Re: [PATCH V7 4/9] Add memcg kswapd thread pool

[Ying Han <yinghan@google.com>]

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On Thu, Apr 21, 2011 at 10:00 PM, KAMEZAWA Hiroyuki <
kamezawa.hiroyu@jp.fujitsu.com> wrote:

> On Thu, 21 Apr 2011 21:49:04 -0700
> Ying Han <yinghan@google.com> wrote:
>
> > On Thu, Apr 21, 2011 at 9:36 PM, KAMEZAWA Hiroyuki <
> > kamezawa.hiroyu@jp.fujitsu.com> wrote:
> >
> > > On Thu, 21 Apr 2011 21:24:15 -0700
> > > Ying Han <yinghan@google.com> wrote:
> > >
> > > > This patch creates a thread pool for memcg-kswapd. All memcg which
> needs
> > > > background recalim are linked to a list and memcg-kswapd picks up a
> memcg
> > > > from the list and run reclaim.
> > > >
> > > > The concern of using per-memcg-kswapd thread is the system overhead
> > > including
> > > > memory and cputime.
> > > >
> > > > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > > Signed-off-by: Ying Han <yinghan@google.com>
> > >
> > > Thank you for merging. This seems ok to me.
> > >
> > > Further development may make this better or change thread pools (to
> some
> > > other),
> > > but I think this is enough good.
> > >
> >
> > Thank you for reviewing and Acking. At the same time, I do have wondering
> on
> > the thread-pool modeling which I posted on the cover-letter :)
> >
> > The per-memcg-per-kswapd model
> > Pros:
> > 1. memory overhead per thread, and The memory consumption would be
> 8k*1000 =
> > 8M
> > with 1k cgroup.
> > 2. we see lots of threads at 'ps -elf'
> >
> > Cons:
> > 1. the implementation is simply and straigh-forward.
> > 2. we can easily isolate the background reclaim overhead between cgroups.
> > 3. better latency from memory pressure to actual start reclaiming
> >
> > The thread-pool model
> > Pros:
> > 1. there is no isolation between memcg background reclaim, since the
> memcg
> > threads
> > are shared.
> > 2. it is hard for visibility and debugability. I have been experienced a
> lot
> > when
> > some kswapds running creazy and we need a stright-forward way to identify
> > which
> > cgroup causing the reclaim.
> > 3. potential starvation for some memcgs, if one workitem stucks and the
> rest
> > of work
> > won't proceed.
> >
> > Cons:
> > 1. save some memory resource.
> >
> > In general, the per-memcg-per-kswapd implmentation looks sane to me at
> this
> > point, esepcially the sharing memcg thread model will make debugging
> issue
> > very hard later.
> >
> > Comments?
> >
> Pros <-> Cons ?
>
> My idea is adding trace point for memcg-kswapd and seeing what it's now
> doing.
> (We don't have too small trace point in memcg...)
>
> I don't think its sane to create kthread per memcg because we know there is
> a user
> who makes hundreds/thousands of memcg.
>
> And, I think that creating threads, which does the same job, more than the
> number
> of cpus will cause much more difficult starvation, priority inversion
> issue.
> Keeping scheduling knob/chances of jobs in memcg is important. I don't want
> to
> give a hint to scheduler because of memcg internal issue.
>
> And, even if memcg-kswapd doesn't exist, memcg works (well?).
> memcg-kswapd just helps making things better but not do any critical jobs.
> So, it's okay to have this as best-effort service.
> Of course, better scheduling idea for picking up memcg is welcomed. It's
> now
> round-robin.
>
> Hmm. The concern I have is the debug-ability. Let's say I am running a
system and found memcg-3 running crazy. Is there a way to find out which
memcg it is trying to reclaim pages from? Also, how to count cputime for the
shared memcg to the memcgs if we wanted to.

--Ying


> Thanks,
> -Kame
>
>

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