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

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Date:	Mon, 23 Feb 2009 22:25:45 -0800
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Salman Qazi <sqazi@...gle.com>
Cc:	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>, nickpiggin@...oo.com.au,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: Another Performance Regression in write() syscall

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?  

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.

Could we also see some kind of profile?  What kind of machine are you
seeing this on, btw?

-- Dave

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