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

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Date:	Wed, 30 May 2012 16:14:03 +0100
From:	Pádraig Brady <P@...igBrady.com>
To:	Cong Wang <amwang@...hat.com>
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
Subject: Re: [RFC Patch] fs: implement per-file drop caches

On 05/30/2012 02:38 PM, Cong Wang wrote:
> This is a draft patch of implementing per-file drop caches.
> 
> It introduces a new fcntl command  F_DROP_CACHES to drop
> file caches of a specific file. The reason is that currently
> we only have a system-wide drop caches interface, it could
> cause system-wide performance down if we drop all page caches
> when we actually want to drop the caches of some huge file.

This is useful functionality.
Though isn't it already provided with POSIX_FADV_DONTNEED?

This functionality was added to GNU dd (8.11) a year ago:
http://git.sv.gnu.org/gitweb/?p=coreutils.git;a=commitdiff;h=5f31155

Here are the examples from that patch:

# Advise to drop cache for whole file
dd if=ifile iflag=nocache count=0

# Ensure drop cache for the whole file
dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0

# Drop cache for part of file
dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null

# Stream data using just the read-ahead cache
dd if=ifile of=ofile iflag=nocache oflag=nocache

cheers,
Pádraig.
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