have other drivers that will run in userspace and have to implement
      memory mapping ? like framebuffers ?
    <braunr> or will there be a translation layer such as storeio that will
      handle mapping ?
    <youpi> framebuffers typically will, yes
    <youpi> that'd be antrik's work on drm
    <braunr> hmm
    <braunr> ok
    <youpi> mcsim: so does the implementation work, and do you see performance
      improvement?
    <mcsim> youpi: I haven't tested it yet with large ext2 :/
    <mcsim> youpi: I'm going to finish now moving of ext2 to new interface,
      than other translators in hurd repository and than finish memory policies
      in gnumach. Is it ok?
    <youpi> which new interface?
    <mcsim> Written by neal. I wrote some temporary code to make ext2 work with
      it, but I'm going to change this now.
    <youpi> you mean the old unapplied patch?
    <mcsim> yes
    <youpi> did you have a look at Karim's work?
    <youpi> (I have to say I never found the time to check how it related with
      neal's patch)
    <mcsim> I found only his work in kernel. I didn't see his work in applying
      of neal's patch.
    <youpi> ok
    <youpi> how do they relate with each other?
    <youpi> (I have never actually looked at either of them :/)
    <mcsim> his work in kernel and neal's patch?
    <youpi> yes
    <mcsim> They do not correlate with each other.
    <youpi> ah, I must be misremembering what each of them do
    <mcsim> in kam's patch was changes to support sequential reading in reverse
      order (as in OSF/Mach), but posix does not support such behavior, so I
      didn't implement this either.
    <youpi> I can't find the pointer to neal's patch, do you have it off-hand?
    <mcsim> http://comments.gmane.org/gmane.os.hurd.bugs/351
    <youpi> thx
    <youpi> I think we are not talking about the same patch from Karim
    <youpi> I mean lists.gnu.org/archive/html/bug-hurd/2010-06/msg00023.html
    <mcsim> I mean this patch:
      http://lists.gnu.org/archive/html/bug-hurd/2010-06/msg00024.html 
    <mcsim> Oh.
    <youpi> ok
    <mcsim> seems, this is just the same
    <youpi> yes
    <youpi> from a non-expert view, I would have thought these patches play
      hand in hand, do they really?
    <mcsim> this patch is completely for kernel and neal's one is completely
      for libpager.
    <youpi> i.e. neal's fixes libpager, and karim's fixes the kernel
    <mcsim> yes
    <youpi> ending up with fixing the whole path?
    <youpi> AIUI, karim's patch will be needed so that your increased readahead
      will end up with clustered page request?
    <mcsim> I will not use kam's patch
    <youpi> is it not needed to actually get pages in together?
    <youpi> how do you tell libpager to fetch pages together?
    <youpi> about the cluster size, I'd say it shouldn't be specified at
      vm_advise() level
    <youpi> in other OSes, it is usually automatically tuned
    <youpi> by ramping it up to a maximum readahead size (which, however, could
      be specified)
    <youpi> that's important for the normal policy, where there are typically
      successive periods of sequential reads, but you don't know in advance for
      how long
    <mcsim> braunr said that there are legal issues with his code, so I cannot
      use it.
    <braunr> did i ?
    <braunr> mcsim: can you give me a link to the code again please ?
    <youpi> see above :)
    <braunr> which one ?
    <youpi> both
    <youpi> they only differ by a typo
    <braunr> mcsim: i don't remember saying that, do you have any link ?
    <braunr> or log ?
    <mcsim> sorry, can you rephrase "ending up with fixing the whole path"?
    <mcsim> cluster_size in vm_advise also could be considered as advise 
    <braunr> no
    <braunr> it must be the third time we're talking about this
    <youpi> mcsim: I mean both parts would be needed to actually achieve
      clustered i/o
    <braunr> again, why make cluster_size a per object attribute ? :(
    <youpi> wouldn't some objects benefit from bigger cluster sizes, while
      others wouldn't?
    <youpi> but again, I believe it should rather be autotuned
    <youpi> (for each object)
    <braunr> if we merely want posix compatibility (and for a first attempt,
      it's quite enough), vm_advise is good, and the kernel selects the
      implementation (and thus the cluster sizes)
    <braunr> if we want finer grained control, perhaps a per pager cluster_size
      would be good, although its efficiency depends on several parameters
    <braunr> (e.g. where the page is in this cluster)
    <braunr> but a per object cluster size is a large waste of memory
      considering very few applications (if not none) would use the "feature"
      ..
    <braunr> (if any*)
    <youpi> there must be a misunderstanding
    <youpi> why would it be a waste of memory?
    <braunr> "per object"
    <youpi> so?
    <braunr> there can be many memory objects in the kernel
    <youpi> so?
    <braunr> so such an overhead must be useful to accept it
    <youpi> in my understanding, a cluster size per object is just a mere
      integer for each object
    <youpi> what overhead?
    <braunr> yes
    <youpi> don't we have just thousands of objects?
    <braunr> for now
    <braunr> remember we're trying to remove the page cache limit :)
    <youpi> that still won't be more than tens of thousands of objects
    <youpi> times an integer
    <youpi> that's completely neglectible
    <mcsim> braunr: Strange, Can't find in logs. Weird things are happening in
      my memory :/ Sorry.
    <braunr> mcsim: i'm almost sure i never said that :/
    <braunr> but i don't trust my memory too much either
    <braunr> youpi: depends
    <youpi> mcsim: I mean both parts would be needed to actually achieve
      clustered i/o
    <mcsim> braunr: I made I call vm_advise that applies policy to memory range
      (vm_map_entry to be specific)
    <braunr> mcsim: good
    <youpi> actually the cluster size should even be per memory range
    <mcsim> youpi: In this sense, yes
    <youpi> k
    <mcsim> sorry, Internet connection lags
    <braunr> when changing a structure used to create many objects, keep in
      mind one thing
    <braunr> if its size gets larger than a threshold (currently, powers of
      two), the cache used by the slab allocator will allocate twice the
      necessary amount
    <youpi> sure
    <braunr> this is the case with most object caching allocators, although
      some can have specific caches for common sizes such as 96k which aren't
      powers of two
    <braunr> anyway, an integer is negligible, but the final structure size
      must be checked
    <braunr> (for both 32 and 64 bits)
    <mcsim> braunr: ok.
    <mcsim> But I didn't understand what should be done with cluster size in
      vm_advise? Should I delete it?
    <braunr> to me, the cluster size is a pager property
    <youpi> to me, the cluster size is a map property
    <braunr> whereas vm_advise indicates what applications want
    <youpi> you could have several process accessing the same file in different
      ways
    <braunr> youpi: that's why there is a policy
    <youpi> isn't cluster_size part of the policy?
    <braunr> but if the pager abilities are limited, it won't change much
    <braunr> i'm not sure
    <youpi> cluster_size is the amount of readahead, isn't it?
    <braunr> no, it's the amount of data in a single transfer
    <mcsim> Yes, it is.
    <braunr> ok, i'll have to check your code
    <youpi> shouldn't transfers permit unbound amounts of data?
    <mcsim> braunr: than I misunderstand what readahead is
    <braunr> well then cluster size is per policy :)
    <braunr> e.g. random => 0, normal => 3, sequential => 15
    <braunr> why make it per map entry ?
    <youpi> because it depends on what the application doezs
    <braunr> let me check the code
    <youpi> if it's accessing randomly, no need for big transfers
    <youpi> just page transfers will be fine
    <youpi> if accessing sequentially, rather use whole MiB of transfers
    <youpi> and these behavior can be for the same file
    <braunr> mcsim: the call is vm_advi*s*e
    <braunr> mcsim: the call is vm_advi_s_e
    <braunr> not advice
    <youpi> yes, he agreed earlier
    <braunr> ok
    <mcsim> cluster_size is the amount of data that I try to read at one time.
    <mcsim> at singe mo_data_request
    <mcsim> *single
    <youpi> which, to me, will depend on the actual map
    <braunr> ok so it is the transfer size
    <youpi> and should be autotuned, especially for normal behavior
    <braunr> youpi: it makes no sense to have both the advice and the actual
      size per map entry
    <youpi> to get big readahead with all apps
    <youpi> braunr: the size is not only dependent on the advice, but also on
      the application behavior
    <braunr> youpi: how does this application tell this ?
    <youpi> even for sequential, you shouldn't necessarily use very big amounts
      of transfers
    <braunr> there is no need for the advice if there is a cluster size
    <youpi> there can be, in the case of sequential, as we said, to clear
      previous pages
    <youpi> but otherwise, indeed
    <youpi> but for me it's the converse
    <youpi> the cluster size should be tuned anyway
    <braunr> and i'm against giving the cluster size in the advise call, as we
      may want to prefetch previous data as well
    <youpi> I don't see how that collides
    <braunr> well, if you consider it's the transfer size, it doesn't
    <youpi> to me cluster size is just the size of a window
    <braunr> if you consider it's the amount of pages following a faulted page,
      it will
    <braunr> also, if your policy says e.g. "3 pages before, 10 after", and
      your cluster size is 2, what happens ?
    <braunr> i would find it much simpler to do what other VM variants do:
      compute the I/O sizes directly from the policy
    <youpi> don't they autotune, and use the policy as a maximum ?
    <braunr> depends on the implementations
    <youpi> ok, but yes I agree
    <youpi> although casting the size into stone in the policy looks bogus to
      me
    <braunr> but making cluster_size part of the kernel interface looks way too
      messy
    <braunr> it is
    <braunr> that's why i would have thought it as part of the pager properties
    <braunr> the pager is the true component besides the kernel that is
      actually involved in paging ...
    <youpi> well, for me the flexibility should still  be per application
    <youpi> by pager you mean the whole pager, not each file, right?
    <braunr> if a pager can page more because e.g. it's a file system with big
      block sizes, why not fetch more ?
    <braunr> yes
    <braunr> it could be each file
    <braunr> but only if we have use for it
    <braunr> and i don't see that currently
    <youpi> well, posix currently doesn't provide a way to set it
    <youpi> so it would be useless atm
    <braunr> i was thinking about our hurd pagers
    <youpi> could we perhaps say that the policy maximum could be a fraction of
      available memory?
    <braunr> why would we want that ?
    <youpi> (total memory, I mean)
    <youpi> to make it not completely cast into stone
    <youpi> as have been in the past in gnumach
    <braunr> i fail to understand :/
    <youpi> there must be a misunderstanding then
    <youpi> (pun not intended)
    <braunr> why do you want to limit the policy maximum ?
    <youpi> how to decide it?
    <braunr> the pager sets it
    <youpi> actually I don't see how a pager could decide it
    <youpi> on what ground does it make the decision?
    <youpi> readahead should ideally be as much as 1MiB
    <braunr> 02:02 < braunr> if a pager can page more because e.g. it's a file
      system with big block sizes, why not fetch more ?
    <braunr> is the example i have in mind
    <braunr> otherwise some default values
    <youpi> that's way smaller than 1MiB, isn't it?
    <braunr> yes
    <braunr> and 1 MiB seems a lot to me :)
    <youpi> for readahead, not really
    <braunr> maybe for sequential
    <youpi> that's what we care about!
    <braunr> ah, i thought we cared about normal
    <youpi> "as much as 1MiB", I said
    <youpi> I don't mean normal :)
    <braunr> right
    <braunr> but again, why limit ?
    <braunr> we could have 2 or more ?
    <youpi> at some point you don't get more efficiency
    <youpi> but eat more memory
    <braunr> having the pager set the amount allows us to easily adjust it over
      time
    <mcsim> braunr: Do you think that readahead should be implemented in
      libpager?
    <youpi> than needed
    <braunr> mcsim: no
    <braunr> mcsim: err
    <braunr> mcsim: can't answer
    <youpi> mcsim: do you read the log of what you have missed during
      disconnection?
    <braunr> i'm not sure about what libpager does actually
    <mcsim> yes
    <braunr> for me it's just mutualisation of code used by pagers
    <braunr> i don't know the details
    <braunr> youpi: yes
    <braunr> youpi: that's why we want these values not hardcoded in the kernel
    <braunr> youpi: so that they can be adjusted by our shiny user space OS
    <youpi> (btw apparently linux uses minimum 16k, maximum 128 or 256k)
    <braunr> that's more reasonable
    <youpi> that's just 4 times less :)
    <mcsim> braunr: You say that pager should decide how much data should be
      read ahead, but each pager can't implement it on it's own as there will
      be too much overhead. So the only way is to implement this in libpager.
    <braunr> mcsim: gni ?
    <braunr> why couldn't they ?
    <youpi> mcsim: he means the size, not the actual implementation
    <youpi> the maximum size, actually
    <braunr> actually, i would imagine it as the pager giving per policy
      parameters
    <youpi> right
    <braunr> like how many before and after
    <youpi> I agree, then
    <braunr> the kernel could limit, sure, to avoid letting pagers use
      completely insane values
    <youpi> (and that's just a max, the kernel autotunes below that)
    <braunr> why not
    <youpi> that kernel limit could be a fraction of memory, then?
    <braunr> it could, yes
    <braunr> i see what you mean now
    <youpi> mcsim: did you understand our discussion?
    <youpi> don't hesitate to ask for clarification
    <mcsim> I supposed cluster_size to be such parameter. And advice will help
      to interpret this parameter (whether data should be read after fault page
      or some data should be cleaned before)
    <youpi> mcsim: we however believe that it's rather the pager than the
      application that would tell that
    <youpi> at least for the default values
    <youpi> posix doesn't have a way to specify it, and I don't think it will
      in the future
    <braunr> and i don't think our own hurd-specific programs will need more
      than that
    <braunr> if they do, we can slightly change the interface to make it a per
      object property
    <braunr> i've checked the slab properties, and it seems we can safely add
      it per object
    <braunr> cf http://www.sceen.net/~rbraun/slabinfo.out
    <braunr> so it would still be set by the pager, but if depending on the
      object, the pager could set different values
    <braunr> youpi: do you think the pager should just provide one maximum size
      ? or per policy sizes ?
    <youpi> I'd say per policy size
    <youpi> so people can increase sequential size like crazy when they know
      their sequential applications need it, without disturbing the normal
      behavior
    <braunr> right
    <braunr> so the last decision is per pager or per object
    <braunr> mcsim: i'd say whatever makes your implementation simpler :)
    <mcsim> braunr: how kernel knows that object are created by specific pager?
    <braunr> that's the kind of things i'm referring to with "whatever makes
      your implementation simpler"
    <braunr> but usually, vm_objects have an ipc port and some properties
      relatedto their pagers
    <braunr> -usually
    <braunr> the problem i had in mind was the locking protocol but our spin
      locks are noops, so it will be difficult to detect deadlocks
    <mcsim> braunr: and for every policy there should be variable in vm_object
      structure with appropriate cluster_size?
    <braunr> if you want it per object, yes
    <braunr> although i really don't think we want it
    <youpi> better keep it per pager for now
    <braunr> let's imagine youpi finishes his 64-bits support, and i can
      successfully remove the page cache limit
    <braunr> we'd jump from 1.8 GiB at most to potentially dozens of GiB of RAM
    <braunr> and 1.8, mostly unused
    <braunr> to dozens almost completely used, almost all the times for the
      most interesting use cases
    <braunr> we may have lots and lots of objects to keep around
    <braunr> so if noone really uses the feature ... there is no point
    <youpi> but also lots and lots of memory to spend on it :)
    <youpi> a lot of objects are just one page, but a lof of them are not
    <braunr> sure
    <braunr> we wouldn't be doing that otherwise :)
    <braunr> i'm just saying there is no reason to add the overhead of several
      integers for each object if they're simply not used at all
    <braunr> hmm, 64-bits, better page cache, clustered paging I/O :>
    <braunr> (and readahead included in the last ofc)
    <braunr> good night !
    <mcsim> than, probably, make system-global max-cluster_size? This will save
      some memory. Also there is usually no sense in reading really huge chunks
      at once.
    <youpi> but that'd be tedious to set
    <youpi> there are only a few pagers, that's no wasted memory
    <youpi> the user being able to set it for his own pager is however a very
      nice feature, which can be very useful for databases, image processing,
      etc.
    <mcsim> In conclusion I have to implement following: 3 memory policies per
      object and per vm_map_entry. Max cluster size for every policy should be
      set per pager.
    <mcsim> So, there should be 2 system calls for setting memory policy and
      one for setting cluster sizes.
    <mcsim> Also amount of data to transfer should be tuned automatically by
      every page fault. 
    <mcsim> youpi: Correct me, please, if I'm wrong.
    <youpi> I believe that's what we ended up to decide, yes


