linux-kernel - Re: seq_file: Use larger buffer to reduce time traversing lists

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Date:	Fri, 01 Jun 2012 14:10:26 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Steven Whitehouse <swhiteho@...hat.com>
Cc:	linux-kernel@...r.kernel.org, cluster-devel@...hat.com,
	Al Viro <viro@...iv.linux.org.uk>, nstraz@...hat.com
Subject: Re: seq_file: Use larger buffer to reduce time traversing lists

On Fri, 2012-06-01 at 11:39 +0100, Steven Whitehouse wrote:
> I've just been taking a look at the seq_read() code, since we've noticed
> that dumping files with large numbers of records can take some
> considerable time. This is due to seq_read() using a buffer which, at
> most is a single page in size, and that it has to find its place again
> on every call to seq_read(). That makes it rather inefficient.
> 
> As an example, I created a GFS2 filesystem with 100k inodes in it, and
> then ran ls -l to get a decent number of cached inodes. This result in
> there being approx 400k lines in the debugfs file containing GFS2's
> glocks. I then timed how long it takes to read this file:
> 
> [root@...woon mnt]# time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks
> of=/dev/null bs=1M
> 0+5769 records in
> 0+5769 records out
> 23273958 bytes (23 MB) copied, 63.3681 s, 367 kB/s

What time do you get if you do

time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks of=/dev/null bs=4k

This patch seems the wrong way to me.

seq_read(size = 1MB) should perform many copy_to_user() calls instead of a single one.

Instead of doing kmalloc(m->size <<= 1, GFP_KERNEL) each time we overflow the buffer,
we should flush its content to user space.



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