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

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Date:   Fri, 6 Jul 2018 23:28:14 +0100
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Waiman Long <longman@...hat.com>
Cc:     Jonathan Corbet <corbet@....net>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-doc@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        "Wangkai (Kevin C)" <wangkai86@...wei.com>
Subject: Re: [PATCH v6 0/7] fs/dcache: Track & limit # of negative dentries

On Fri, Jul 06, 2018 at 03:32:45PM -0400, Waiman Long wrote:

> With a 4.18 based kernel, the positive & negative dentries lookup rates
> (lookups per second) after initial boot on a 2-socket 24-core 48-thread
> 64GB memory system with and without the patch were as follows: `
> 
>   Metric                    w/o patch  neg_dentry_pc=0  neg_dentry_pc=1
>   ------                    ---------  ---------------  ---------------
>   Positive dentry lookup      584299       586749	   582670
>   Negative dentry lookup     1422204      1439994	  1438440
>   Negative dentry creation    643535       652194	   641841
> 
> For the lookup rate, there isn't any signifcant difference with or
> without the patch or with a zero or non-zero value of neg_dentry_pc.

Sigh...  What I *still* don't see (after all the iterations of the patchset)
is any performance data on workloads that would be likely to feel the impact.
Anything that seriously hits INCLUDE_PATH, for starters...