## IRC, freenode, #hurd, 2012-07-02

    <braunr> is it safe to say that all memory objects implemented by external
      pagers have "file" semantics ?
    <braunr> i wonder if the current memory manager interface is suitable for
      device pagers
    <mcsim> braunr: What does "file" semantics mean?
    <braunr> mcsim: anonymous memory doesn't have the same semantics as a file
      for example
    <braunr> anonymous memory that is discontiguous in physical memory can be
      contiguous in swap
    <braunr> and its location can change with time
    <braunr> whereas with a memory object, the data exchanged with pagers is
      identified with its offset
    <braunr> in (probably) all other systems, this way of specifying data is
      common to all files, whatever the file system
    <braunr> linux uses the struct vm_file name, while in BSD/Solaris they are
      called vnodes (the link between a file system inode and virtual memory)
    <braunr> my question is : can we implement external device pagers with the
      current interface, or is this interface really meant for files ?
    <braunr> also
    <braunr> mcsim: something about what you said yesterday
    <braunr> 02:39 < mcsim> In conclusion I have to implement following: 3
      memory policies per object and per vm_map_entry. Max cluster size for
      every policy should be set per pager.
    <braunr> not per object
    <braunr> one policy per map entry
    <braunr> transfer parameters (pages before and after the faulted page) per
      policy, defined by pagers
    <braunr> 02:39 < mcsim> So, there should be 2 system calls for setting
      memory policy and one for setting cluster sizes.
    <braunr> adding one call for vm_advise is good because it mirrors the posix
      call
    <braunr> but for the parameters, i'd suggest changing an already existing
      call
    <braunr> not sure which one though
    <mcsim> braunr: do you know how mo_change_attributes implemented in
      OSF/Mach?
    <braunr> after a quick reading of the reference manual, i think i
      understand why they made it per object
    <braunr> mcsim: no
    <braunr> did they change the call to include those paging parameters ?
    <mcsim> it accept two parameters: flavor and pointer to structure with
      parameters.
    <mcsim> flavor determines semantics of structure with parameters.
    <mcsim>
      http://www.darwin-development.org/cgi-bin/cvsweb/osfmk/src/mach_kernel/vm/memory_object.c?rev=1.1
    <mcsim> structure can have 3 different views and what exect view will be is
      determined by value of flavor
    <mcsim> So, I thought about implementing similar call that could be used
      for various purposes.
    <mcsim> like ioctl
    <braunr> "pointer to structure with parameters" <= which one ?
    <braunr> mcsim: don't model anything anywhere like ioctl please
    <mcsim> memory_object_info_t	attributes
    <braunr> ioctl is the very thing we want NOT to have on the hurd
    <braunr> ok attributes
    <braunr> and what are the possible values of flavour, and what kinds of
      attributes ?
    <mcsim> and then appears something like this on each case: behave =
      (old_memory_object_behave_info_t) attributes;
    <braunr> ok i see
    <mcsim> flavor could be  OLD_MEMORY_OBJECT_BEHAVIOR_INFO,
      MEMORY_OBJECT_BEHAVIOR_INFO, MEMORY_OBJECT_PERFORMANCE_INFO etc
    <braunr> i don't really see the point of flavour here, other than
      compatibility
    <braunr> having attributes is nice, but you should probably add it as a
      call parameter, not inside a structure
    <braunr> as a general rule, we don't like passing structures too much
      to/from the kernel, because handling them with mig isn't very clean
    <mcsim> ok
    <mcsim> What policy parameters should be defined by pager?
    <braunr> i'd say number of pages to page-in before and after the faulted
      page
    <mcsim> Only pages before and after the faulted page?
    <braunr> for me yes
    <braunr> youpi might have different things in mind
    <braunr> the page cleaning in sequential mode is something i wouldn't do
    <braunr> 1/ applications might want data read sequentially to remain in the
      cache, for other sequential accesses
    <braunr> 2/ applications that really don't want to cache anything should
      use O_DIRECT
    <braunr> 3/ it's complicated, and we're in july
    <braunr> i'd rather have a correct and stable result than too many unused
      features
    <mcsim> braunr: MADV_SEQUENTIAL Expect page references in sequential order.
      (Hence, pages in the given range can be aggressively read ahead, and may
      be freed soon after they are accessed.)
    <mcsim> this is from linux man
    <mcsim> braunr: Can I at least make keeping in mind that it could be
      implemented?
    <mcsim> I mean future rpc interface
    <mcsim> braunr: From behalf of kernel pager is just a port.
    <mcsim> That's why it is not clear for me how I can make in kernel
      per-pager policy
    <braunr> mcsim: you can't
    <braunr> 15:19 < braunr> after a quick reading of the reference manual, i
      think i understand why they made it per object
    <braunr>
      http://pubs.opengroup.org/onlinepubs/009695399/functions/posix_madvise.html
    <braunr> POSIX_MADV_SEQUENTIAL
    <braunr> Specifies that the application expects to access the specified
      range sequentially from lower addresses to higher addresses.
    <braunr> linux might free pages after their access, why not, but this is
      entirely up to the implementation
    <mcsim> I know, when but applications might want data read sequentially to
      remain in the cache, for other sequential accesses this kind of access
      could be treated rather normal or random
    <braunr> we can do differently
    <braunr> mcsim: no
    <braunr> sequential means the access will be sequential
    <braunr> so aggressive readahead (e.g. 0 pages before, many after), should
      be used
    <braunr> for better performance
    <braunr> from my pov, it has nothing to do with caching
    <braunr> i actually sometimes expect data to remain in cache
    <braunr> e.g. before playing a movie from sshfs, i sometimes prefetch it
      using dd
    <braunr> then i use mplayer
    <braunr> i'd be very disappointed if my data didn't remain in the cache :)
    <mcsim> At least these pages could be placed into inactive list to be first
      candidates for pageout.
    <braunr> that's what will happen by default
    <braunr> mcsim: if we need more properties for memory objects, we'll adjust
      the call later, when we actually implement them
    <mcsim> so, first call is vm_advise and second is changed
      mo_change_attributes?
    <braunr> yes
    <mcsim> there will appear 3 new parameters in mo_c_a: policy, pages before
      and pages after?
    <mcsim> braunr: With vm_advise I didn't understand one thing. This call is
      defined in defs file, so that should mean that vm_advise is ordinal rpc
      call. But on the same time it is defined as syscall in mach internals (in
      mach_trap_table).
    <braunr> mcsim: what ?
    <braunr> were is it "defined" ? (it doesn't exit in gnumach currently)
    <mcsim> Ok, let consider vm_map
    <mcsim> I define it both in mach_trap_table and in defs file.
    <mcsim> But why?
    <braunr> uh ?
    <braunr> let me see
    <mcsim> Why defining in defs file is not enough?
    <mcsim> and previous question: there will appear 3 new parameters in
      mo_c_a: policy, pages before and pages after?
    <braunr> mcsim: give me the exact file paths please
    <braunr> mcsim: we'll discuss the new parameters after
    <mcsim> kern/syscall_sw.c
    <braunr> right i see
    <mcsim> here mach_trap_table in defined
    <braunr> i think they're not used
    <braunr> they were probably introduced for performance
    <mcsim> and ./include/mach/mach.defs
    <braunr> don't bother adding vm_advise as a syscall
    <braunr> about the parameters, it's a bit more complicated
    <braunr> you should add 6 parameters
    <braunr> before and after, for the 3 policies
    <braunr> but
    <braunr> as seen in the posix page, there could be more policies ..
    <braunr> ok forget what i said, it's stupid
    <braunr> yes, the 3 parameters you had in mind are correct
    <braunr> don't forget a "don't change" value for the policy though, so the
      kernel ignores the before/after values if we don't want to change that
    <mcsim> ok
    <braunr> mcsim: another reason i asked about "file semantics" is the way we
      handle the cache
    <braunr> mcsim: file semantics imply data is cached, whereas anonymous and
      device memory usually isn't
    <braunr> (although having the cache at the vm layer instead of the pager
      layer allows nice things like the swap cache)
    <mcsim> But this shouldn't affect possibility of implementing of device
      pager.
    <braunr> yes it may
    <braunr> consider how a fault is actually handled by a device
    <braunr> mach must use weird fictitious pages for that
    <braunr> whereas it would be better to simply let the pager handle the
      fault as it sees fit
    <mcsim> setting may_cache to false should resolve the issue
    <braunr> for the caching problem, yes
    <braunr> which is why i still think it's better to handle the cache at the
      vm layer, unlike UVM which lets the vnode pager handle its own cache, and
      removes the vm cache completely
    <mcsim> The only issue with pager interface I see is implementing of
      scatter-gather DMA (as current interface does not support non-consecutive
      access)
    <braunr> right
    <braunr> but that's a performance issue
    <braunr> my problem with device pagers is correctness
    <braunr> currently, i think the kernel just asks pagers for "data"
    <braunr> whereas a device pager should really map its device memory where
      the fault happen
    <mcsim> braunr: You mean that every access to memory should cause page
      fault?
    <mcsim> I mean mapping of device memory
    <braunr> no
    <braunr> i mean a fault on device mapped memory should directly access a
      shared region
    <braunr> whereas file pagers only implement backing store
    <braunr> let me explain a bit more
    <braunr> here is what happens with file mapped memory
    <braunr> you map it, access it (some I/O is done to get the page content in
      physical memory), then later it's flushed back
    <braunr> whereas with device memory, there shouldn't be any I/O, the device
      memory should directly be mapped (well, some devices need the same
      caching behaviour, while others provide direct access)
    <braunr> one of the obvious consequences is that, when you map device
      memory (e.g. a framebuffer), you expect changes in your mapped memory to
      be effective right away
    <braunr> while with file mapped memory, you need to msync() it
    <braunr> (some framebuffers also need to be synced, which suggests greater
      control is needed for external pagers)
    <mcsim> Seems that I understand you. But how it is implemented in other
      OS'es? Do they set something in mmu?
    <braunr> mcsim: in netbsd, pagers have a fault operatin in addition to get
      and put
    <braunr> the device pager sets get and put to null and implements fault
      only
    <braunr> the fault callback then calls the d_mmap callback of the specific
      driver
    <braunr> which usually results in the mmu being programmed directly
    <braunr> (e.g. pmap_enter or similar)
    <braunr> in linux, i think raw device drivers, being implemented as
      character device files, must provide raw read/write/mmap/etc.. functions
    <braunr> so it looks pretty much similar
    <braunr> i'd say our current external pager interface is insufficient for
