On Tue 24-01-12 11:26:05, Hillf Danton wrote:

Hi all

On Tue, Jan 24, 2012 at 3:14 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.

If soft limits are set to be limited and there are no excessors, who
are consuming physical pages? The consumers maybe those with soft
unlimited.

You might have many small groups which are all under their soft limit
but their total usage triggers global reclaim for example.

If so, they should be punished first, based on the assumption that the
unlimited is treated with no guarantee.
Then soft limit guarantee could be assured without changes in the
current default setting of soft limit, no?

How would you prioritize between over soft limit groups (which should
give at least some protection from over reclaim AFAIU from your
suggestion) from those that are unlimited?

Also, when we start talking about guarantees, why would somebody who
is soft unlimited be punished at all? Soft unlimited basically means
soft_limit >= hard_limit which says please do not reclaim from me unless
really really really necessary.

With soft limit available, victims are only selected from excessors, I think.

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.

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.

Johannes did great cleanup, why barriered?

Barriered?

-- 
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
Lihovarska 1060/12
190 00 Praha 9    
Czech Republic

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