Re: [RFC][PATCH 3/4] memcg: softlimit caller via kswapd

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

On Wed, 11 Mar 2009 00:32:42 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:

> * KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> [2009-03-09 16:42:18]:
> 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> Looks like a dirty hack, replacing sc-> fields this way. I've
> experimented a lot with per zone balancing and soft limits and it does
> not work well. The reasons
> 
> 1. zone watermark balancing has a different goal than soft limit. Soft
> limits are more of a mem cgroup feature rather than node/zone feature.
I can't catch what you want to say, here.

> IIRC, you called reclaim as hot-path for soft limit reclaim, my
> experimentation is beginning to show changed behaviour
> 
> On a system with 4 CPUs and 4 Nodes, I find all CPUs spending time
> doing reclaim, putting the hook in the reclaim path, makes the reclaim
> dependent on the number of tasks and contention.
> 
> What does your test data/experimentation show?
> 

Not done very pricrse test but I admit that can happen.
(BTW, 1 cpu per 1 node ?)
How to call this is the my main concern, now.

BTW, when you don't use softlimit, CPUs won't spend time in reclaim ?
If 1 cpu per 1 node, 1 kswapd per 1 node. So, it doesn't sound strange.

If it's better to add a softlimitd() for the system (means 1 thread for
the whole system), modification is not difficult.
(I think my code doesn't assume the caller is kswapd() other than /* comment */)

Thanks,
-Kame

> > +		scan -= sc->nr_scanned;
> > +	}
> > +	return;
> > +}
> >  /*
> >   * For kswapd, balance_pgdat() will work across all this node's zones until
> >   * they are all at pages_high.
> > @@ -1776,6 +1813,8 @@ static unsigned long balance_pgdat(pg_da
> >  	 */
> >  	int temp_priority[MAX_NR_ZONES];
> > 
> > +	/* Refill softlimit queue */
> > +	mem_cgroup_reschedule(pgdat->node_id);
> >  loop_again:
> >  	total_scanned = 0;
> >  	sc.nr_reclaimed = 0;
> > @@ -1856,6 +1895,9 @@ loop_again:
> >  					       end_zone, 0))
> >  				all_zones_ok = 0;
> >  			temp_priority[i] = priority;
> > +			/* Try soft limit at first */
> > +			shrink_zone_softlimit(&sc, zone, order, priority,
> > +					       8 * zone->pages_high, end_zone);
> >  			sc.nr_scanned = 0;
> >  			note_zone_scanning_priority(zone, priority);
> >  			/*
> > 
> > --
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> 
> -- 
> 	Balbir
> 

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