linux-kernel - Re: [PATCH 23/35] Update NR_FREE

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Date:	Mon, 16 Mar 2009 16:42:39 +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 23/35] Update NR_FREE_PAGES only as necessary

On Mon, Mar 16, 2009 at 12:17:15PM -0400, Christoph Lameter wrote:
> On Mon, 16 Mar 2009, Mel Gorman wrote:
> 
> > When pages are being freed to the buddy allocator, the zone
> > NR_FREE_PAGES counter must be updated. In the case of bulk per-cpu page
> > freeing, it's updated once per page. This retouches cache lines more
> > than necessary. Update the counters one per per-cpu bulk free.
> 
> Not sure about the reasoning here since the individual updates are batched

Each update take places between lots of other work with different cache
lines. With enough buddy merging, I believed the line holding the counters
could at least get pushed out of L1 although probably not L2 cache.

> and you are touching the same cacheline as the pcp you are operating on
> and have to touch anyways.
> 
> But if its frequent that __rmqueue_smallest() and free_pages_bulk() are
> called with multiple pages then its always a win.
> 

It's frequent enough that it showed up in profiles

> Reviewed-by: Christoph Lameter <cl@...ux-foundation.org>
> 
> > +	__mod_zone_page_state(zone, NR_FREE_PAGES, -(1UL << order) * i);
> 
> A multiplication? Okay with contemporary cpus I guess.
> 

Replaced with

__mod_zone_page_state(zone, NR_FREE_PAGES, -(i << order));

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