linux-kernel - Re: [patch 3/8] mm: vmscan: save work scanning (almost) empty LRU lists

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Message-ID: <20121213193332.GB6317@cmpxchg.org>
Date:	Thu, 13 Dec 2012 14:33:32 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 3/8] mm: vmscan: save work scanning (almost) empty LRU
 lists

On Thu, Dec 13, 2012 at 10:41:04AM +0000, Mel Gorman wrote:
> On Wed, Dec 12, 2012 at 04:43:35PM -0500, Johannes Weiner wrote:
> > In certain cases (kswapd reclaim, memcg target reclaim), a fixed
> > minimum amount of pages is scanned from the LRU lists on each
> > iteration, to make progress.
> > 
> > Do not make this minimum bigger than the respective LRU list size,
> > however, and save some busy work trying to isolate and reclaim pages
> > that are not there.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> This looks like a corner case where the LRU size would have to be smaller
> than SWAP_CLUSTER_MAX. Is that common enough to care? It looks correct,
> I'm just curious.

We have one lruvec per memcg per zone, so consider memory cgroups in a
NUMA environment: NR_MEMCG * (NR_NODES - 1) * NR_LRU_LISTS permanently
empty lruvecs, assuming the memory of one cgroup is bound to one node.
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