linux-kernel - Re: -rt dbench scalabiltiy issue

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Date:	Tue, 01 Dec 2009 17:53:34 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	"Theodore Ts'o" <tytso@....edu>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhltc@...ibm.com>,
	Clark Williams <williams@...hat.com>,
	"Paul E. McKenney" <paulmck@...ibm.com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, cmm@...ibm.com
Subject: Re: -rt dbench scalabiltiy issue

On Thu, 2009-11-26 at 07:20 +0100, Nick Piggin wrote:
> On Wed, Nov 25, 2009 at 02:20:33PM -0800, john stultz wrote:
> > On Wed, 2009-11-25 at 08:18 +0100, Nick Piggin wrote:
> > > If you're using acls or something on ext2 then lock free path walk
> > > might fail more often.
> > 
> > CC'ed Ted and Mingming as they might be interested:
> > 
> > Got ext4 data up:
> > http://sr71.net/~jstultz/dbench-scalability/graphs/ext4-scalability.png
> 
> Ahh, looks much nicer than ext3, at least on non-rt. -rt seems to
> be running into journal lock contention.

Yep.

> 
> > Looks pretty similar to ext2. I'm also seeing path_get contention as
> > well with your patch on ext4 in the perflogs:
> > http://sr71.net/~jstultz/dbench-scalability/perflogs/2.6.32-rc7-nick.ext4.perflog
> 
> That's *all* coming from reading /proc/mounts by the looks. I don't
> think we're going to bother trying to make d_path incredibly scalable,
> the fix is to fix glibc's statvfs call.
> 
> As I said, you can work around this by changing dbench's statvfs call
> to statfs.
> 
> After that, the same journal locks look like they might hit next, but
> it should get quite a lot further.

Thanks for reminding me. I had tried this for ext2, but didn't see much
change, so I forgot to try it for ext4. 

But your right. I've verified changing dbench to use statfs() does cause
the path_get contention to fall out for the mainline ext4 case. It
doesn't change too much in the -rt case, but it does help (and gives a
really nice boost for the non-rt case).

See:
http://sr71.net/~jstultz/dbench-scalability/graphs/ext4-statfs-scalability.png
vs
http://sr71.net/~jstultz/dbench-scalability/graphs/ext4-scalability.png

Perflogs here:
http://sr71.net/~jstultz/dbench-scalability/perflogs/

So yea from -rt's perspective, with this patchset we're down to journal
lock contention for ext3 and ext4 as the main issue now.

Nick: So what's your plan to upstream this work?  With 2.6.32 around the
corner and 2.6.32-rt likely following shortly, I don't think pushing the
backport to 2.6.31-rt make much sense right at this moment. But when
2.6.32-rt does arrive, it might be nice to have the broken out patches.

Thomas/Ingo: Any thoughts on how receptive you guys would be to picking
up these changes for -rt (maybe for 2.6.32-rt)? Or should they go
mainline first?

thanks
-john

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