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SubjectRe: seq_file: Use larger buffer to reduce time traversing lists
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Hi,

On Fri, 2012-06-01 at 14:14 +0200, Eric Dumazet wrote:
> On Fri, 2012-06-01 at 14:10 +0200, Eric Dumazet wrote:
> > 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@chywoon 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.
> >
> >
>
> by the way, is the following command even working ?
>
> time dd if=/sys/kernel/debug/gfs2/unity\:myfs/glocks of=/dev/null bs=16M
>
> I guess not, it probably returns -ENOMEM
>
>
>

Why would it return -ENOMEM? It works for me, at worst it will fall back
to a single page buffer size unless we are really stuck for memory, and
in that case, all bets are off,

Steve.




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