Re: [Jfs-discussion] benchmark results

From: tytso@mit.edu
To: Larry McVoy <lm@bitmover.com>,
	Christian Kujau <lists@nerdbynature.de>,
	jfs-discussion@lists.sourceforge.net,
	linux-nilfs@vger.kernel.org, xfs@oss.sgi.com,
	reiserfs-devel@vger.kernel.org,
	Peter Grandi <pg_jf2@jf2.for.sabi.co.UK>,
	ext-users <ext3-users@redhat.com>,
	linux-ext4@vger.kernel.org, linux-btrfs@vger.kernel.org
Subject: Re: [Jfs-discussion] benchmark results
Date: Fri, 25 Dec 2009 11:33:41 -0500	[thread overview]
Message-ID: <20091225163341.GE32757@thunk.org> (raw)
In-Reply-To: <20091225162238.GB19303@bitmover.com>

On Fri, Dec 25, 2009 at 08:22:38AM -0800, Larry McVoy wrote:
> 
> Dudes, sync() doesn't flush the fs cache, you have to unmount for that.
> Once upon a time Linux had an ioctl() to flush the fs buffers, I used
> it in lmbench.  
> 
> 	ioctl(fd, BLKFLSBUF, 0);
> 
> No idea if that is still supported, but sync() is a joke for benchmarking.

Depends on what you are trying to do (flush has multiple meanings, so
using can be ambiguous).  BLKFLSBUF will write out any dirty buffers,
*and* empty the buffer cache.  I use it when benchmarking e2fsck
optimization.  It doesn't do anything for the page cache.  If you are
measuring the time to write a file, using fsync() or sync() will
include the time to actually write the data to disk.  It won't empty
caches, though; if you are going to measure read as well as writes,
then you'll probably want to do something like "echo 3 >
/proc/sys/vm/drop-caches".

						- Ted

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