path: root/open_issues/resource_management_problems/pagers.mdwn

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[[!tag open_issue_gnumach]]

[[!toc]]


# IRC, freenode, #hurd, 2011-09-14

Coming from [[translators_set_up_by_untrusted_users]], 2011-09-14 discussion:

    <slpz> antrik: I think a tunable option for preventing non-root users from
      creating pagers and attaching translators could also be desirable
    <antrik> slpz: why would you want to prevent creating pagers and attaching
      translators?
    <tschwinge> Preventing resource exhaustion, I guess.
    <slpz> antrik: security and (as tschwinge says) for prevent a rouge pager
      from exhausting the system.
    <slpz> antrik: without the ability to use translators for non-root users,
      Hurd can provide (almost) the same level of resource protection than
      other *nixes

See also: [[translators_set_up_by_untrusted_users]],
[[hurd/translator/tmpfs/tmpfs_vs_defpager]].

    <braunr> the hurd is about that though
    <slpz> there should be also a limit on the number of outstanding requests
      that a task can have, and some other easily traceable values
    <braunr> port messages queues have limits
    <antrik> slpz: anything can exhaust the system. there are much more basic
      limits that are missing... and I don't see how translators or pagers are
      special in that regard
    <slpz> braunr: that's what I said tunable. If I don't share my computer
      with untrusted users, I want full functionality. Otherwise, I can enable
      that limitation
    <slpz> braunr: but I think those limits are on reception
    <braunr> that's a wrong solution
    <slpz> antrik: because pagers are external memory objects, and those are
      treated differently
    <braunr> compared to what ?
    <braunr> and yes, the limit is on the message queue, on reception
    <braunr> why is that a problem ?
    <slpz> antrik: forbidding the use of translator was for security, to avoid
      the problem of traversing an untrusted FS
    <slpz> braunr: compared to anonymous memory
    <slpz> braunr: because if the limit is on reception, a task can easily do a
      DoS against a server
    <braunr> hm actually, the problems we have with swap handling is that
      anonymous memory is handled in a very similar way as other objects
    <slpz> braunr: I want to limit the number of outstanding (unprocessed
      messages in queues) requests
    <braunr> slpz: the solution isn't about forbidding the use of translators,
      but changing common code (libc i guess) not to use them, they can still
      run beside
    <slpz> braunr: that's because, currently, the external page limit is not
      enforced
    <braunr> i'm also not sure about DoS attacks
    <braunr> if i'm right, there is often one port for each managed object,
      which usually exist per client
    <slpz> braunr: yes, that could an option too (for translators, not for
      pagers)
    <braunr> i don't see how pagers wouldn't be translators on the hurd
    <slpz> braunr: all pagers are translators, but not all translators are
      pagers ;-)
    <braunr> so if it works for translators, it also works for pagers
    <slpz> braunr: it would fix the security issue, but not the resource
      exhaustion problem, with only affects to pagers
    <braunr> i just don't see a point in implementing resource limits before
      even fixing other fundamental issues
    <braunr> the only way to avoid resource exhaustion is resource limits
    <antrik> slpz: just not following untrusted translators is much more useful
      than forbidding them alltogether
    <braunr> and the main problem of mach is resource accounting
    <braunr> so first, fix that, using the critique as a starting point

[[hurd/critique]].

    <slpz> braunr: i'm not saying that this should be implemented right now,
      i'm just pointing out this possibility
    <braunr> i think we're all mostly aware of it
    <slpz> braunr: resource accounting, as it's expressed in the critique,
      would be wonderful, but it's just too complex IMHO
    <braunr> it requires carefully designed changes to the interface yes
    <slpz> to the interface, to the internals, to user space tasks...
    <braunr> the internals wouldn't be impacted that much
    <braunr> user space tasks would mostly include hurd servers
    <braunr> if the changes are centralized in libraries, it should be easy to
      provide to the servers


# IRC, freenode, #hurd, 2011-09-22

    <slpz> antrik: I've also implemented a simple resource control on dirty
      pages and changed pageout_scan to free external pages, and only touch
      anonymous memory if it's really needed
    <slpz> antrik: those combined make the system work better under heavy load
    <slpz> antrik: 1.5 GB of RAM and another 1.5 GB of swap helps a lot, too
      :-)
    <antrik> hm... I'm not sure what these things mean exactly TBH... but I
      wonder whether some of these could fix the performance degradation (and
      ultimate crash) I described recently...

[[/open_issues/default_pager]], [[system performance degradation
(?)|performance/degradation]].

