linux-kernel - Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries

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

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