Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v6)

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Tue, 17 Mar 2009 10:10:16 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> >   - vm.softlimit_ratio
> > 
> > If vm.softlimit_ratio = 99%, 
> >   when sum of all usage of memcg is over 99% of system memory,
> >   softlimit runs and reclaim memory until the whole usage will be below 99%.
> >    (or some other trigger can be considered.)
> > 
> > Then,
> >  - We don't have to take care of misc. complicated aspects of memory reclaiming
> >    We reclaim memory based on our own logic, then, no influence to global LRU.
> > 
> > I think this approach will hide the all corner case and make merging softlimit 
> > to mainline much easier. If you use this approach, RB-tree is the best one
> > to go with (and we don't have to care zone's status.)
> 
> I like the idea in general, but I have concerns about
> 
> 1. Tracking all cgroup memory, it can quickly get expensive (tracking
> to check for vm.soft_limit_ratio and for usage)

Not so expensive because we already tracks them all by default cgroup.
Then, what we need is "fast" counter.
Maybe percpu coutner (lib/percpu_counter.c) gives us enough codes for counting.

Checking value ratio is ...how about "once per 1000 increment per cpu" or some ?

> 2. Finding a good default for the sysctl (might not be so hard)
> 
I think some parameter like high-low watermark is good and we can find
good value as
  - low watermak .... max_memory - (sum of all zone->high) * 16 of memory.
  - high watermark .... max_memory - (sum_of all zone->high) * 8
(just an example but not so bad.)

> Even today our influence on global LRU is very limited, only when we
> come under reclaim, we do an additional step of seeing if we can get
> memory from soft limit groups first.
> 
> (1) is a real concern.

Maybe yes. But all memcg will call "charge" "uncharge" codes so, problem is
just "counter". I think percpu coutner works enough.

Thanks,
-Kame 

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