    <antrik> care to explain them to a noob like me?
    <slpz> probably not. During my tests, I've noticed that, at some points,
      the system performance starts to degrade, and this doesn't change until
      it's restarted
    <slpz> but I wasn't able to create a test case to reproduce the bug...
    <slpz> antrik: Sure. First, I've changed GNU Mach to:
    <slpz>  - Classify all pages from data_supply as external, and count them
      in vm_page_external_count (previously, this variable was always zero)

[[/open_issues/mach_vm_pageout]]

    <slpz>  - Count all pages for which a  data_unlock has been requested as
      potentially dirty pages
    <antrik> there is one important bit I forgot to mention in my recent
      report: one "reliable" way to cause growing swap usage is simply
      installing a lot of debian packages (e.g. running an apt-get upgrade)
    <antrik> some other kinds of I/O also seem to have such an effect, but I
      wasn't able to pinpoint specific situations
    <slpz>  - Establish a limit on how many potentially dirty pages are
      allowed. If it's reached, a notification (right now it's just a bogus
      m_o_data_unlock, to avoid implementing a new RPC) it's sent to the pager
      which has generated the page fault
    <slpz>  - Establish a hard limit on those dirt pages. If it's reached,
      threads asking for a data_unlock are blocked until someone cleans some
      pages. This should be improved with a forced pageout, if needed.
    <slpz>  - And finally, in vm_pageout_scan, run over the inactive queue
      searching for clean, external pages, freeing them. If it's not possible
      to free enough pages, or if vm_page_external_count is less than 10% of
      system's memory, the "normal" pageout is used.
    <slpz> I need to clean up things a little, but I want to send a preliminary
      patch to bug-hurd ASAP, to have more people testing it.
    <slpz> antrik: Do you thing that performance degradation can be related
      with the number of threads of your ext2fs translators?
    <antrik> slpz: hm... I didn't watch that recently; but in the past, I
      observe that the thread count is pretty constant after it reaches
      something like 14000 on heavy load...
    <antrik> err... wait, 14000 was ports :-)
    <antrik> I doubt my system would survive 14000 threads ;-)
    <antrik> don't remember thread count... I guess I should start watching
      this again
    <slpz> antrik: I was thinking that 14000 threads sound like a lot :-)
    <slpz> what I know for sure, is that when operating with large files, the
      deactivation of all pages of the memory object which is done after every
      operation really hurts to performance
    <antrik> right now my root FS has 5100 ports and a mere 71 thread... but
      then, it's almost freshly booted :-)
    <slpz> that's why I've just commented that operation in my code, since it's
      not really needed anymore :-)
    <slpz> anyway, after submitting all my pending mails to bug-hurd, I'll try
      to hunt that bug. Sounds funny.
    <antrik> regarding your explanation, I'm still trying to wrap my head
      around some of the details. I must admit that I don't remember what
      data_unlock does... or maybe I never fully understood it
    <antrik> the limit on dirty pages is global?
    <slpz> yes, right now it's global
    <marcusb> I try to find the old discussion of the thread storm stuff
    <marcusb> there was some concern about deadlocks
    <slpz> marcusb: yes, because we were talking about putting an static limit
      for the server threads of a translators
    <slpz> marcusb: and that was wrong (my fault, I was even dumber back then
      :-P)
    <marcusb> oh boy digging in old mail is no fun.  first I see mistakes in my
      english.  then I see quite complicated pager stuff I don't ever remember
      touching.  but there is a patch, and it has my name on it
    <marcusb> I think I lost a couple of the early years of my hurd hacking :)
    <antrik> hm... I reread the chapter on locking, and it's still above me :-(
    <marcusb> not sure what you are talking about, but if there are any
      specific questions...
    <antrik> marcusb: external pager interface

[[microkernel/mach/external_pager_mechanism]].