      device pagers
    <braunr> but antrik may know more since he worked on ggi
    <braunr> antrik: ^
    <mcsim> braunr: Seems he used io_map
    <braunr> mcsim: where ar eyou looking at ? the incubator ?
    <mcsim> his master's thesis
    <braunr> ah the thesis
    <braunr> but where ? :)
    <mcsim> I'll give you a link
    <mcsim> http://dl.dropbox.com/u/36519904/kgi_on_hurd.pdf
    <braunr> thanks
    <mcsim> see p 158
    <braunr> arg, more than 200 pages, and he says he's lazy :/
    <braunr> mcsim: btw, have a look at m_o_ready
    <mcsim> braunr: This is old form of mo_change attributes
    <mcsim> I'm not going to change it
    <braunr> mcsim: these are actually the default object parameters right ?
    <braunr> mcsim: if you don't change it, it means the kernel must set
      default values until the pager changes them, if it does
    <mcsim> yes.
    <antrik> mcsim: madvise() on Linux has a separate flag to indicate that
      pages won't be reused. thus I think it would *not* be a good idea to
      imply it in SEQUENTIAL
    <antrik> braunr: yes, my KMS code relies on mapping memory objects for the
      framebuffer
    <antrik> (it should be noted though that on "modern" hardware, mapping
      graphics memory directly usually gives very poor performance, and drivers
      tend to avoid it...)
    <antrik> mcsim: BTW, it was most likely me who warned about legal issues
      with KAM's work. AFAIK he never managed to get the copyright assignment
      done :-(
    <antrik> (that's not really mandatory for the gnumach work though... only
      for the Hurd userspace parts)
    <antrik> also I'd like to point out again that the cluster_size argument
      from OSF Mach was probably *not* meant for advice from application
      programs, but rather was supposed to reflect the cluster size of the
      filesystem in question. at least that sounds much more plausible to me...
    <antrik> braunr: I have no idea whay you mean by "device pager". device
      memory is mapped once when the VM mapping is established; there is no
      need for any fault handling...
    <antrik> mcsim: to be clear, I think the cluster_size parameter is mostly
      orthogonal to policy... and probably not very useful at all, as ext2
      almost always uses page-sized clusters. I'm strongly advise against
      bothering with it in the initial implementation
    <antrik> mcsim: to avoid confusion, better use a completely different name
      for the policy-decided readahead size
    <mcsim> antrik: ok
    <antrik> braunr: well, yes, the thesis report turned out HUGE; but the
      actual work I did on the KGI port is fairly tiny (not more than a few
      weeks of actual hacking... everything else was just brooding)
    <antrik> braunr: more importantly, it's pretty much the last (and only
      non-trivial) work I did on the Hurd :-(
    <antrik> (also, I don't think I used the word "lazy"... my problem is not
      laziness per se; but rather inability to motivate myself to do anything
      not providing near-instant gratification...)
    <braunr> antrik: right
    <braunr> antrik: i shouldn't consider myself lazy either
    <braunr> mcsim: i agree with antrik, as i told you weeks ago
    <braunr> about
    <braunr> 21:45 < antrik> mcsim: to be clear, I think the cluster_size
      parameter is mostly orthogonal to policy... and probably not very useful
      at all, as ext2 almost always uses page-sized clusters. I'm strongly
      advise against bothering with it
    <braunr>                 in the initial implementation
    <braunr> antrik: but how do you actually map device memory ?
    <braunr> also, strangely enough, here is the comment in dragonflys
      madvise(2)
    <braunr> 21:45 < antrik> mcsim: to be clear, I think the cluster_size
      parameter is mostly orthogonal to policy... and probably not very useful
      at all, as ext2 almost always uses page-sized clusters. I'm strongly
      advise against bothering with it
    <braunr>                 in the initial implementation
    <braunr> arg
    <braunr> MADV_SEQUENTIAL  Causes the VM system to depress the priority of
      pages immediately preceding a given page when it is faulted in.
    <antrik> braunr: interesting...
    <antrik> (about SEQUENTIAL on dragonfly)
    <antrik> as for mapping device memory, I just use to device_map() on the
      mem device to map the physical address space into a memory object, and
      then through vm_map into the driver (and sometimes application) address
      space
    <antrik> formally, there *is* a pager involved of course (implemented
      in-kernel by the mem device), but it doesn't really do anything
      interesting
    <antrik> thinking about it, there *might* actually be page faults involved
      when the address ranges are first accessed... but even then, the handling
      is really trivial and not terribly interesting
    <braunr> antrik: it does the most interesting part, create the physical
      mapping
    <braunr> and as trivial as it is, it requires a special interface
    <braunr> i'll read about device_map again
    <braunr> but yes, the fact that it's in-kernel is what solves the problem
      here
    <braunr> what i'm interested in is to do it outside the kernel :)
    <antrik> why would you want to do that?
    <antrik> there is no policy involved in doing an MMIO mapping
    <antrik> you ask for the pysical memory region you are interested in, and
      that's it
    <antrik> whether the kernel adds the page table entries immediately or on
      faults is really an implementation detail
    <antrik> braunr: ^
    <braunr> yes it's a detail
    <braunr> but do we currently have the interface to make such mappings from
      userspace ?
    <braunr> and i want to do that because i'd like as many drivers as possible
      outside the kernel of course
    <antrik> again, the userspace driver asks the kernel to establish the
      mapping (through device_map() and then vm_map() on the resulting memory
      object)
    <braunr> hm i'm missing something
    <braunr>
      http://www.gnu.org/software/hurd/gnumach-doc/Device-Map.html#Device-Map
      <= this one ?
    <antrik> yes, this one
    <braunr> but this implies the device is implemented by the kernel
    <antrik> the mem device is, yes
    <antrik> but that's not a driver
    <braunr> ah
    <antrik> it's just the interface for doing MMIO
    <antrik> (well, any physical mapping... but MMIO is probably the only real
      use case for that)
    <braunr> ok
    <braunr> i was thinking about completely removing the device interface from
      the kernel actually
    <braunr> but it makes sense to have such devices there
    <antrik> well, in theory, specific kernel drivers can expose their own
      device_map() -- but IIRC the only one that does (besides mem of course)
      is maptime -- which is not a real driver either...
    <braunr> oh btw, i didn't know you had a blog :)
    <antrik> well, it would be possible to replace the device interface by
      specific interfaces for the generic pseudo devices... I'm not sure how
      useful that would be
    <braunr> there are lots of interesting stuff there
    <antrik> hehe... another failure ;-)
    <braunr> failure ?
    <antrik> well, when I realized that I'm speding a lot of time pondering
      things, and never can get myself to actually impelemnt any of them, I had
      the idea that if I write them down, there might at least be *some* good
      from it...
    <antrik> unfortunately it turned out that I need so much effort to write
      things down, that most of the time I can't get myself to do that either
      :-(
    <braunr> i see
    <braunr> well it's still nice to have it
    <antrik> (notice that the latest entry is two years old... and I haven't
      even started describing most of my central ideas :-( )
    <braunr> antrik: i tried to create a blog once, and found what i wrote so
      stupid i immediately removed it
    <antrik> hehe
    <antrik> actually some of my entries seem silly in retrospect as well...
    <antrik> but I guess that's just the way it is ;-)
    <braunr> :)
    <braunr> i'm almost sure other people would be interested in what i had to
      say
    <antrik> BTW, I'm actually not sure whether the Mach interfaces are
      sufficient to implement GEM/TTM... we would certainly need kernel support
      for GART (as for any other kind IOMMU in fact); but beyond that it's not
      clear to me
    <braunr> GEM ? TTM ? GART ?
    <antrik> GEM = Graphics Execution Manager. part of the "new" DRM interface,
      closely tied with KMS
    <antrik> TTM = Translation Table Manager. does part of the background work
      for most of the GEM drivers
    <braunr> "The Graphics Execution Manager (GEM) is a computer software
      system developed by Intel to do memory management for device drivers for
      graphics chipsets." hmm
    <antrik> (in fact it was originally meant to provide the actual interface;
      but the Inter folks decided that it's not useful for their UMA graphics)
    <antrik> GART = Graphics Aperture
    <antrik> kind of an IOMMU for graphics cards
    <antrik> allowing the graphics card to work with virtual mappings of main
      memory
    <antrik> (i.e. allowing safe DMA)
    <braunr> ok
    <braunr> all this graphics stuff looks so complex :/
    <antrik> it is
    <antrik> I have a whole big chapter on that in my thesis... and I'm not
      even sure I got everything right
    <braunr> what is nvidia using/doing (except for getting the finger) ?
    <antrik> flushing out all the details for KMS, GEM etc. took the developers
      like two years (even longer if counting the history of TTM)
    <antrik> Nvidia's proprietary stuff uses a completely own kernel interface,
      which is of course not exposed or docuemented in any way... but I guess
      it's actually similar in what it does)
    <braunr> ok
    <antrik> (you could ask the nouveau guys if you are truly
      interested... they are doing most of their reverse engineering at the
      kernel interface level)
    <braunr> it seems graphics have very special needs, and a lot of them
    <braunr> and the interfaces are changing often
    <braunr> so it's not that much interesting currently
    <braunr> it just means we'll probably have to change the mach interface too
    <braunr> like you said
    <braunr> so the answer to my question, which was something like "do mach
      external pagers only implement files ?", is likely yes
    <antrik> well, KMS/GEM had reached some stability; but now there are
      further changes ahead with the embedded folks coming in with all their
      dedicated hardware, calling for unified buffer management across the
      whole pipeline (from capture to output)
    <antrik> and yes: graphics hardware tends to be much more complex regarding
      the interface than any other hardware. that's because it's a combination
      of actual I/O (like most other devices) with a very powerful coprocessor
    <antrik> and the coprocessor part is pretty much unique amongst peripherial
      devices
    <antrik> (actually, the I/O part is also much more complex than most other
      hardware... but that alone would only require a more complex driver, not
      special interfaces)
    <antrik> embedded hardware makes it more interesting in that the I/O
      part(s) are separate from the coprocessor ones; and that there are often
      several separate specialised ones of each... the DRM/KMS stuff is not
      prepared to deal with this
    <antrik> v4l over time has evolved to cover such things; but it's not
      really the right place to implement graphics drivers... which is why
      there are not efforts to unify these frameworks. funny times...


