linux-kernel - Re: [PATCH 0/3] Shrinkers and proportional reclaim

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Message-ID: <alpine.LSU.2.11.1405271406520.4317@eggly.anvils>
Date:	Tue, 27 May 2014 14:17:59 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Dave Chinner <david@...morbit.com>
cc:	Mel Gorman <mgorman@...e.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Yuanhan Liu <yuanhan.liu@...ux.intel.com>,
	Bob Liu <bob.liu@...cle.com>, Jan Kara <jack@...e.cz>,
	Rik van Riel <riel@...hat.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linux-FSDevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 0/3] Shrinkers and proportional reclaim

On Tue, 27 May 2014, Dave Chinner wrote:
> On Mon, May 26, 2014 at 02:44:29PM -0700, Hugh Dickins wrote:
> > 
> > [PATCH 4/3] fs/superblock: Avoid counting without __GFP_FS
> > 
> > Don't waste time counting objects in super_cache_count() if no __GFP_FS:
> > super_cache_scan() would only back out with SHRINK_STOP in that case.
> > 
> > Signed-off-by: Hugh Dickins <hughd@...gle.com>
> 
> While you might think that's a good thing, it's not.  The act of
> shrinking is kept separate from the accounting of how much shrinking
> needs to take place.  The amount of work the shrinker can't do due
> to the reclaim context is deferred until the shrinker is called in a
> context where it can do work (eg. kswapd)
> 
> Hence not accounting for work that can't be done immediately will
> adversely impact the balance of the system under memory intensive
> filesystem workloads. In these worklaods, almost all allocations are
> done in the GFP_NOFS or GFP_NOIO contexts so not deferring the work
> will will effectively stop superblock cache reclaim entirely....

Thanks for filling me in on that.  At first I misunderstood you,
and went off looking in the wrong direction.  Now I see what you're
referring to: the quantity that shrink_slab_node() accumulates in
and withdraws from shrinker->nr_deferred[nid].

Right: forget my super_cache_count() __GFP_FS patch!

Hugh
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