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

[Balbir Singh <balbir@linux.vnet.ibm.com>]

* KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-17 13:47:27]:

> 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 ?

That is not true..we don't track them to default cgroup unless
memory.use_hiearchy is enabled in the root cgroup. To do what you
suggest, we have to iterate through all mem cgroups, which is not
desirable at all.

> 
> > 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.)
>

OK..

[offtopic] I liked the per-mem cgroup watermark patches as well. I
think we should look at them later on, after soft limits and some other items.
 
> > 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.
>

This scheme adds more overhead due to (1), we'll need a global counter
and need to protect it, which will serialize all res_counters. 

-- 
	Balbir

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