## IRC, freenode, #hurd, 2012-07-03

    <braunr> mcsim: vm_for_every_page should be static
    <mcsim> braunr: ok
    <braunr> mcsim: see http://gcc.gnu.org/onlinedocs/gcc/Inline.html
    <braunr> and it looks big enough that you shouldn't make it inline
    <braunr> let the compiler decide for you (which is possible only if the
      function is static)
    <braunr> (otherwise a global symbol needs to exist)
    <braunr> mcsim: i don't know where you copied that comment from, but you
      should review the description of the vm_advice call in mach.Defs
    <mcsim> braunr: I see
    <mcsim> braunr: It was vm_inherit :)
    <braunr> mcsim: why isn't NORMAL defined in vm_advise.h ?
    <braunr> mcsim: i figured actually ;)
    <mcsim> braunr: I was going to do it later when.
    <braunr> mcsim: for more info on inline, see
      http://www.kernel.org/doc/Documentation/CodingStyle
    <braunr> arg that's an old one
    <mcsim> braunr: I know that I do not follow coding style
    <braunr> mcsim: this one is about linux :p
    <braunr> mcsim: http://lxr.linux.no/linux/Documentation/CodingStyle should
      have it
    <braunr> mcsim: "Chapter 15: The inline disease"
    <mcsim> I was going to fix it later during refactoring when I'll merge
      mplaneta/gsoc12/working to mplaneta/gsoc12/master
    <braunr> be sure not to forget :p
    <braunr> and the best not to forget is to do it asap
    <braunr> +way
    <mcsim> As to inline. I thought that even if I specify function as inline
      gcc makes final decision about it.
    <mcsim> There was a specifier that made function always inline, AFAIR.
    <braunr> gcc can force a function not to be inline, yes
    <braunr> but inline is still considered as a strong hint


## IRC, freenode, #hurd, 2012-07-05

    <mcsim1> braunr: hello. You've said that pager has to supply 2 values to
      kernel to give it an advice how execute page fault. These two values
      should be number of pages before and after the page where fault
      occurred. But for sequential policy number of pager before makes no
      sense. For random policy too. For normal policy it would be sane to make
      readahead symmetric. Probably it would be sane to make pager supply
      cluster_size (if it is necessary to supply any) that w
    <mcsim1> *that will be advice for kernel of least sane value? And maximal
      value will be f(free_memory, map_entry_size)?
    <antrik> mcsim1: I doubt symmetric readahead would be a good default
      policy... while it's hard to estimate an optimum over all typical use
      cases, I'm pretty sure most situtations will benefit almost exclusively
      from reading following pages, not preceeding ones
    <antrik> I'm not even sure it's useful to read preceding pages at all in
      the default policy -- the use cases are probably so rare that the penalty
      in all other use cases is not justified. I might be wrong on that
      though...
    <antrik> I wonder how other systems handle that
    <LarstiQ> antrik: if there is a mismatch between pages and the underlying
      store, like why changing small bits of data on an ssd is slow?
    <braunr> mcsim1: i don't see why not
    <braunr> antrik: netbsd reads a few pages before too
    <braunr> actually, what netbsd does vary on the version, some only mapped
      in resident pages, later versions started asynchronous transfers in the
      hope those pages would be there
    <antrik> LarstiQ: not sure what you are trying to say
    <braunr> in linux :
    <braunr> 321  *  MADV_NORMAL - the default behavior is to read clusters.
      This
    <braunr> 322  *              results in some read-ahead and read-behind.
    <braunr> not sure if it's actually what the implementation does
    <antrik> well, right -- it's probably always useful to read whole clusters
      at a time, especially if they are the same size as pages... that doesn't
      mean it always reads preceding pages; only if the read is in the middle
      of the cluster AIUI
    <LarstiQ> antrik: basically what braunr just pasted
    <antrik> and in most cases, we will want to read some *following* clusters
      as well, but probably not preceding ones
    * LarstiQ nods
    <braunr> antrik: the default policy is usually rather sequential
    <braunr> here are the numbers for netbsd
    <braunr>  166 static struct uvm_advice uvmadvice[] = {
    <braunr>  167         { MADV_NORMAL, 3, 4 },
    <braunr>  168         { MADV_RANDOM, 0, 0 },
    <braunr>  169         { MADV_SEQUENTIAL, 8, 7},
    <braunr>  170 };
    <braunr> struct uvm_advice {
    <braunr>          int advice;
    <braunr>          int nback;
    <braunr>          int nforw;
    <braunr> };
    <braunr> surprising isn't it ?
    <braunr> they may suggest sequential may be backwards too
    <braunr> makes sense
    <antrik> braunr: what are these numbers? pages?
    <braunr> yes
    <antrik> braunr: I suspect the idea behind SEQUENTIAL is that with typical
      sequential access patterns, you will start at one end of the file, and
      then go towards the other end -- so the extra clusters in the "wrong"
      direction do not actually come into play
    <antrik> only situation where some extra clusters are actually read is when
      you start in the middle of a file, and thus do not know yet in which
      direction the sequential read will go...
    <braunr> yes, there are similar comments in the linux code
    <braunr> mcsim1: so having before and after numbers seems both
      straightforward and in par with other implementations
    <antrik> I'm still surprised about the almost symmetrical policy for NORMAL
      though
    <antrik> BTW, is it common to use heuristics for automatically recognizing
      random and sequential patterns in the absence of explicit madise?
    <braunr> i don't know
    <braunr> netbsd doesn't use any, linux seems to have different behaviours
      for anonymous and file memory
    <antrik> when KAM was working on this stuff, someone suggested that...
    <braunr> there is a file_ra_state struct in linux, for per file read-ahead
      policy
    <braunr> now the structure is of course per file system, since they all use
      the same address
    <braunr> (which is why i wanted it to be per pager in the first place)
    <antrik> mcsim1: as I said before, it might be useful for the pager to
      supply cluster size, if it's different than page size. but right now I
      don't think this is something worth bothering with...
    <antrik> I seriously doubt it would be useful for the pager to supply any
      other kind of policy
    <antrik> braunr: I don't understand your remark about using the same
      address...
    <antrik> braunr: pre-mapping seems the obvious way to implement readahead
      policy
    <antrik> err... per-mapping
    <braunr> the ra_state (read ahead state) isn't the policy
    <braunr> the policy is per mapping, parts of the implementation of the
      policy is per file system
    <mcsim1> braunr: How do you look at following implementation of NORMAL
      policy: We have fault page that is current. Than we have maximal size of
      readahead block. First we find first absent pages before and after
      current. Than we try to fit block that will be readahead into this
      range. Here could be following situations: in range RBS/2 (RBS -- size of
      readahead block) there is no any page, so readahead will be symmetric; if
      current page is first absent page than all 
    <mcsim1> RBS block will consist of pages that are after current; on the
      contrary if current page is last absent than readahead will go backwards.
    <mcsim1> Additionally if current page is approximately in the middle of the
      range we can decrease RBS, supposing that access is random.
    <braunr> mcsim1: i think your gsoc project is about readahead, we're in
      july, and you need to get the job done
    <braunr> mcsim1: grab one policy that works, pages before and after are
      good enough
    <braunr> use sane default values, let the pagers decide if they want
      something else
    <braunr> and concentrate on the real work now
    <antrik> braunr: I still don't see why pagers should mess with that... only
      complicates matters IMHO
    <braunr> antrik: probably, since they almost all use the default
      implementation
    <braunr> mcsim1: just use sane values inside the kernel :p
    <braunr> this simplifies things by only adding the new vm_advise call and
      not change the existing external pager interface