    <marcusb> uuuuh ;)
    <antrik> memory_object_lock_request(), memory_object_lock_completed(),
      memory_object_data_unlock()
    <marcusb> is that from the mach manual?
    <antrik> yes
    <antrik> I didn't really understand that part when I first read it a couple
      of years ago, and I still don't understand it now :-(
    <marcusb> I am sure I didn't understand it either
    <marcusb> and maybe I missed my window :)
    <marcusb> let's see
    <antrik> hehe
    <antrik> slpz: what exactly do you mean by "the pager which has generated
      the page fault"?
    <antrik> marcusb: essentially I'm trying to understand the explanation of
      the changes slpz did, but there are several bits totally obscure to me
      :-(
    <slpz> antrik: when a I/O operation is requested to ext2fs, it maps the
      object in question to it's own space, and then memcpy's from/to there
    <slpz> antrik: so the translator (which is also a pager) is the one who
      generates the page fault
    <marcusb> yeah
    <marcusb> antrik: it's important to understand which messages are sent by
      the kernel to the manager and which are sent the other way
    <marcusb> if the dest port is memory_object_t, that indicates a msg from
      kernel to manager.  if it is memory_object_control_t, it's a msg from
      manager to kernel
    <slpz> antrik: m_o_lock_request it's used by the pager to "settle" the
      status of a memory object, m_o_lock_completed is the answer from the
      kernel when the lock has been completed (only if the client has requested
      to be notified), and m_o_data_unlock is a request from the kernel to
      change the level of protection for a page (it's called from vm_fault.c)
    <marcusb> slpz: but it's not pagers generating page faults, but users of
      the memory object on the other side
    <antrik> marcusb: well, I think the direction is clear to me... but the
      purpose not really :-)
    <marcusb> ie a client that mapped a file
    <slpz> antrik: in ext2fs, all pages are initially provided to the kernel
      (via data_supply) write protected. When a write operation is done over
      one of those pages, a page fault it's generated, which sends a
      m_o_data_unlock to the pager, which answers (if convenient) which a
      page_lock decreasing the protection level
    <marcusb> antrik: one use of lock_request is when you want to shut down
      cleanly and want to get the dirty pages written back to you from the
      kernel.
    <marcusb> antrik: the other thing may be COW strategies
    <slpz> marcusb: well, pagers and clients are in the same task for most
      translators, like ext2fs
    <marcusb> slpz: oh.
    <slpz> marcusb: but yes, a read operation in a mmap'ed file would trigger
      the fault in a client user task
    <marcusb> slpz: I think I forgot everything about pagers :)
    <slpz> marcusb: pager-memcpy.c is the key :-)
    <marcusb> slpz: what becomes of the fault then?  the kernel sees it's a
      mapped memory object.  will it then talk to the manager or to a pager? 
    <antrik> slpz: the translator causes the faults itself when it handles
      io_read()/io_write() requests I suppose, as opposed to clients accessing
      mmap()ed objects which then generate the faults?...
    <antrik> ah, that's actually what you already said above :-)
    <slpz> marcusb: I'm not sure what do you mean by "manager"...
    <marcusb> manager == memory object
    <marcusb> mh
    <slpz> marcusb: for all external objects, it will ask to their current
      pager
    <marcusb> slpz: I think I am missing a couple of details, so nevermind.
      It's starting to come back to me, but I am a bit afraid of that ;)
    <marcusb> what I love about the Hurd is how damn readable the code is
    <marcusb> considering it's an object system, it's so much nicer to read
      than gtk stuff
    <slpz> when you get the big picture, it's actually somewhat fun to see how
      data moves around just to fulfill a simple read()
    <marcusb> you should make a diagram!
    <marcusb> bonus point for animated video ;)

[[hurd/IO_path]].

    <slpz> marcusb: heh, take a look at the hurd specific parts of glibc... I
      cry in pain every time a do that...
    <marcusb> slpz: oh yeah, rdwr-internal.
    <marcusb> oh man
    <marcusb> slpz: funny thing, I just looked at them the other day because of
      the security issue
    <slpz> marcusb: I think there was one, maybe a slice from someone's
      presentation...
    <marcusb> I think I was always confused about the pager/memobj/kernel
      interactions
    <slpz> marcusb: I'm barely able to read Roland's glibc code. I think it's
      out of my reach.
    <antrik> marcusb: I think part of the problem is confusing terminology
    <marcusb> it's good that you are instrumenting the mach kernel to see
      what's actually going on in there.  it was a black book for me, but neal
      too a peek and got a much better understanding of the performance issues
      than I ever did
    <antrik> when talking about "pager", we usually mean the process doing the
      paging; but in mach terminology this actually seems to be the "manager",
      while a "pager" is an individual object in the manager process... or
      something like that ;-)
    <marcusb> antrik: I just never took a look at the big picture.  I look at
      the parts
    <marcusb> I knew the tail, ears, and legs of the elephant.
    <marcusb> it's a lot of code for a beginner
    <antrik> I never understood the distinction between "pager" and "memory
      object" though...
    <antrik> maybe "pager" refers to the object in the external pager, while
      "memory object" is the part managed in Mach itself?...
    <marcusb> memory object is a real object, to which you can send messages.
      it's implemented in the server
    <antrik> hm... maybe it's the other way around then ;-)
    <marcusb> there is also the default pager
    <marcusb> I think the pager is just another name for the process that
      serves the memory object (default pager == memory object for anonymous
      memory == swap)
    <marcusb> but!
    <marcusb> there is also libpager

[[hurd/libpager]]

    <marcusb> and that's a more complicated beast
    <antrik> actually, the correct term seems to be "default memory manager"...
    <marcusb> yeah
    <marcusb> from mach's pov
    <marcusb> we always called it default pager in the Hurd
    <antrik> marcusb: problem is that "pager" is sometimes used in the Mach
      documentation to refer to memory object ports IIRC
    <marcusb> isn't it defpager executable?
    <marcusb> could be
    <marcusb> it's the same thing, really
    <antrik> indeed, the program implementing the default memory manager is
      called "default pager"... so the terminology is really inconsistent
    <marcusb> the hurd's pager library is a high level abstraction for mach's
      external memory object interface.
    <marcusb> i wouldn't worry about it too much
    <antrik> I never looked at libpager
    <marcusb> you should!
    <marcusb> it's an important beast
    <antrik> never seemed relevant to anything I did so far...
    <antrik> though maybe it would help understanding
    <marcusb> it's related to what you are looking now :)