On Tue 24-01-12 09:18:21, Balbir Singh wrote:

On Tue, Jan 24, 2012 at 12:44 AM, Ying Han [off-list ref] wrote:

quoted

On Mon, Jan 23, 2012 at 5:02 AM, Michal Hocko [off-list ref] wrote:

quoted

On Sat 21-01-12 22:49:23, Hillf Danton wrote:

quoted

In soft limit reclaim, overreclaim occurs when pages are reclaimed from mem
group that is under its soft limit, or when more pages are reclaimd than the
exceeding amount, then performance of reclaimee goes down accordingly.

First of all soft reclaim is more a help for the global memory pressure
balancing rather than any guarantee about how much we reclaim for the
group.
We need to do more changes in order to make it a guarantee.
For example you implementation will cause severe problems when all
cgroups are soft unlimited (default conf.) or when nobody is above the
limit but the total consumption triggers the global reclaim. Therefore
nobody is in excess and you would skip all groups and only bang on the
root memcg.

True, ideally soft reclaim should not turn on and allow global reclaim
to occur in the scenario mentioned.

quoted

quoted

Ying Han has a patch which basically skips all cgroups which are under
its limit until we reach a certain reclaim priority but even for this we
need some additional changes - e.g. reverse the current default setting
of the soft limit.

I'd be wary of that approach, because it might be harder to explain
the working of soft limits,

This is an attempt to turn the soft reclaim into a "guarantee". Changing
the default value from unlimited to 0 basically says that everybody will
be considered under memory pressure unless the soft limit setting says
otherwise.
This btw. has been the case with the double (global and per-cgroup) LRUs
as well. It was just hidden.

I'll look at the discussion thread mentioned earlier for the benefits
of that approach.

quoted

quoted

Anyway, I like the nr_to_reclaim reduction idea because we have to do
this in some way because the global reclaim starts with ULONG
nr_to_scan.

Agree with Michal where there are quite a lot changes we need to get
in for soft limit before any further optimization.

Hillf, please refer to the patch from Johannes
https://lkml.org/lkml/2012/1/13/99 which got quite a lot recent
discussions. I am expecting to get that in before further soft limit
changes.

Balbir
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Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
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