## IRC, freenode, #hurd, 2012-07-12

    <braunr> mcsim: so, to begin with, tell us what state you've reached please
    <mcsim> braunr: I'm writing code for hurd and gnumach. For gnumach I'm
      implementing memory policies now. RANDOM and NORMAL seems work, but in
      hurd I found error that I made during editing ext2fs. So for now ext2fs
      does not work
    <braunr> policies ?
    <braunr> what about mechanism ?
    <mcsim> also I moved some translators to new interface.
    <mcsim> It works too
    <braunr> well that's impressive
    <mcsim> braunr: I'm not sure yet that everything works
    <braunr> right, but that's already a very good step
    <braunr> i thought you were still working on the interfaces to be honest
    <mcsim> And with mechanism I didn't implement moving pages to inactive
      queue
    <braunr> what do you mean ?
    <braunr> ah you mean with the sequential policy ?
    <mcsim> yes
    <braunr> you can consider this a secondary goal
    <mcsim> sequential I was going to implement like you've said, but I still
      want to support moving pages to inactive queue
    <braunr> i think you shouldn't
    <braunr> first get to a state where clustered transfers do work fine
    <mcsim> policies are implemented in function calculate_clusters
    <braunr> then, you can try, and measure the difference
    <mcsim> ok. I'm now working on fixing ext2fs
    <braunr> so, except from bug squashing, what's left to do ?
    <mcsim> finish policies and ext2fs; move fatfs, ufs, isofs to new
      interface; test this all; edit patches from debian repository, that
      conflict with my changes; rearrange commits and fix code indentation;
      update documentation;
    <braunr> think about measurements too
    <tschwinge> mcsim: Please don't spend a lot of time on ufs.  No testing
      required for that one.
    <braunr> and keep us informed about your progress on bug fixing, so we can
      test soon
    <mcsim> Forgot about moving system to new interfaces (I mean determine form
      of vm_advise and memory_object_change_attributes)
    <braunr> s/determine/final/
    <mcsim> braunr: ok.
    <braunr> what do you mean "moving system to new interfaces" ?
    <mcsim> braunr: I also pushed code changes to gnumach and hurd git
      repositories
    <mcsim> I met an issue with memory_object_change_attributes when I tried to
      use it as I have to update all applications that use it. This includes
      libc and translators that are not in hurd repository or use debian
      patches. So I will not be able to run system with new
      memory_object_change_attributes interface, until I update all software
      that use this rpc
    <braunr> this is a bit like the problem i had with my change
    <braunr> the solution is : don't do it
    <braunr> i mean, don't change the interface in an incompatible way
    <braunr> if you can't change an existing call, add a new one
    <mcsim> temporary I changed memory_object_set_attributes as it isn't used
      any more.
    <mcsim> braunr: ok. Adding new call is a good idea :)


## IRC, freenode, #hurd, 2012-07-16

    <braunr> mcsim: how did you deal with multiple page transfers towards the
      default pager ?
    <mcsim> braunr: hello. Didn't handle this yet, but AFAIR default pager
      supports multiple page transfers.
    <braunr> mcsim: i'm almost sure it doesn't
    <mcsim> braunr: indeed
    <mcsim> braunr: So, I'll update it just other translators.
    <braunr> like other translators you mean ?
    <mcsim> *just as
    <mcsim> braunr: yes
    <braunr> ok
    <braunr> be aware also that it may need some support in vm_pageout.c in
      gnumach
    <mcsim> braunr: thank you
    <braunr> if you see anything strange in the default pager, don't hesitate
      to talk about it
    <mcsim> braunr: ok. I didn't finish with ext2fs yet.
    <braunr> so it's a good thing you're aware of it now, before you begin
      working on it :)
    <mcsim> braunr: I'm working on ext2 now.
    <braunr> yes i understand
    <braunr> i meant "before beginning work on the default pager"
    <mcsim> ok

    <antrik> mcsim: BTW, we were mostly talking about readahead (pagein) over
      the past weeks, so I wonder what the status on clustered page*out* is?...
    <mcsim> antrik: I don't work on this, but following, I think, is an example
      of *clustered* pageout: _pager_seqnos_memory_object_data_return: object =
      113, seqno = 4, control = 120, start_address = 0, length = 8192, dirty =
      1. This is an example of debugging printout that shows that pageout
      manipulates with chunks bigger than page sized.
    <mcsim> antrik: Another one with bigger length
      _pager_seqnos_memory_object_data_return: object = 125, seqno = 124,
      control = 132, start_address = 131072, length = 126976, dirty = 1, kcopy
    <antrik> mcsim: that's odd -- I didn't know the functionality for that even
      exists in our codebase...
    <antrik> my understanding was that Mach always sends individual pageout
      requests for ever single page it wants cleaned...
    <antrik> (and this being the reason for the dreadful thread storms we are
      facing...)
    <braunr> antrik: ok
    <braunr> antrik: yes that's what is happening
    <braunr> the thread storms aren't that much of a problem now
    <braunr> (by carefully throttling pageouts, which is a task i intend to
      work on during the following months, this won't be an issue any more)


## IRC, freenode, #hurd, 2012-07-19

    <mcsim> I moved fatfs, ufs, isofs to new interface, corrected some errors
      in other that I already moved, moved kernel to new interface (renamed
      vm_advice to vm_advise and added rpcs memory_object_set_advice and
      memory_object_get_advice). Made some changes in mechanism and tried to
      finish ext2 translator.
    <mcsim> braunr: I've got an issue with fictitious pages...
    <mcsim> When I determine bounds of cluster in external object I never know
      its actual size. So, mo_data_request call could ask data that are behind
      object bounds. The problem is that pager returns data that it has and
      because of this fictitious pages that were allocated are not freed.
    <braunr> why don't you know the size ?
    <mcsim> I see 2 solutions. First one is do not allocate fictitious pages at
      all (but I think that there could be issues). Another lies in allocating
      fictitious pages, but then freeing them with mo_data_lock.
    <mcsim> braunr: Because pages does not inform kernel about object size.
    <braunr> i don't understand what you mean
    <mcsim> I think that second way is better.
    <braunr> so how does it happen ?
    <braunr> you get a page fault
    <mcsim> Don't you understand problem or solutions?
    <braunr> then a lookup in the map finds the map entry
    <braunr> and the map entry gives you the link to the underlying object
    <mcsim> from vm_object.h: 	vm_size_t		size;		/*
      Object size (only valid if internal)				 */
    <braunr> mcsim: ugh
    <mcsim> For external they are either 0x8000 or 0x20000...
    <braunr> and for internal ?
    <braunr> i'm very surprised to learn that
    <mcsim> braunr: for internal size is actual
    <braunr> right sorry, wrong question
    <braunr> did you find what 0x8000 and 0x20000 are ?
    <mcsim> for external I met only these 2 magic numbers when printed out
      arguments of functions _pager_seqno_memory_object_... when they were
      called.
    <braunr> yes but did you try to find out where they come from ?
    <mcsim> braunr: no. I think that 0x2000(many zeros) is maximal possible
      object size.
    <braunr> what's the exact value ?
    <mcsim> can't tell exactly :/ My hurd box has broken again.
    <braunr> mcsim: how does the vm find the backing content then ?
    <mcsim> braunr: Do you know if it is guaranteed that map_entry size will be
      not bigger than external object size?
    <braunr> mcsim: i know it's not
    <braunr> but you can use the map entry boundaries though
    <mcsim> braunr: vm asks pager
    <braunr> but if the page is already present
    <braunr> how does it know ?
    <braunr> it must be inside a vm_object ..
    <mcsim> If I can use these boundaries than the problem, I described is not
      actual.
    <braunr> good
    <braunr> it makes sense to use these boundaries, as the application can't
      use data outside the mapping
    <mcsim> I ask page with vm_page_lookup
    <braunr> it would matter for shared objects, but then they have their own
      faults :p
    <braunr> ok
    <braunr> so the size is actually completely ignord
    <mcsim> if it is present than I stop expansion of cluster.
    <braunr> which makes sense
    <mcsim> braunr: yes, for external.
    <braunr> all right
    <braunr> use the mapping boundaries, it will do
    <braunr> mcsim: i have only one comment about what i could see
    <braunr> mcsim: there are 'advice' fields in both vm_map_entry and
      vm_object
    <braunr> there should be something else in vm_object
    <braunr> i told you about pages before and after
    <braunr> mcsim: how are you using this per object "advice" currently ?
    <braunr> (in addition, using the same name twice for both mechanism and
      policy is very sonfusing)
    <braunr> confusing*
    <mcsim> braunr: I try to expand cluster as much as it possible, but not
      much than limit
    <mcsim> they both determine policy, but advice for entry has bigger
      priority
    <braunr> that's wrong
    <braunr> mapping and content shouldn't compete for policy
    <braunr> the mapping tells the policy (=the advice) while the content tells
      how to implement (e.g. how much content)
    <braunr> IMO, you could simply get rid of the per object "advice" field and
      use default values for now
    <mcsim> braunr: What sense these values for number of pages before and
      after should have?
    <braunr> or use something well known, easy, and effective like preceding
      and following pages
    <braunr> they give the vm the amount of content to ask the backing pager
    <mcsim> braunr: maximal amount, minimal amount or exact amount?
    <braunr> neither
    <braunr> that's why i recommend you forget it for now
    <braunr> but
    <braunr> imagine you implement the three standard policies (normal, random,
      sequential)
    <braunr> then the pager assigns preceding and following numbers for each of
      them, say [5;5], [0;0], [15;15] respectively
    <braunr> these numbers would tell the vm how many pages to ask the pagers
      in a single request and from where
    <mcsim> braunr: but in fact there could be much more policies.
    <braunr> yes
    <mcsim> also in kernel context there is no such unit as pager.
    <braunr> so there should be a call like memory_object_set_advice(int
      advice, int preceding, int following);
    <braunr> for example
    <braunr> what ?
    <braunr> the pager is the memory manager
    <braunr> it does exist in kernel context
    <braunr> (or i don't understand what you mean)
    <mcsim> there is only port, but port could be either pager or something
      else
    <braunr> no, it's a pager
    <braunr> it's a port whose receive right is hold by a task implementing the
      pager interface
    <braunr> either the default pager or an untrusted task
    <braunr> (or null if the object is anonymous memory not yet sent to the
      default pager)
    <mcsim> port is always pager?
    <braunr> the object port is, yes
    <braunr>         struct ipc_port         *pager;         /* Where to get
      data */
    <mcsim> So, you suggest to keep set of advices for each object?
    <braunr> i suggest you don't change anything in objects for now
    <braunr> keep the advice in the mappings only, and implement default
      behaviour for the known policies
    <braunr> mcsim: if you understand this point, then i have nothing more to
      say, and we should let nowhere_man present his work
    <mcsim> braunr: ok. I'll implement only default behaviors for know policies
      for now.
    <braunr> (actually, using the mapping boundaries is slightly unoptimal, as
      we could have several mappings for the same content, e.g. a program with
      read only executable mapping, then ro only)
    <braunr> mcsim: another way to know the "size" is to actually lookup for
      pages in objects
    <braunr> hm no, that's not true
    <mcsim> braunr: But if there is no page we have to ask it
    <mcsim> and I don't understand why using mappings boundaries is unoptimal
    <braunr> here is bash
    <braunr> 0000000000400000    868K r-x--  /bin/bash
    <braunr> 00000000006d9000     36K rw---  /bin/bash
    <braunr> two entries, same file
    <braunr> (there is the anonymous memory layer for the second, but it would
      matter for the first cow faults)


