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

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Date:	Tue, 24 Feb 2009 17:13:29 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Salman Qazi <sqazi@...gle.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Performance regression in write() syscall


* Andi Kleen <andi@...stfloor.org> wrote:

> On Mon, Feb 23, 2009 at 06:03:04PM -0800, Salman Qazi wrote:

> > -	return __copy_user_nocache(dst, src, size, 1);
> > +	if (likely(size >= PAGE_SIZE))
> > +		return __copy_user_nocache(dst, src, size, 1);
> > +	else
> > +		return __copy_from_user(dst, src, size);
> 
> I think you disabled it completely, the kernel never really 
> does any copies larger than page size because all its internal 
> objects are page sized only.

No, look again, it's not disabled completely - the check now 
basically special-cases 4K writes _only_, and makes them 
non-temporal. That still covers the big/midsize file case.

And that kind of 4K limit makes a lot of sense. A small file 
write will unlikely to have a perfect 4K sized copy. Big file 
writes (and raw/direct IO related copies, etc.) will be chunked 
down to 4K sized units.

	Ingo
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