linux-kernel - Re: [PATCH 1/3] page-allocator: Split per-cpu list into one-list-per-migrate-type

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Message-ID: <20090818131024.GD31469@csn.ul.ie>
Date:	Tue, 18 Aug 2009 14:10:24 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] page-allocator: Split per-cpu list into
	one-list-per-migrate-type

On Tue, Aug 18, 2009 at 01:43:35PM +0200, Nick Piggin wrote:
> On Tue, Aug 18, 2009 at 12:16:00PM +0100, Mel Gorman wrote:
> > Currently the per-cpu page allocator searches the PCP list for pages of the
> > correct migrate-type to reduce the possibility of pages being inappropriate
> > placed from a fragmentation perspective. This search is potentially expensive
> > in a fast-path and undesirable. Splitting the per-cpu list into multiple
> > lists increases the size of a per-cpu structure and this was potentially
> > a major problem at the time the search was introduced. These problem has
> > been mitigated as now only the necessary number of structures is allocated
> > for the running system.
> > 
> > This patch replaces a list search in the per-cpu allocator with one list per
> > migrate type. The potential snag with this approach is when bulk freeing
> > pages. We round-robin free pages based on migrate type which has little
> > bearing on the cache hotness of the page and potentially checks empty lists
> > repeatedly in the event the majority of PCP pages are of one type.
> 
> Seems OK I guess. Trading off icache and branches for dcache and
> algorithmic gains. Too bad everything is always a tradeoff ;)
> 

Tell me about it. The dcache overhead of this is a problem although I
tried to limit the damage using pahole to see how much padding I had to
play with and staying within it where possible.

> But no I think this is a good idea.
> 

Thanks. Is that an Ack?

> > <SNIP>

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