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

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Date:	Thu, 4 Jul 2013 12:00:44 +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 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 think that readahead path is one of the most used path, so this penalty looks
> endurable. And after supporting this feature, we can find more use cases.

What about page faults? I would oppose that page faults are _much_ more
frequent than read ahead so you really cannot slow them down.

[...]
-- 
Michal Hocko
SUSE Labs
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