linux-kernel - Re: Performance regression in scsi sequential throughput (iozone) due to "e084b - page-allocator: preserve PFN ordering when __GFP

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Date:	Mon, 15 Feb 2010 17:59:14 +1100
From:	Nick Piggin <npiggin@...e.de>
To:	Christian Ehrhardt <ehrhardt@...ux.vnet.ibm.com>
Cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	epasch@...ibm.com, SCHILLIG@...ibm.com,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	christof.schmitt@...ibm.com, thoss@...ibm.com, hare@...e.de,
	gregkh@...ell.com
Subject: Re: Performance regression in scsi sequential throughput (iozone)
 due to "e084b - page-allocator: preserve PFN ordering when	__GFP_COLD
 is set"

On Fri, Feb 12, 2010 at 09:05:19PM +1100, Nick Piggin wrote:
> > This leaves me with two ideas what the real issue could be:
> > 1. either really the 6th parameter as this is the first one that needs to go on stack and that way might open a race and rob gcc a big pile of optimization chances
> 
> It must be something to do wih code generation AFAIKS. I'm surprised
> the function isn't inlined, giving exactly the same result regardless
> of the patch.
> 
> Unlikely to be a correctness issue with code generation, but I'm
> really surprised that a small difference in performance could have
> such a big (and apparently repeatable) effect on behaviour like this.
> 
> What's the assembly look like?

Oh, and what happens if you always_inline it? I would have thought
gcc should be able to generate the same code in both cases -- unless
I'm really missing something and there is a functional change in
there?

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