linux-kernel - Re: [RFC PATCH 1/5] mm, page_alloc: support multiple pages allocation

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Message-ID: <20130716003754.GB2430@lge.com>
Date:	Tue, 16 Jul 2013 09:37:54 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Dave Hansen <dave@...1.net>
Cc:	Dave Hansen <dave.hansen@...el.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 1/5] mm, page_alloc: support multiple pages allocation

On Fri, Jul 12, 2013 at 09:31:42AM -0700, Dave Hansen wrote:
> On 07/10/2013 11:12 PM, Joonsoo Kim wrote:
> > On Wed, Jul 10, 2013 at 10:38:20PM -0700, Dave Hansen wrote:
> >> You're probably right for small numbers of pages.  But, if we're talking
> >> about things that are more than, say, 100 pages (isn't the pcp batch
> >> size clamped to 128 4k pages?) you surely don't want to be doing
> >> buffered_rmqueue().
> > 
> > Yes, you are right.
> > Firstly, I thought that I can use this for readahead. On my machine,
> > readahead reads (maximum) 32 pages in advance if faulted. And batch size
> > of percpu pages list is close to or larger than 32 pages
> > on today's machine. So I didn't consider more than 32 pages before.
> > But to cope with a request for more pages, using rmqueue_bulk() is
> > a right way. How about using rmqueue_bulk() conditionally?
> 
> How about you test it both ways and see what is faster?

It is not easy to test which one is better, because a difference may be
appeared on certain circumstances only. Do not grab the global lock
as much as possible is preferable approach to me.

> 
> > Hmm, rmqueue_bulk() doesn't stop until all requested pages are allocated.
> > If we request too many pages (1024 pages or more), interrupt latency can
> > be a problem.
> 
> OK, so only call it for the number of pages you believe allows it to
> have acceptable interrupt latency.  If you want 200 pages, and you can
> only disable interrupts for 100 pages, then just do it in two batches.
> 
> The point is that you want to avoid messing with the buffering by the
> percpu structures.  They're just overhead in your case.

Okay.

Thanks.

> 
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