## IRC, freenode, #hurd, 2012-08-02

    <mcsim> braunr: You said that I probably need some support in vm_pageout.c
      to make defpager work with clustered page transfers, but TBH I thought
      that I have to implement only pagein. Do you expect from me implementing
      pageout either? Or I misunderstand role of vm_pageout.c?
    <braunr> no
    <braunr> you're expected to implement only pagins for now
    <braunr> pageins
    <mcsim> well, I'm finishing merging of ext2fs patch for large stores and
      work on defpager in parallel.
    <mcsim> braunr: Also I didn't get your idea about configuring of paging
      mechanism on behalf of pagers.
    <braunr> which one ?
    <mcsim> braunr: You said that pager has somehow pass size of desired
      clusters for different paging policies.
    <braunr> mcsim: i said not to care about that
    <braunr> and the wording isn't correct, it's not "on behalf of pagers"
    <mcsim> servers?
    <braunr> pagers could tell the kernel what size (before and after a faulted
      page) they prefer for each existing policy
    <braunr> but that's one way to do it
    <braunr> defaults work well too
    <braunr> as shown in other implementations


## IRC, freenode, #hurd, 2012-08-09

    <mcsim> braunr: I'm still debugging ext2 with large storage patch
    <braunr> mcsim: tough problems ?
    <mcsim> braunr: The same issues as I always meet when do debugging, but it
      takes time.
    <braunr> mcsim: so nothing blocking so far ?
    <mcsim> braunr: I can't tell you for sure that I will finish up to 13th of
      August and this is unofficial pencil down date.
    <braunr> all right, but are you blocked ?
    <mcsim> braunr: If you mean the issues that I can not even imagine how to
      solve than there is no ones.
    <braunr> good
    <braunr> mcsim: i'll try to review your code again this week end
    <braunr> mcsim: make sure to commit everything even if it's messy
    <mcsim> braunr: ok
    <mcsim> braunr: I made changes to defpager, but I haven't tried
      them. Commit them too?
    <braunr> mcsim: sure
    <braunr> mcsim: does it work fine without the large storage patch ?
    <mcsim> braunr: looks fine, but TBH I can't even run such things like fsx,
      because even without my changes it failed mightily at once.
    <braunr> mcsim: right, well, that will be part of another task :)


## IRC, freenode, #hurd, 2012-08-13

    <mcsim> braunr: hello. Seems ext2fs with large store patch works.


## IRC, freenode, #hurd, 2012-08-19

    <mcsim> hello. Consider such situation. There is a page fault and kernel
      decided to request pager for several pages, but at the moment pager is
      able to provide only first pages, the rest ones are not know yet. Is it
      possible to supply only one page and regarding rest ones tell the kernel
      something like: "Rest pages try again later"?
    <mcsim> I tried pager_data_unavailable && pager_flush_some, but this seems
      does not work.
    <mcsim> Or I have to supply something anyway?
    <braunr> mcsim: better not provide them
    <braunr> the kernel only really needs one page
    <braunr> don't try to implement "try again later", the kernel will do that
      if other page faults occur for those pages
    <mcsim> braunr: No, translator just hangs
    <braunr> ?
    <mcsim> braunr: And I even can't deattach it without reboot
    <braunr> hangs when what 
    <braunr> ?
    <braunr> i mean, what happens when it hangs ?
    <mcsim> If kernel request 2 pages and I provide one, than when page fault
      occurs in second page translator hangs.
    <braunr> well that's a bug
    <braunr> clustered pager transfer is a mere optimization, you shouldn't
      transfer more than you can just to satisfy some requested size
    <mcsim> I think that it because I create fictitious pages before calling
      mo_data_request
    <braunr> as placeholders ?
    <mcsim> Yes. Is it correct if I will not grab fictitious pages?
    <braunr> no
    <braunr> i don't know the details well enough about fictitious pages
      unfortunately, but it really feels wrong to use them where real physical
      pages should be used instead
    <braunr> normally, an in-transfer page is simply marked busy
    <mcsim> But If page is already marked busy kernel will not ask it another
      time.
    <braunr> when the pager replies, you unbusy them
    <braunr> your bug may be that you incorrectly use pmap
    <braunr> you shouldn't create mmu mappings for pages you didn't receive
      from the pagers
    <mcsim> I don't create them
    <braunr> ok so you correctly get the second page fault
    <mcsim> If pager supplies only first pages, when asked were two, than
      second page will not become un-busy.
    <braunr> that's a bug
    <braunr> your code shouldn't assume the pager will provide all the pages it
      was asked for
    <braunr> only the main one
    <mcsim> Will it be ok if I will provide special attribute that will keep
      information that page has been advised?
    <braunr> what for ?
    <braunr> i don't understand "page has been advised"
    <mcsim> Advised page is page that is asked in cluster, but there wasn't a
      page fault in it.
    <mcsim> I need this attribute because if I don't inform kernel about this
      page anyhow, than kernel will not change attributes of this page.
    <braunr> why would it change its attributes ?
    <mcsim> But if page fault will occur in page that was asked than page will
      be already busy by the moment.
    <braunr> and what attribute ?
    <mcsim> advised
    <braunr> i'm lost
    <braunr> 08:53 < mcsim> I need this attribute because if I don't inform
      kernel about this page anyhow, than kernel will not change attributes of
      this page.
    <braunr> you need the advised attribute because if you don't inform the
      kernel about this page, the kernel will not change the advised attribute
      of this page ?
    <mcsim> Not only advised, but busy as well.
    <mcsim> And if page fault will occur in this page, kernel will not ask it
      second time. Kernel will just block.
    <braunr> well that's normal
    <mcsim> But if kernel will block and pager is not going to report somehow
      about this page, than translator will hang.
    <braunr> but the pager is going to report
    <braunr> and in this report, there can be less pages then requested
    <mcsim> braunr: You told not to report
    <braunr> the kernel can deduce it didn't receive all the pages, and mark
      them unbusy anyway
    <braunr> i told not to transfer more than requested
    <braunr> but not sending data can be a form of communication
    <braunr> i mean, sending a message in which data is missing
    <braunr> it simply means its not there, but this info is sufficient for the
      kernel
    <mcsim> hmmm... Seems I understood you. Let me try something.
    <mcsim> braunr: I informed kernel about missing page as follows:
      pager_data_supply (pager, precious, writelock, i, 1, NULL, 0); Am I
      right?
    <braunr> i don't know the interface well
    <braunr> what does it mean 
    <braunr> ?
    <braunr> are you passing NULL as the data for a missing page ?
    <mcsim> yes
    <braunr> i see
    <braunr> you shouldn't need a request for that though, avoiding useless ipc
      is a good thing
    <mcsim> i is number of page, 1 is quantity
    <braunr> but if you can't find a better way for now, it will do
    <mcsim> But this does not work :(
    <braunr> that's a bug
    <braunr> in your code probably
    <mcsim> braunr: supplying NULL as data returns MACH_SEND_INVALID_MEMORY
    <braunr> but why would it work ?
    <braunr> mach expects something
    <braunr> you have to change that
    <mcsim> It's mig who refuses data. Mach does not even get the call.
    <braunr> hum
    <mcsim> That's why I propose to provide new attribute, that will keep
      information regarding whether the page was asked as advice or not.
    <braunr> i still don't understand why
    <braunr> why don't you fix mig so you can your null message instead ?
    <braunr> +send
    <mcsim> braunr: because usually this is an error
    <braunr> the kernel will decide if it's an erro
    <braunr> r
    <braunr> what kinf of reply do you intend to send the kernel with for these
      "advised" pages ?
    <mcsim> no reply. But when page fault will occur in busy page and it will
      be also advised, kernel will not block, but ask this page another time.
    <mcsim> And how kernel will know that this is an error or not?
    <braunr> why ask another time ?!
    <braunr> you really don't want to flood pagers with useless messages
    <braunr> here is how it should be
    <braunr> 1/ the kernel requests pages from the pager
    <braunr> it know the range
    <braunr> 2/ the pager replies what it can, full range, subset of it, even
      only one page
    <braunr> 3/ the kernel uses what the pager replied, and unbusies the other
      pages
    <mcsim> First time page was asked because page fault occurred in
      neighborhood. And second time because PF occurred in page. 
    <braunr> well it shouldn't
    <braunr> or it should, but then you have a segfault
    <mcsim> But kernel does not keep bound of range, that it asked.
    <braunr> if the kernel can't find the main page, the one it needs to make
      progress, it's a segfault
    <mcsim> And this range could be supplied in several messages.
    <braunr> absolutely not
    <braunr> you defeat the purpose of clustered pageins if you use several
      messages
    <mcsim> But interface supports it
    <braunr> interface supported single page transfers, doesn't mean it's good
    <braunr> well, you could use several messages
    <braunr> as what we really want is less I/O
    <mcsim> Noone keeps bounds of requested range, so it couldn't be checked
      that range was split 
    <braunr> but it would be so much better to do it all with as few messages
      as possible
    <braunr> does the kernel knows the main page ?
    <braunr> know*
    <mcsim> Splitting range is not optimal, but it's not an error.
    <braunr> i assume it does
    <braunr> doesn't it ?
    <mcsim> no, that's why I want to provide new attribute.
    <braunr> i'm sorry i'm lost again
    <braunr> how does the kernel knows a page fault has been serviced ?
    <braunr> know*
    <mcsim> It receives an interrupt
    <braunr> ?
    <braunr> let's not mix terms
    <mcsim> oh.. I read as received. Sorry
    <mcsim> It get mo_data_supply message. Than it replaces fictitious pages
      with real ones.
    <braunr> so you get a message
    <braunr> and you kept track of the range using fictitious pages
    <braunr> use the busy flag instead, and another way to retain the range
    <mcsim> I allocate fictitious pages to reserve place. Than if page fault
      will occur in this page fictitious page kernel will not send another
      mo_data_request call, it will wait until fictitious page unblocks.
    <braunr> i'll have to check the code but it looks unoptimal to me
    <braunr> we really don't want to allocate useless objects when a simple
      busy flag would do
    <mcsim> busy flag for what? There is no page yet
    <braunr> we're talking about mo_data_supply
    <braunr> actually we're talking about the whole page fault process
    <mcsim> We can't mark nothing as busy, that's why kernel allocates
      fictitious page and marks it as busy until real page would be supplied.
    <braunr> what do you mean "nothing" ?
    <mcsim> VM_PAGE_NULL
    <braunr> uh ?
    <braunr> when are physical pages allocated ?
    <braunr> on request or on reply from the pager ?
    <braunr> i'm reading mo_data_supply, and it looks like the page is already
      busy at that time
    <mcsim> they are allocated by pager and than supplied in reply
    <mcsim> Yes, but these pages are fictitious
    <braunr> show me please
    <braunr> in the master branch, not yours
    <mcsim> that page is fictitious?
    <braunr> yes
    <braunr> i'm referring to the way mach currently does things
    <mcsim> vm/vm_fault.c:582
    <braunr> that's memory_object_lock_page
    <braunr> hm wait
    <braunr> my bad
    <braunr> ah that damn object chaining :/
    <braunr> ok
    <braunr> the original code is stupid enough to use fictitious pages all the
      time, you probably have to do the same
    <mcsim> hm... Attributes will be useless, pager should tell something about
      pages, that it is not going to supply.
    <braunr> yes
    <braunr> that's what null is for
    <mcsim> Not null, null is error.
    <braunr> one problem i can think of is making sure the kernel doesn't
      interpret missing as error
    <braunr> right
    <mcsim> I think better have special value for mo_data_error
    <braunr> probably


