linux-kernel - Re: Performance regression in write() syscall

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Message-ID: <20090224162443.GW26292@one.firstfloor.org>
Date:	Tue, 24 Feb 2009 17:24:43 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <nickpiggin@...oo.com.au>,
	Salman Qazi <sqazi@...gle.com>, davem@...emloft.net,
	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>
Subject: Re: Performance regression in write() syscall

On Tue, Feb 24, 2009 at 07:52:34AM -0800, Linus Torvalds wrote:
> 
> 
> On Tue, 24 Feb 2009, Nick Piggin wrote:
> >
> > > it does make some kind of sense to try to avoid the noncached versions for
> > > small writes - because small writes tend to be for temp-files.
> > 
> > I don't see the significance of a temp file. If the pagecache is truncated,
> > then the cachelines remain dirty and so you can't avoid an eventual store
> > back to RAM? 
> 
> No, because many small files end up being used as scratch-pads (think 
> shell script sequences etc), and get read back immediately again. Doing 
> non-temporal stores might just be bad simply because trying to play games 
> with caching may simply do the wrong thing.

I usually recommend to use shmfs for /tmp. Perhaps that should be done
more widely.

BTW I got some upcomming patches to improve c_*_u() and memcpy
on modern Intel x86 CPUs.

-Andi

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