linux-kernel - Re: [PATCH 11/20] Inline get_page_from

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Message-ID: <20090224150327.GB5364@csn.ul.ie>
Date:	Tue, 24 Feb 2009 15:03:28 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH 11/20] Inline get_page_from_freelist() in the fast-path

On Wed, Feb 25, 2009 at 01:08:10AM +1100, Nick Piggin wrote:
> On Wednesday 25 February 2009 00:32:53 Mel Gorman wrote:
> > On Tue, Feb 24, 2009 at 02:32:37AM +1100, Nick Piggin wrote:
> > > On Monday 23 February 2009 10:17:20 Mel Gorman wrote:
> > > > In the best-case scenario, use an inlined version of
> > > > get_page_from_freelist(). This increases the size of the text but
> > > > avoids time spent pushing arguments onto the stack.
> > >
> > > I'm quite fond of inlining ;) But it can increase register pressure as
> > > well as icache footprint as well. x86-64 isn't spilling a lot more
> > > registers to stack after these changes, is it?
> >
> > I didn't actually check that closely so I don't know for sure. Is there a
> > handier way of figuring it out than eyeballing the assembly? In the end
> 
> I guess the 5 second check is to look at how much stack the function
> uses. OTOH I think gcc does do a reasonable job at register allocation.
> 

FWIW, 6 registers get pushed onto the stack from the calling function from
a glance of the assembly. According to the profile, about 7% of the cost of
the get_page_from_freelist() function is incurred by setting up and making
the function call. This is 2755 samples out of 35266. To compare, the cost
of zeroing was 192574 samples.

So, it's a good chunk of time, but in the grand scheme of things, time is
better spent optimising elsewhere for now.

> 
> > I dropped the inline of this function anyway. It means the patches
> > reduce rather than increase text size which is a bit more clear-cut.
> 
> Cool, clear cut patches for round 1 should help to get things moving.
> 

Indeed

> 
> > > In which case you will get extra icache footprint. What speedup does
> > > it give in the cache-hot microbenchmark case?
> >
> > I wasn't measuring with a microbenchmark at the time of writing so I don't
> > know. I was going entirely by profile counts running kernbench and the
> > time spent running the benchmark.
> 
> OK. Well seeing as you have dropped this for the moment, let's not
> dwell on it ;)
> 

Agreed.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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