On Fri, Apr 22, 2011 at 6:35 PM, Johannes Weiner <hannes@xxxxxxxxxxx> 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@xxxxxxxxxxx>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 isOf course, I am not arguing against the watermarks. I am just
> > 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.
(violently) against making them configurable from userspace.
And my point is that this should not be a problem at all! If the
> 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.
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.
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.
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.
I used to have per-memcg wmark_ratio api which controls both high/low_wmark based on hard_limit, but the two APIs seems give finer granularity.
--Ying
(I fixed up the following quotation, please be more careful when
replying, this makes it so hard to follow your emails. thanks!)
I don't understand what you mean by that. Could you elaborate?
> > 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.
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.
Thanks,
Hannes