linux-kernel - Re: [RFC Patch] fs: implement per-file drop caches

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Date:	Thu, 31 May 2012 14:28:16 +0800
From:	Cong Wang <amwang@...hat.com>
To:	John Stoffel <john@...ffel.org>
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Cong Wang <xiyou.wangcong@...il.com>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	Matthew Wilcox <matthew@....cx>, linux-fsdevel@...r.kernel.org,
	linux-mm@...ck.org, Keiichi Kii <k-keiichi@...jp.nec.com>
Subject: Re: [RFC Patch] fs: implement per-file drop caches

On Wed, 2012-05-30 at 11:12 -0400, John Stoffel wrote:
> Cong> This is a draft patch of implementing per-file drop caches.
> 
> Interesting.  So can I do this from outside a process?  I'm a
> SysAdmin, so my POV is from noticing, finding and fixing performance
> problems when the system is under pressure.  

Yes, sure, we need to write a utility (or patch an existing one) to do
this for you admins.

> 
> Cong> It introduces a new fcntl command  F_DROP_CACHES to drop
> Cong> file caches of a specific file. The reason is that currently
> Cong> we only have a system-wide drop caches interface, it could
> Cong> cause system-wide performance down if we drop all page caches
> Cong> when we actually want to drop the caches of some huge file.
> 
> How can I tell how much cache is used by a file?  And what is the
> performance impact of this when run on a busy system?  And what does
> this patch buy us since I figure the VM should already be dropping
> caches once the system comes under mem pressure...
> 

AFAIK, we don't export such information to user-space, we only have
system-wide statistics.

Keiichi (in Cc) once wrote a patch to implement page cache tracepoint:
http://marc.info/?l=linux-mm&m=131102496904326&w=3

but the patches are still not in upstream.

Thanks!

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