Issues relating to system behavior under memory pressure.

gnumach page cache policy

IRC, freenode, #hurd, 2012-07-08

<braunr> am i mistaken or is the default pager simply not vm privileged ?
<braunr> (which would explain the hangs when memory is very low)
<youpi> no idea
<youpi> but that's very possible
<youpi> we start it by hand from the init scripts
<braunr> actually, i see no way provided by mach to set that
<braunr> i'd assume it would set the property when a thread would register
  itself as the default pager, but it doesn't
<braunr> i'll check at runtime and see if fixing helps
<youpi> thread_wire(host, thread, 1) ?
<youpi> ./hurd/mach-defpager/wiring.c:  kr =
<braunr> no
<braunr> look in cprocs.c
<braunr> iir
<braunr> iirc
<braunr> iiuc, it sets a 1:1 kernel/user mapping
<youpi> ??
<youpi> thread_wire, not cthread_wire
<braunr> ah
<braunr> right, i'm getting tired
<braunr> youpi: do you understand the comment in default_pager_thread() ?
<youpi> well, I'm not sure to know what external vs internal is
<braunr> i'm almost sure the default pager is blocked because of a relation
  with an unprivlege thread
<braunr> +d
<braunr> when hangs happen, the pageout daemon is still running, waiting
  for an event so he can continue
<braunr> it*

<braunr> all right, our pageout stuff completely sucks
<braunr> when you think the system is hanged, it's actually not
<pinotree> and what's happening instead?
<braunr> instead, it seems it's in a very complex resursive state which
  ends in the slab allocator not being able to allocate kernel map entries
<braunr> recursive*
<braunr> the pageout daemon, unable to continue, progressively slows
<braunr> in hope the default pager is able to service the pageout requests,
  but it's not
<braunr> probably the most complicated deadlock i've seen :)
<braunr> luckily !
<braunr> i've been playing with some tunables involved in waking up the
  pageout daemon
<braunr> and got good results so far
<braunr> (although it's clearly not a proper solution)
<braunr> one thing the kernel lacks is a way to separate clean from dirty
<braunr> this stupid kernel doesn't try to free clean pages first .. :)
<braunr> hm
<braunr> now i can see the system recover, but some applications are still
  stuck :(
<braunr> (but don't worry, my tests are rather aggressive)
<braunr> what i mean by aggressive is several builds and various dd of a
  few hundred MiB in parallel, on various file systems
<braunr> so far the file systems have been very resilient
<braunr> ok, let's try running the hurd with 64 MiB of RAM
<braunr> after some initial swapping, it runs smoothly :)
<braunr> uh ?
<braunr> ah no, i'm still doing my parallel builds
<braunr> although less
<braunr> gcc: internal compiler error: Resource lost (program as)
<braunr> arg
<braunr> lol
<braunr> the file system crashed under the compiler
<pinotree> too much memory required during linking? or ram+swap should have
  been enough?
<braunr> there is a lot of swap, i doubt it
<braunr> the hurd is such a dumb and impressive system at the same time
<braunr> pinotree: what does this tell you ?
<braunr> git: hurdsig.c:948: post_signal: Unexpected error: (os/kern)
<pinotree> something samuel spots often during the builds of haskell

Probably also the sigpost case mentioned in id:"".

<braunr> actually i should be asking jkoenig
<braunr> it seems the lack of memory has a strong impact on signal delivery
<braunr> which is bad
<antrik> braunr: I have a vague recollection of slpz also saying something
  about missing dirty page tracking a while back... I might be confusing
  stuff though
<braunr> pinotree: yes it happens often during links
<braunr> which makes sense
<pinotree> braunr: "happens often" == "hurdsig.c:948: post_signal: ..."?
<braunr> yes
<pinotree> if you can reproduce it often, what about debugging it? :P
<braunr> i mean, the few times i got it, it was often during a link :p
<braunr> i'd rather debug the pageout deadlock :(
<braunr> but it's hard