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[[!tag open_issue_gnumach]]

There is a [[!FF_project 268]][[!tag bounty]] on this task.

IRC, freenode, #hurd, 2011-04-23

    <braunr> youpi: is there any use of the port renaming facility ?
    <youpi> I don't know
    <braunr> at least, did you see such use ?
    <braunr> i wonder why mach mach_port_insert_right() lets the caller specify
      the port name
    <youpi> ../hurd-debian/hurd/serverboot/default_pager.c:	kr =
      mach_port_rename(	default_pager_self,
    <braunr> mach_port_rename() is used only once, in the default pager
    <braunr> so it's not that important
    <braunr> but mach_port_insert_right() lets userspace task decide the port
      name value
    <youpi> just to repeat myself again, I don't know port stuff very much :)
    <braunr> well you know that a port denotes a right, which denotes a port
    <youpi> yes, but I don't have any real experience with it
    <braunr> err
    <braunr> port name
    <braunr> the only reason I see is that the caller, say /hurd/exec running a
      fork()
    <braunr> hm
    <braunr> no, i don't even see the reason here
    <braunr> port names should be allocated by the kernel only, like file
      descriptors
    <youpi> you can choose file descriptor values too
    <braunr> really ?
    <youpi> with dup2, yes
    <braunr> oh
    <braunr> hm
    <braunr> what's the data structure in current unices to store file
      descriptors ?
    <braunr> a hash table ?
    <youpi> I don't know
    <braunr> i'll have to look at that
    <braunr> FYI, i'm asking these questions because i'm thinking of reworking
      ipc spaces
    <braunr> i believe the use of splay trees completely destroys performance
      of tasks with many many port names such as the root file system
    <youpi> that can be a problem yes
    <youpi> since there are 3 ports per opened file, and like 3 per thread too
    <braunr> + the page cache
    <youpi> with a few thousand opened files and threads, that makes a lot
    <youpi> by "opened file" I meant page cache actually
    <braunr> i saw numbers up to 30k
    <braunr> ok
    <youpi> on buildds I easily see 100k ports
    <braunr> for a single task ?
    <braunr> wow
    <youpi> yes
    <youpi> the page cache is 4k files
    <braunr> so that's definitely worth the try
    <youpi> so that already makes 12k ports
    <youpi> and 4k is not so big
    <braunr> it's limited to 4K ?
    <youpi> I haven't been able to check where the 100k come from yet
    <youpi> braunr: yas
    <braunr> could be leaks :/
    <youpi> yes
    <braunr> omg, a hard limit on the page cache ..
    <youpi> vm/vm_object.c:int		vm_object_cached_max = 4000;	/* may
      be patched*/
    <braunr> mach is really old :(
    <youpi> I've raised it
    <youpi> before it was 200
    <youpi> ...
    <braunr> oO
    <youpi> I tried to dro pthe limit, but then I was lacking memory
    <youpi> which I believe have fixed the other day, but I have to test again
    <braunr> that implementation doesn't know how to deal with memory pressure
    <youpi> yes
    <braunr> i saw your recent changes about adding warnings in such cases
    <braunr> so, back to ipc spaces
    <braunr> i think splay trees 1/ can get very unbalanced easily
    <braunr> which isn't hard to imagine
    <braunr> and 2/ make poor usage of the cpu caches because they're BST and
      write a lot to memory
    <youpi> maybe you could write a patch which would dump statistics on that?
    <braunr> that's part of the job i'm assigning to myself
    <youpi> ok
    <braunr> i'd like to try replacing splay trees with radix trees
    <youpi> I can run it on the buildds
    <youpi> buildds are very good stress-tests :)
    <braunr> :)
    <youpi> 22h building -> 77k ports
    <youpi> 26h building -> 97k ports
    <youpi> the problem is that when I add leak debugging (backtraces), I'm
      getting out of memory :)
    <braunr> that will be a small summer of code outside the gsoc :p
    <braunr> :/
    <braunr> backtraces are very consuming
    <youpi> but that's only because of hardcoded limits
    <youpi> I'll have to test again with bigger limits
    <braunr> again ..
    <braunr> evil hard limits
    <youpi> well, actually we could as well just drop them
    <youpi> but we'd also need to easily get statistics on zone/vm_maps usage
    <youpi> because else we don't see leaks
    <youpi> (except that the machine eventually crashes)
    <braunr> hm
    <braunr> i haven't explained why i was asking my questions actually
    <braunr> so, i want radix trees, because they're nice
    <braunr> they reduce the paths lengths
    <braunr> they don't get too unbalanced (they're invariant wrt the order of
      operations)
    <braunr> they don't need to write to memory on lookups
    <braunr> the only drawback is that they can create much overhead if their
      usage pattern isn't appropriate
    <braunr> elements in such a structure should be close, so that they share
      common nodes
    <youpi> the common usage pattern in ext2fs is a big bunch of ever-open
      ports :)
    <braunr> if there is one entry per node, it's a big waste
    <braunr> yes
    <youpi> there are 3, actually
    <braunr> but the port names have low values
    <braunr> they're allocated sequentially, beginning at 0
    <braunr> (or 1 actually)
    <braunr> which is perfect for radix trees
    <youpi> yes
    <youpi>  97989: send
    <braunr> but if anyone can rename
    <braunr> this introduces a new potential weakness
    <youpi> ah, if it's just a weakness it's probably not a problem
    <youpi> I thought it was even a no-go
    <braunr> i think so
    <youpi> I guess port rename is very seldom
    <braunr> but in a future version, it would be nice not to allow port
      renaming
    <braunr> unless there are similar issues in current unix kernels
    <braunr> in which case i'd say it's acceptable
    <youpi> there are
    <braunr> of that order ?
    <youpi> and it'd be useful for e.g. processing
      tracing/debugging/tweaking/whatever
    <youpi> it's also used to hide fds from a process
    <braunr> port renaming you mean ?
    <youpi> you allocate them very high
    <youpi> yes
    <braunr> ok
    <youpi> choosing your port name, generally
    <youpi> to match what the process expects for instance
    <braunr> then it would be a matter of resource limiting (which we totally
      lack afaik)
    <braunr> along the number of maximum open files, you would have a number of
      maximum rights
    <braunr> does that seem fine to you ?
    <youpi> if done throught rlimits, sure
    <braunr> something similar yes
    <youpi> (_no_ PORTS_MAX ;) )
    <braunr> oh and, in addition, i remember gnumach has a special
      configuration of the processor in which caching is limited
    <braunr> like write-through only
    <youpi> didn't I fix that recently ?
    <braunr> i don't know :)
    <braunr> CR0=e001003b
    <braunr> i don't think it's fixed
    <youpi> I mean, in the git
    <braunr> ah
    <youpi> not in the debian package
    <braunr> didn't tried the git version yet
    <braunr> last time i tried (which was a long time ago), it made the kernel
      crash
    <braunr> have you figured why ?
    <youpi> I'm not aware of that
    <braunr> anyway, splay trees write a lot, and most trees write a lot even
      at insertion/removal to rebalance
    <youpi> braunr: Mmm, there's no clearance of CD in the kernel actually
    <braunr> with radix trees, even if caching can't be fully enabled, it would
      make much better use of it
    <braunr> so if port renaming isn't a true issue, i'll choose that data
      structure
    <youpi> that'd probably be better yes
    <youpi> I'm surprised by the CD, I do remember fixing something like this
      lately
    <braunr> there are several levels where CD can be set
    <braunr> the processors ORs all those if i'm right
    <braunr> to determine if caching is enabled 
    <youpi> I know
    <braunr> ok
    <youpi> but in my memory that was at the CR* level, precisely
    <braunr> maybe for xen only ?
    <youpi> no
    <braunr> well good luck if you hunt that one, i'm off, see you :)
    <youpi> braunr: ah, no, it was the PGE flag that I had fixed

