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Date: Mon, 16 Jul 2018 11:10:40 +0200
From: Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries
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To: James Bottomley <James.Bottomley@HansenPartnership.com>
Cc: Dave Chinner <david@fromorbit.com>, Linus Torvalds <torvalds@linux-foundation.org>, Matthew Wilcox <willy@infradead.org>, Waiman Long <longman@redhat.com>, Al Viro <viro@zeniv.linux.org.uk>, Jonathan Corbet <corbet@lwn.net>, "Luis R. Rodriguez" <mcgrof@kernel.org>, Kees Cook <keescook@chromium.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-fsdevel <linux-fsdevel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>, Jan Kara <jack@suse.cz>, Paul McKenney <paulmck@linux.vnet.ibm.com>, Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@kernel.org>, Miklos Szeredi <mszeredi@redhat.com>, Larry Woodman <lwoodman@redhat.com>, "Wangkai (Kevin,C)" <wangkai86@huawei.com>

On Fri 13-07-18 08:46:52, James Bottomley wrote:
> On Fri, 2018-07-13 at 10:36 +1000, Dave Chinner wrote:
> > On Thu, Jul 12, 2018 at 12:57:15PM -0700, James Bottomley wrote:
> > > What surprises me most about this behaviour is the steadiness of
> > > the page cache ... I would have thought we'd have shrunk it
> > > somewhat given the intense call on the dcache.
> > 
> > Oh, good, the page cache vs superblock shrinker balancing still
> > protects the working set of each cache the way it's supposed to
> > under heavy single cache pressure. :)
> 
> Well, yes, but my expectation is most of the page cache is clean, so
> easily reclaimable.  I suppose part of my surprise is that I expected
> us to reclaim the clean caches first before we started pushing out the
> dirty stuff and reclaiming it.  I'm not saying it's a bad thing, just
> saying I didn't expect us to make such good decisions under the
> parameters of this test.

This is indeed unepxected. Especially when the current LRU reclaim balancing
logic is highly pagecache biased. Are you sure you were not running in a
memcg with a small amount of the pagecache?
-- 
Michal Hocko
SUSE Labs