linux-kernel - Re: [RFC PATCH 0/5] Support multiple pages allocation

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Date:	Wed, 10 Jul 2013 13:27:37 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Zhang Yanfei <zhangyanfei.yes@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>,
	David Rientjes <rientjes@...gle.com>,
	Glauber Costa <glommer@...il.com>,
	Johannes Weiner <hannes@...xchg.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Rik van Riel <riel@...hat.com>,
	Hugh Dickins <hughd@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	Jiang Liu <jiang.liu@...wei.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/5] Support multiple pages allocation

On Wed 10-07-13 18:55:33, Joonsoo Kim wrote:
> On Wed, Jul 10, 2013 at 11:17:03AM +0200, Michal Hocko wrote:
> > On Wed 10-07-13 09:31:42, Joonsoo Kim wrote:
> > > On Thu, Jul 04, 2013 at 12:00:44PM +0200, Michal Hocko wrote:
[...]
> > > > Which benchmark you are using for this testing?
> > > 
> > > I use my own module which do allocation repeatedly.
> > 
> > I am not sure this microbenchmark will tell us much. Allocations are
> > usually not short lived so the longer time might get amortized.
> > If you want to use the multi page allocation for read ahead then try to
> > model your numbers on read-ahead workloads.
> 
> Of couse. In later, I will get the result on read-ahead workloads or
> vmalloc workload which is recommended by Zhang.
> 
> I think, without this microbenchmark, we cannot know this modification's
> performance effect to single page allocation accurately. Because the impact
> to single page allocation is relatively small and it is easily hidden by
> other factors.

The main thing is whether the numbers you get from an artificial
microbenchmark matter at all. You might see a regression which cannot be
hit in practice because other effects are of magnitude more significant.
-- 
Michal Hocko
SUSE Labs
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