### IRC, freenode, #hurd, 2012-08-20

    <antrik> braunr: I think it's useful to allow supplying the data in several
      batches. the kernel should *not* assume that any data missing in the
      first batch won't be supplied later.
    <braunr> antrik: it really depends
    <braunr> i personally prefer synchronous approaches
    <antrik> demanding that all data is supplied at once could actually turn
      readahead into a performace killer
    <mcsim> antrik: Why? The only drawback I see is higher response time for
      page fault, but it also leads to reduced overhead.
    <braunr> that's why "it depends"
    <braunr> mcsim: it brings benefit only if enough preloaded pages are
      actually used to compensate for the time it took the pager to provide
      them
    <braunr> which is the case for many workloads (including sequential access,
      which is the common case we want to optimize here)
    <antrik> mcsim: the overhead of an extra RPC is negligible compared to
      increased latencies when dealing with slow backing stores (such as disk
      or network)
    <mcsim> antrik: also many replies lead to fragmentation, while in one reply
      all data is gathered in one bunch. If all data is placed consecutively,
      than it may be transferred next time faster.
    <braunr> mcsim: what kind of fragmentation ?
    <antrik> I really really don't think it's a good idea for the page to hold
      back the first page (which is usually the one actually blocking) while
      it's still loading some other pages (which will probably be needed only
      in the future anyways, if at all)
    <antrik> err... for the pager to hold back
    <braunr> antrik: then all pagers should be changed to handle asynchronous
      data supply
    <braunr> it's a bit late to change that now
    <mcsim> there could be two cases of data placement in backing store: 1/ all
      asked data is placed consecutively; 2/ it is spread among backing
      store. If pager gets data in one message it more like place it
      consecutively. So to have data consecutive in each pager, each pager has
      to try send data in one message. Having data placed consecutive is
      important, since reading of such data is much more faster.
    <braunr> mcsim: you're confusing things ..
    <braunr> or you're not telling them properly
    <mcsim> Ok. Let me try one more time
    <braunr> since you're working *only* on pagein, not pageout, how do you
      expect spread pages being sent in a single message be better than
      multiple messages ?
    <mcsim> braunr: I think about future :)
    <braunr> ok
    <braunr> but antrik is right, paging in too much can reduce performance
    <braunr> so the default policy should be adjusted for both the worst case
      (one page) and the average/best (some/mane contiguous pages)
    <braunr> through measurement ideally
    <antrik> mcsim: BTW, I still think implementing clustered pageout has
      higher priority than implementing madvise()... but if the latter is less
      work, it might still make sense to do it first of course :-)
    <braunr> many*
    <braunr> there aren't many users of madvise, true
    <mcsim> antrik: Implementing madvise I expect to be very simple. It should
      just translate call to vm_advise
    <antrik> well, that part is easy of course :-) so you already implemented
      vm_advise itself I take it?
    <mcsim> antrik: Yes, that was also quite easy.
    <antrik> great :-)
    <antrik> in that case it would be silly of course to postpone implementing
      the madvise() wrapper. in other words: never mind my remark about
      priorities :-)


## IRC, freenode, #hurd, 2012-09-03

    <mcsim> I try a test with ext2fs. It works, than I just recompile ext2fs
      and it stops working, than I recompile it again several times and each
      time the result is unpredictable.
    <braunr> sounds like a concurrency issue
    <mcsim> I can run the same test several times and ext2 works until I
      recompile it. That's the problem. Could that be concurrency too?
    <braunr> mcsim: without bad luck, yes, unless "several times" is a lot
    <braunr> like several dozens of tries


## IRC, freenode, #hurd, 2012-09-04

    <mcsim> hello. I want to tell that ext2fs translator, that I work on,
      replaced for my system old variant that processed only single pages
      requests. And it works with partitions bigger than 2 Gb.
    <mcsim> Probably I'm not for from the end.
    <mcsim> But it's worth to mention that I didn't fix that nasty bug that I
      told yesterday about.
    <mcsim> braunr: That bug sometimes appears after recompilation of ext2fs
      and always disappears after sync or reboot. Now I'm going to finish
      defpager and test other translators.


## IRC, freenode, #hurd, 2012-09-17

    <mcsim> braunr: hello. Do you remember that you said that pager has to
      inform kernel about appropriate cluster size for readahead?
    <mcsim> I don't understand how kernel store this information, because it
      does not know about such unit as "pager".
    <mcsim> Can you give me an advice about how this could be implemented?
    <youpi> mcsim: it can store it in the object
    <mcsim> youpi: It too big overhead
    <mcsim> youpi: at least from my pow
    <mcsim> *pov
    <braunr> mcsim: we discussed this already
    <braunr> mcsim: there is no "pager" entity in the kernel, which is a defect
      from my PoV
    <braunr> mcsim: the best you can do is follow what the kernel already does
    <braunr> that is, store this property per object$
    <braunr> we don't care much about the overhead for now
    <braunr> my guess is there is already some padding, so the overhead is
      likely to be amortized by this
    <braunr> like youpi said
    <mcsim> I remember that discussion, but I didn't get than whether there
      should be only one or two values for all policies. Or each policy should
      have its own values?
    <mcsim> braunr: ^
    <braunr> each policy should have its own values, which means it can be
      implemented with a simple static array somewhere
    <braunr> the information in each object is a policy selector, such as an
      index in this static array
    <mcsim> ok
    <braunr> mcsim: if you want to minimize the overhead, you can make this
      selector a char, and place it near another char member, so that you use
      space that was previously used as padding by the compiler
    <braunr> mcsim: do you see what i mean ?
    <mcsim> yes
    <braunr> good


