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

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Date:	Wed, 10 Jul 2013 11:17:03 +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 09:31:42, Joonsoo Kim wrote:
> On Thu, Jul 04, 2013 at 12:00:44PM +0200, Michal Hocko wrote:
> > On Thu 04-07-13 13:24:50, Joonsoo Kim wrote:
> > > On Thu, Jul 04, 2013 at 12:01:43AM +0800, Zhang Yanfei wrote:
> > > > On 07/03/2013 11:51 PM, Zhang Yanfei wrote:
> > > > > On 07/03/2013 11:28 PM, Michal Hocko wrote:
> > > > >> On Wed 03-07-13 17:34:15, Joonsoo Kim wrote:
> > > > >> [...]
> > > > >>> For one page allocation at once, this patchset makes allocator slower than
> > > > >>> before (-5%). 
> > > > >>
> > > > >> Slowing down the most used path is a no-go. Where does this slow down
> > > > >> come from?
> > > > > 
> > > > > I guess, it might be: for one page allocation at once, comparing to the original
> > > > > code, this patch adds two parameters nr_pages and pages and will do extra checks
> > > > > for the parameter nr_pages in the allocation path.
> > > > > 
> > > > 
> > > > If so, adding a separate path for the multiple allocations seems better.
> > > 
> > > Hello, all.
> > > 
> > > I modify the code for optimizing one page allocation via likely macro.
> > > I attach a new one at the end of this mail.
> > > 
> > > In this case, performance degradation for one page allocation at once is -2.5%.
> > > I guess, remained overhead comes from two added parameters.
> > > Is it unreasonable cost to support this new feature?
> > 
> > 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.
-- 
Michal Hocko
SUSE Labs
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