linux-kernel - Re: Another Performance Regression in write() syscall

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Message-Id: <200902251449.38361.nickpiggin@yahoo.com.au>
Date:	Wed, 25 Feb 2009 14:49:37 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
	Dave Hansen <haveblue@...ibm.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Another Performance Regression in write() syscall

On Wednesday 25 February 2009 03:58:52 Dave Hansen wrote:
> On Tue, 2009-02-24 at 19:47 +1100, Nick Piggin wrote:
> > On Tuesday 24 February 2009 17:25:45 Dave Hansen wrote:
> > > On Mon, 2009-02-23 at 22:05 -0800, Salman Qazi wrote:
> > > > Analysis of profile data has led us to believe that the commit
> > > > 3d733633a633065729c9e4e254b2e5442c00ef7e has caused a performance
> > > > regression.  This commit provides for tracking of writers so that
> > > > read only bind mounts function correctly.
> > > >
> > > > We can verify this regression by applying the following patch to
> > > > partially disable the above-mentioned commit and then running the
> > > > fstime component of Unixbench.  The settings used were 256 byte
> > > > writes with MAX_BLOCK of 2000.
> > >
> > > I'm a bit surprised that write() is what is regressing.  Unless I
> > > screwed up, we do all the expensive accounting at open()/close() time.
> > > Is this a test that gets run in parallel on multiple cpus?
> >
> > Don't forget touch_atime...
>
> Yeah, that's a good point.  Are we sure that's what is happening here,
> though?  That's one thing a profile would hopefully help with.
>
> > Still, open/close isn't unimportant either.
>
> Yeah, that's true.  But, what I noticed was that all of the other
> open/close activity masked out any overhead from mnt_want/drop_write()
> since a big chunk of the overhead was just going and bringing the
> vfsmount pieces into the cache.

That's very true.


> > > Could you take a look at Nick's patches to speed this stuff up?
> > >
> > > 	http://thread.gmane.org/gmane.linux.file-systems/28186
> > >
> > > We may need to dust those off, although I'm still a bit worried about
> > > the complexities of open-coding all the barriers.
> >
> > I really need to do something about trying to push them upstream again
> > actually because we've got them in SLES11 tree.
>
> Were the patches that you integrated any different from the ones you
> posted a few months ago?

Don't think they were significantly changed. I'll take another look
and repost them.

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