Re: [PATCH v5 2/6] memcg: stop vmscan when enough done.

[KAMEZAWA Hiroyuki]

On Wed, 10 Aug 2011 16:14:25 +0200
Michal Hocko <mhocko@xxxxxxx> wrote:

> On Tue 09-08-11 19:09:33, KAMEZAWA Hiroyuki wrote:
> > memcg :avoid node fallback scan if possible.
> > 
> > Now, try_to_free_pages() scans all zonelist because the page allocator
> > should visit all zonelists...but that behavior is harmful for memcg.
> > Memcg just scans memory because it hits limit...no memory shortage
> > in pased zonelist.
> > 
> > For example, with following unbalanced nodes
> > 
> >      Node 0    Node 1
> > File 1G        0
> > Anon 200M      200M
> > 
> > memcg will cause swap-out from Node1 at every vmscan.
> > 
> > Another example, assume 1024 nodes system.
> > With 1024 node system, memcg will visit 1024 nodes
> > pages per vmscan... This is overkilling. 
> > 
> > This is why memcg's victim node selection logic doesn't work
> > as expected.
> > 
> > This patch is a help for stopping vmscan when we scanned enough.
> > 
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>
> 
> OK, I see the point. At first I was afraid that we would make a bigger
> pressure on the node which triggered the reclaim but as we are selecting
> t dynamically (mem_cgroup_select_victim_node) - round robin at the
> moment - it should be fair in the end. More targeted node selection
> should be even more efficient.
> 
> I still have a concern about resize_limit code path, though. It uses
> memcg direct reclaim to get under the new limit (assuming it is lower
> than the current one). 
> Currently we might reclaim nr_nodes * SWAP_CLUSTER_MAX while
> after your change we have it at SWAP_CLUSTER_MAX. This means that
> mem_cgroup_resize_mem_limit might fail sooner on large NUMA machines
> (currently it is doing 5 rounds of reclaim before it gives up). I do not
> consider this to be blocker but maybe we should enhance
> mem_cgroup_hierarchical_reclaim with a nr_pages argument to tell it how
> much we want to reclaim (min(SWAP_CLUSTER_MAX, nr_pages)).
> What do you think?
> 

Hmm,

> mem_cgroup_resize_mem_limit might fail sooner on large NUMA machines

mem_cgroup_resize_limit() just checks (curusage < prevusage), then, 
I agree reducing the number of scan/reclaim will cause that.

I agree to pass nr_pages to try_to_free_mem_cgroup_pages().


Thanks,
-Kame

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