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Date: Tue, 27 May 2014 14:17:59 -0700 (PDT)
From: Hugh Dickins <hughd@google.com>
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To: Dave Chinner <david@fromorbit.com>
cc: Mel Gorman <mgorman@suse.de>, Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
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        Bob Liu <bob.liu@oracle.com>, Jan Kara <jack@suse.cz>,
        Rik van Riel <riel@redhat.com>,
        Linux Kernel <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linux-FSDevel <linux-fsdevel@vger.kernel.org>
Subject: Re: [PATCH 0/3] Shrinkers and proportional reclaim
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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@google.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