    <antrik> braunr: explicit port naming is used for example to pass some
      initial ports to a new task at well-known places IIRC
    <antrik> braunr: but these tend to be low numbers, so I don't see a problem
      there
    <antrik> (I'm not familiar with radix trees... why would high numbers be a
      problem?)

    <youpi> braunr: iirc the ipc space is limited to ~192k ports

    <braunr> antrik: in most cases i've seen, the insert_right() call is used
      on task_self()
    <braunr> and if there really are special ports (like the bootstrap or
      device ports), they should have special names
    <braunr> IIRC, these ports are given through command line expansion by the
      kernel at boot time
    <braunr> but it seems reasonable to think of port renaming as a potentially
      useful feature
    <braunr> antrik: the problem with radix trees isn't them being high, it's
      them being sparse
    <braunr> you get the most efficient trees when entries have keys that are
      close to each other
    <braunr> because radix trees are a type of tries (the path in the tree is
      based on the elements composing the key)
    <braunr> so the more common prefixes you have, the less external nodes you
      need
    <braunr> here, keys are port names, but they can be memory addresses or
      offsets in memory objects (like in the page cache)
    <braunr> the radix algorithm takes a few bits, say 4 or 6, at a time from a
      key, and uses that as an index in a node
    <braunr> if keys are sparse, there can be as little as one entry per node
    <braunr> IIRC, the worst case (on entry per node with the maximum possible
      number of nodes for a 32-bits key) is 2% entries
    <braunr> the reste being null entries and almost-empty nodes containing
      them
    <braunr> so if you leave the ability to give port rights the names you
      want, you can create such worst case trees
    <braunr> which may consume several MiB of memory per tree
    <braunr> tens of MiB i'd say
    <braunr> on the other hand, in the current state, almost all hurd
      applications use sequentially allocated port names, close to 0 (which
      allows a nice optimization)
    <braunr> so a radix ree would be the most efficient
    <antrik> well, if some processes really feel they must use random numbers
      for port names, they *ought* to be penalized ;-)