linux-kernel - Re: -rt dbench scalabiltiy issue

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Date:	Thu, 26 Nov 2009 07:20:20 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	john stultz <johnstul@...ibm.com>
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 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.

 
> 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.


> > Anyway, I think all these problems should largely go away when path
> > walk fall back is improved.
> 
> Great, let me know when you have a rough shot at it ready for testing.

Will do
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