Re: [PATCH V6 00/10] memcg: per cgroup background reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Apr 22, 2011 at 07:10:25PM -0700, Ying Han wrote:
> On Fri, Apr 22, 2011 at 6:35 PM, Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> > On Wed, Apr 20, 2011 at 10:28:17PM -0700, Ying Han wrote:
> > > On Wed, Apr 20, 2011 at 10:08 PM, Johannes Weiner <hannes@cmpxchg.org
> > >wrote:
> > > > On Thu, Apr 21, 2011 at 01:00:16PM +0900, KAMEZAWA Hiroyuki wrote:
> > > > > I don't think its a good idea to kick kswapd even when free memory is
> > > > enough.
> > > >
> > > > This depends on what kswapd is supposed to be doing.  I don't say we
> > > > should reclaim from all memcgs (i.e. globally) just because one memcg
> > > > hits its watermark, of course.
> > > >
> > > > But the argument was that we need the watermarks configurable to force
> > > > per-memcg reclaim even when the hard limits are overcommitted, because
> > > > global reclaim does not do a fair job to balance memcgs.
> > >
> > > There seems to be some confusion here. The watermark we defined is
> > > per-memcg, and that is calculated
> > > based on the hard_limit. We need the per-memcg wmark the same reason of
> > > per-zone wmart which triggers
> > > the background reclaim before direct reclaim.
> >
> > Of course, I am not arguing against the watermarks.  I am just
> > (violently) against making them configurable from userspace.
> >
> > > There is a patch in my patchset which adds the tunable for both
> > > high/low_mark, which gives more flexibility to admin to config the host.
> > In
> > > over-commit environment, we might never hit the wmark if all the wmarks
> > are
> > > set internally.
> >
> > And my point is that this should not be a problem at all!  If the
> > watermarks are not physically reachable, there is no reason to reclaim
> > on behalf of them.
> >
> > In such an environment, global memory pressure arises before the
> > memcgs get close to their hard limit, and global memory pressure
> > reduction should do the right thing and equally push back all memcgs.
> >
> > Flexibility in itself is not an argument.  On the contrary.  We commit
> > ourselves to that ABI and have to maintain this flexibility forever.
> > Instead, please find a convincing argument for the flexibility itself,
> > other than the need to workaround the current global kswapd reclaim.

[fixed following quotation]

> Ok, I tend to agree with you now that the over-commit example i gave
> early is a weak argument. We don't need to provide the ability to
> reclaim from a memcg before it is reaching its wmarks in over-commit
> environment.

Yep.  If it is impossible to reach the hard limit, it can't possibly
be a source of latency.

> However, i still think there is a need from the admin to have some controls
> of which memcg to do background reclaim proactively (before global memory
> pressure) and that was the initial logic behind the API.

That sounds more interesting.  Do you have a specific use case that
requires this?

min_free_kbytes more or less indirectly provides the same on a global
level, but I don't think anybody tunes it just for aggressiveness of
background reclaim.

> > (I fixed up the following quotation, please be more careful when
> > replying, this makes it so hard to follow your emails.  thanks!)

^^^^

> > > > My counter proposal is to fix global reclaim instead and apply
> > > > equal pressure on memcgs, such that we never have to tweak
> > > > per-memcg > > watermarks to achieve the same thing.
> > >
> > > We still need this and that is the soft_limit reclaim under global
> > > background reclaim.
> >
> > I don't understand what you mean by that.  Could you elaborate?
> 
> Sorry I think I misunderstood your early comment. What I pointed out here
> was that we need both per-memcg
> background reclaim and global soft_limit reclaim. I don't think we have
> disagreement on that at this point.

Ah, got you, thanks.

	Hannes

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