linux-kernel - Re: [PATCH 31/35] Optimistically check the first page on the PCP free list is suitable

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Message-ID: <20090316165220.GO24293@csn.ul.ie>
Date:	Mon, 16 Mar 2009 16:52:20 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Christoph Lameter <cl@...ux-foundation.org>
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>,
	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>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 31/35] Optimistically check the first page on the PCP
	free list is suitable

On Mon, Mar 16, 2009 at 12:33:44PM -0400, Christoph Lameter wrote:
> On Mon, 16 Mar 2009, Mel Gorman wrote:
> 
> > diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> > index bb5bd5e..8568284 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -1212,6 +1212,12 @@ again:
> >  				if (pcp_page_suit(page, order))
> >  					break;
> >  		} else {
> > +			/* Optimistic before we start a list search */
> > +			page = list_entry(list->next, struct page, lru);
> > +			if (pcp_page_suit(page, order))
> > +				goto found;
> > +
> > +			/* Do the search */
> >  			list_for_each_entry(page, list, lru)
> >  				if (pcp_page_suit(page, order))
> >  					break;
> 
> I am not convinced that this is beneficial. If it would then the compiler
> would unroll the loop.
> 

It hit a large number of times when I checked (although I don't have the
figures any more) and the list_for_each_entry() does a prefetch
possibly fetching in a line we don't need.

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