linux-kernel - Re: [patch 2/7] mm: vmscan: save work scanning (almost) empty LRU lists

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Date:	Thu, 20 Dec 2012 22:02:36 -0500
From:	Johannes Weiner <hannes@...xchg.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>, Michal Hocko <mhocko@...e.cz>,
	Mel Gorman <mgorman@...e.de>, Hugh Dickins <hughd@...gle.com>,
	Satoru Moriya <satoru.moriya@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 2/7] mm: vmscan: save work scanning (almost) empty LRU
 lists

On Wed, Dec 19, 2012 at 03:59:01PM -0800, Andrew Morton wrote:
> On Mon, 17 Dec 2012 13:12:32 -0500
> Johannes Weiner <hannes@...xchg.org> 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.
> > 
> > Empty LRU lists are quite common with memory cgroups in NUMA
> > environments because there exists a set of LRU lists for each zone for
> > each memory cgroup, while the memory of a single cgroup is expected to
> > stay on just one node.  The number of expected empty LRU lists is thus
> > 
> >   memcgs * (nodes - 1) * lru types
> > 
> > Each attempt to reclaim from an empty LRU list does expensive size
> > comparisons between lists, acquires the zone's lru lock etc.  Avoid
> > that.
> > 
> > ...
> >
> > -#define SWAP_CLUSTER_MAX 32
> > +#define SWAP_CLUSTER_MAX 32UL
> 
> You made me review the effects of this change.  It looks OK.  A few
> cleanups are possible, please review.
> 
> I wonder what happens in __setup_per_zone_wmarks() if we set
> SWAP_CLUSTER_MAX greater than 128.

In the current clamp() implementation max overrides min, so...

BUILD_BUG_ON()?  Probably unnecessary, it seems like a rather
arbitrary range to begin with.

> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: mm/page_alloc.c:__setup_per_zone_wmarks: make min_pages unsigned long
> 
> `int' is an inappropriate type for a number-of-pages counter.
> 
> While we're there, use the clamp() macro.
> 
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Satoru Moriya <satoru.moriya@....com>
> Cc: Simon Jeons <simon.jeons@...il.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

Acked-by: Johannes Weiner <hannes@...xchg.org>

> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: mm/vmscan.c:shrink_lruvec(): switch to min()
> 
> "mm: vmscan: save work scanning (almost) empty LRU lists" made
> SWAP_CLUSTER_MAX an unsigned long.
> 
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Satoru Moriya <satoru.moriya@....com>
> Cc: Simon Jeons <simon.jeons@...il.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

Acked-by: Johannes Weiner <hannes@...xchg.org>

> From: Andrew Morton <akpm@...ux-foundation.org>
> Subject: mm/vmscan.c:__zone_reclaim(): replace max_t() with max()
> 
> "mm: vmscan: save work scanning (almost) empty LRU lists" made
> SWAP_CLUSTER_MAX an unsigned long.
> 
> Cc: Johannes Weiner <hannes@...xchg.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Michal Hocko <mhocko@...e.cz>
> Cc: Hugh Dickins <hughd@...gle.com>
> Cc: Satoru Moriya <satoru.moriya@....com>
> Cc: Simon Jeons <simon.jeons@...il.com>
> Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>

Acked-by: Johannes Weiner <hannes@...xchg.org>
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