Re: [PATCH 4/4] Memory controller soft limit reclaim on contention (v4)

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

On Fri, 6 Mar 2009 15:31:55 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:


> > > +		if (wait)
> > > +			wait_for_completion(&mem->wait_on_soft_reclaim);
> > >  	}
> > What ???? Why we have to wait here...holding mmap->sem...This is too bad.
> >
> 
> Since mmap_sem is no longer used for pthread_mutex*, I was not sure.
> That is why I added the comment asking for more review and see what
> people think about it. We get here only when
> 
> 1. The memcg is over its soft limit
> 2. Tasks/threads belonging to memcg are faulting in more pages
> 
> The idea is to throttle them. If we did reclaim inline, like we do for
> hard limits, we can still end up holding mmap_sem for a long time.
> 
This "throttle" is hard to measuer the effect and IIUC, not implemneted in
vmscan.c ...for global try_to_free_pages() yet.
Under memory shortage. before reaching here, the thread already called
try_to_free_pages() or check some memory shorage conditions because
it called alloc_pages(). So, waiting here is redundant and gives it
too much penaly.


> > > +	/*
> > > +	 * This loop can run a while, specially if mem_cgroup's continuously
> > > +	 * keep exceeding their soft limit and putting the system under
> > > +	 * pressure
> > > +	 */
> > > +	do {
> > > +		mem = mem_cgroup_get_largest_soft_limit_exceeding_node();
> > > +		if (!mem)
> > > +			break;
> > > +		usage = mem_cgroup_get_node_zone_usage(mem, zone, nid);
> > > +		if (!usage)
> > > +			goto skip_reclaim;
> > 
> > Why this works well ? if "mem" is the laragest, it will be inserted again
> > as the largest. Do I miss any ?
> >
> 
> No that is correct, but when reclaim is initiated from a different
> zone/node combination, we still want mem to show up. 
....
your logic is
==
   nr_reclaimd = 0;
   do {
      mem = select victim.
      remvoe victim from the RBtree (the largest usage one is selected)
      if (victim is not good)
          goto  skip this.
      reclaimed += shirnk_zone.
      
skip_this:
      if (mem is still exceeds soft limit)
           insert RB tree again.
   } while(!nr_reclaimed)
==
When this exits loop ?

Thanks,
-Kame

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