## IRC, freenode, #hurd, 2012-09-17

    <mcsim> hello. May I add function krealloc to slab.c?
    <braunr> mcsim: what for ?
    <mcsim> braunr: It is quite useful for creating dynamic arrays
    <braunr> you don't want dynamic arrays
    <mcsim> why?
    <braunr> they're expensive
    <braunr> try other data structures
    <mcsim> more expensive than linked lists?
    <braunr> depends
    <braunr> but linked lists aren't the only other alternative
    <braunr> that's why btrees and radix trees (basically trees of arrays)
      exist
    <braunr> the best general purpose data structure we have in mach is the red
      black tree currently
    <braunr> but always think about what you want to do with it
    <mcsim> I want to store there sets of sizes for different memory
      policies. I don't expect this array to be big. But for sure I can use
      rbtree for it.
    <braunr> why not a static array ?
    <braunr> arrays are perfect for known data sizes
    <mcsim> I expect from pager to supply its own sizes. So at the beginning in
      this array is only default policy. When pager wants to supply it own
      policy kernel lookups table of advice. If this policy is new set of sizes
      then kernel creates new entry in table of advice.
    <braunr> that would mean one set of sizes for each object
    <braunr> why don't you make things simple first ?
    <mcsim> Object stores only pointer to entry in this table.
    <braunr> but there is no pager object shared by memory objects in the
      kernel
    <mcsim> I mean struct vm_object
    <braunr> so that's what i'm saying, one set per object
    <braunr> it's useless overhead
    <braunr> i would really suggest using a global set of policies for now
    <mcsim> Probably, I don't understand you. Where do you want to store this
      static array?
    <braunr> it's a global one
    <mcsim> "for now"? It is not a problem to implement a table for local
      advice, using either rbtree or dynamic array.
    <braunr> it's useless overhead
    <braunr> and it's not a single integer, you want a whole container per
      object
    <braunr> don't do anything fancy unless you know you really want it
    <braunr> i'll link the netbsd code again as a very good example of how to
      implement global policies that work more than decently for every file
      system in this OS
    <braunr>
      http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/uvm/uvm_fault.c?rev=1.194&content-type=text/x-cvsweb-markup&only_with_tag=MAIN
    <braunr> look for uvmadvice
    <mcsim> But different translators have different demands. Thus changing of
      global policy for one translator would have impact on behavior of another
      one.
    <braunr> i understand
    <braunr> this isn't l4, or anything experimental
    <braunr> we want something that works well for us
    <mcsim> And this is acceptable?
    <braunr> until you're able to demonstrate we need different policies, i'd
      recommend not making things more complicated than they already are and
      need to be
    <braunr> why wouldn't it ?
    <braunr> we've been discussing this a long time :/
    <mcsim> because every process runs in isolated environment and the fact
      that there is something outside this environment, that has no rights to
      do that, does it surprises me.
    <braunr> ?
    <mcsim> ok. let me dip in uvm code. Probably my questions disappear
    <braunr> i don't think it will
    <braunr> you're asking about the system design here, not implementation
      details
    <braunr> with l4, there are as you'd expect well defined components
      handling policies for address space allocation, or paging, or whatever
    <braunr> but this is mach
    <braunr> mach has a big shared global vm server with in kernel policies for
      it
    <braunr> so it's ok to implement a global policy for this
    <braunr> and let's be pragmatic, if we don't need complicated stuff, why
      would we waste time on this ?
    <mcsim> It is not complicated.
    <braunr> retaining a whole container for each object, whereas they're all
      going to contain exactly the same stuff for years to come seems overly
      complicated for me
    <mcsim> I'm not going to create separate container for each object.
    <braunr> i'm not following you then
    <braunr> how can pagers upload their sizes in the kernel ?
    <mcsim> I'm going to create a new container only for combination of cluster
      sizes that are not present in table of advice.
    <braunr> that's equivalent
    <braunr> you're ruling out the default set, but that's just an optimization
    <braunr> whenever a file system decides to use other sizes, the problem
      will arise
    <mcsim> Before creating a container I'm going to lookup a table. And only
      than create
    <braunr> a table ?
    <mcsim> But there will be the same container for a huge bunch of objects
    <braunr> how do you select it ?
    <braunr> if it's a per pager container, remember there is no shared pager
      object in the kernel, only ports to external programs
    <mcsim> I'll give an example
    <mcsim> Suppose there are only two policies. At the beginning we have table
      {{random = 4096, sequential = 8096}}. Than pager 1 wants to add new
      policy where random cluster size is 8192. He asks kernel to create it and
      after this table will be following: {{random = 4096, sequential = 8192},
      {random = 8192, sequential = 8192}}. If pager 2 wants to create the same
      policy as pager 1, kernel will lockup table and will not create new
      entry. So the table will be the same.
    <mcsim> And each object has link to appropriate table entry
    <braunr> i'm not sure how this can work
    <braunr> how can pagers 1 and 2 know the sizes are the same for the same
      policy ?
    <braunr> (and actually they shouldn't)
    <mcsim> For faster lookup there will be create hash keys for each entry
    <braunr> what's the lookup key ?
    <mcsim> They do not know
    <mcsim> The kernel knows
    <braunr> then i really don't understand
    <braunr> and how do you select sizes based on the policy ?
    <braunr> and how do you remove unused entries ?
    <braunr> (ok this can be implemented with a simple ref counter)
    <mcsim> "and how do you select sizes based on the policy ?" you mean at
      page fault?
    <braunr> yes
    <mcsim> entry or object keeps pointer to appropriate entry in the table
    <braunr> ok your per object data is a pointer to the table entry and the
      policy is the index inside
    <braunr> so you really need a ref counter there
    <mcsim> yes
    <braunr> and you need to maintain this table
    <braunr> for me it's uselessly complicated
    <mcsim> but this keeps design clear
    <braunr> not for me
    <braunr> i don't see how this is clearer
    <braunr> it's just more powerful
    <braunr> a power we clearly don't need now
    <braunr> and in the following years
    <braunr> in addition, i'm very worried about the potential problems this
      can introduce
    <mcsim> In fact I don't feel comfortable from the thought that one
      translator can impact on behavior of another.
    <braunr> simple example: the table is shared, it needs a lock, other data
      structures you may have added in your patch may also need a lock
    <braunr> but our locks are noop for now, so you just can't be sure there is
      no deadlock or other issues
    <braunr> and adding smp is a *lot* more important than being able to select
      precisely policy sizes that we're very likely not to change a lot
    <braunr> what do you mean by "one translator can impact another" ?
    <mcsim> As I understand your idea (I haven't read uvm code yet) that there
      is a global table of cluster sizes for different policies. And every
      translator can change values in this table. That is what I mean under one
      translator will have an impact on another one.
    <braunr> absolutely not
    <braunr> translators *can't* change sizes
    <braunr> the sizes are completely static, assumed to be fit all
    <braunr> -be
    <braunr> it's not optimial but it's very simple and effective in practice
    <braunr> optimal*
    <braunr> and it's not a table of cluster sizes
    <braunr> it's a table of pages before/after the faulted one
    <braunr> this reflects the fact tha in mach, virtual memory (implementation
      and policy) is in the kernel
    <braunr> translators must not be able to change that
    <braunr> let's talk about pagers here, not translators
    <mcsim> Finally I got you. This is an acceptable tradeoff.
    <braunr> it took some time :)
    <braunr> just to clear something
    <braunr> 20:12 < mcsim> For faster lookup there will be create hash keys
      for each entry
    <braunr> i'm not sure i understand you here
    <mcsim> To found out if there is such policy (set of sizes) in the table we
      can lookup every entry and compare each value. But it is better to create
      a hash value for set and thus find equal policies.
    <braunr> first, i'm really not comfortable with hash tables
    <braunr> they really need careful configuration
    <braunr> next, as we don't expect many entries in this table, there is
      probably no need for this overhead
    <braunr> remember that one property of tables is locality of reference
    <braunr> you access the first entry, the processor automatically fills a
      whole cache line
    <braunr> so if your table fits on just a few, it's probably faster to
      compare entries completely than to jump around in memory
    <mcsim> But we can sort hash keys, and in this way find policies quickly.
    <braunr> cache misses are way slower than computation
    <braunr> so unless you have massive amounts of data, don't use an optimized
      container
    <mcsim> (20:38:53) braunr: that's why btrees and radix trees (basically
      trees of arrays) exist
    <mcsim> and what will be the key?
    <braunr> i'm not saying to use a tree instead of a hash table
    <braunr> i'm saying, unless you have many entries, just use a simple table
    <braunr> and since pagers don't add and remove entries from this table
      often, it's on case reallocation is ok
    <braunr> one*
    <mcsim> So here dynamic arrays fit the most?
    <braunr> probably
    <braunr> it really depends on the number of entries and the write ratio
    <braunr> keep in mind current processors have 32-bits or (more commonly)
      64-bits cache line sizes
    <mcsim> bytes probably?
    <braunr> yes bytes
    <braunr> but i'm not willing to add a realloc like call to our general
      purpose kernel allocator
    <braunr> i don't want to make it easy for people to rely on it, and i hope
      the lack of it will make them think about other solutions instead :)
    <braunr> and if they really want to, they can just use alloc/free
    <mcsim> Under "other solutions" you mean trees?
    <braunr> i mean anything else :)
    <braunr> lists are simple, trees are elegant (but add non negligible
      overhead)
    <braunr> i like trees because they truely "gracefully" scale
    <braunr> but they're still O(log n)
    <braunr> a good hash table is O(1), but must be carefully measured and
      adjusted
    <braunr> there are many other data structures, many of them you can find in
      linux
    <braunr> but in mach we don't need a lot of them
    <mcsim> Your favorite data structures are lists and trees. Next, what
      should you claim, is that lisp is your favorite language :)
    <braunr> functional programming should eventually rule the world, yes
    <braunr> i wouldn't count lists are my favorite, which are really trees
    <braunr> as*
    <braunr> there is a reason why red black trees back higher level data
      structures like vectors or maps in many common libraries ;)
    <braunr> mcsim: hum but just to make it clear, i asked this question about
      hashing because i was curious about what you had in mind, i still think
      it's best to use static predetermined values for policies
    <mcsim> braunr: I understand this.
    <braunr> :)
    <mcsim> braunr: Yeah. You should be cautious with me :)


## IRC, freenode, #hurd, 2012-09-21

    <antrik> mcsim: there is only one cluster size per object -- it depends on
      the properties of the backing store, nothing else.
    <antrik> (while the readahead policies depend on the use pattern of the
      application, and thus should be selected per mapping)
    <antrik> but I'm still not convinced it's worthwhile to bother with cluster
      size at all. do other systems even do that?...


## IRC, freenode, #hurd, 2012-09-23

    <braunr> mcsim: how long do you think it will take you to polish your gsoc
      work ?
    <braunr> (and when before you begin that part actually, because we'll to
      review the whole stuff prior to polishing it)
    <mcsim> braunr: I think about 2 weeks
    <mcsim> But you may already start review it, if you're intended to do it
      before I'll rearrange commits.
    <mcsim> Gnumach, ext2fs and defpager are ready. I just have to polish the
      code.
    <braunr> mcsim: i don't know when i'll be able to do that
    <braunr> so expect a few weeks on my (our) side too
    <mcsim> ok
    <braunr> sorry for being slow, that's how hurd development is :)
    <mcsim> What should I do with libc patch that adds madvise support?
    <mcsim> Post it to bug-hurd?
    <braunr> hm probably the same i did for pthreads, create a topic branch in
      glibc.git
    <mcsim> there is only one commit
    <braunr> yes
    <braunr> (mine was a one liner :p)
    <mcsim> ok
    <braunr> it will probably be a debian patch before going into glibc anyway,
      just for making sure it works
    <mcsim> But according to term. I expect that my study begins in a week and
      I'll have to do some stuff then, so actually probably I'll need a week
      more.
    <braunr> don't worry, that's expected
    <braunr> and that's the reason why we're slow
    <mcsim> And what should I do with large store patch?
    <braunr> hm good question
    <braunr> what did you do for now ?
    <braunr> include it in your work ?
    <braunr> that's what i saw iirc
    <mcsim> Yes. It consists of two parts.
    <braunr> the original part and the modificaionts ?
    <braunr> modifications*
    <braunr> i think youpi would know better about that
    <mcsim> First (small) adds notification to libpager interface and second
      one adds support for large stores.
    <braunr> i suppose we'll probably merge the large store patch at some point
      anyway
    <mcsim> Yes both original and modifications
    <braunr> good
    <mcsim> I'll split these parts to different commits and I'll try to make
      support for large stores independent from other work.
    <braunr> that would be best
    <braunr> if you can make it so that, by ommitting (or including) one patch,
      we can add your patches to the debian package, it would be great
    <braunr> (only with regard to the large store change, not other potential
      smaller conflicts)
    <mcsim> braunr: I also found several bugs in defpager, that I haven't fixed
      since winter.
    <braunr> oh
    <mcsim> seems nobody hasn't expect them.
    <braunr> i'm very interested in those actually (not too soon because it
      concerns my work on pageout, which is postponed after pthreads and
      select)
    <mcsim> ok. than I'll do it first.


## IRC, freenode, #hurd, 2012-09-24

    <braunr> mcsim: what is vm_get_advice_info ?
    <mcsim> braunr: hello. It should supply some machine specific parameters
      regarding clustered reading. At the moment it supplies only maximal
      possible size of cluster.
    <braunr> mcsim: why such a need ?
    <mcsim> It is used by defpager, as it can't allocate memory dynamically and
      every thread has to allocate maximal size beforehand 
    <braunr> mcsim: i see


## IRC, freenode, #hurd, 2012-10-05

    <mcsim> braunr: I think it's not worth to separate large store patch for
      ext2 and patch for moving it to new libpager interface. Am I right?
    <braunr> mcsim: it's worth separating, but not creating two versions
    <braunr> i'm not sure what you mean here
    <mcsim> First, I applied large store patch, and than I was changing patched
      code, to make it work with new libpager interface. So changes to make
      ext2 work with new interface depend on large store patch.
    <mcsim> braunr: ^
    <braunr> mcsim: you're not forced to make each version resulting from a new
      commit work
    <braunr> but don't make big commits
    <braunr> so if changing an interface requires its users to be updated
      twice, it doesn't make sense to do that
    <braunr> just update the interface cleanly, you'll have one or more commits
      that produce intermediate version that don't build, that's ok
    <braunr> then in another, separate commit, adjust the users
    <mcsim> braunr: The only user now is ext2. And the problem with ext2 is
      that I updated not the version from git repository, but the version, that
      I've got after applying the large store patch. So in other words my
      question is follows: should I make a commit that moves to new interface
      version of ext2fs without large store patch?
    <braunr> you're asking if you can include the large store patch in your
      work, and by extension, in the main branch
    <braunr> i would say yes, but this must be discussed with others