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

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Date:	Thu, 11 Jul 2013 10:02:48 +0900
From:	Joonsoo Kim <iamjoonsoo.kim@....com>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	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 Wed, Jul 10, 2013 at 03:52:42PM -0700, Dave Hansen wrote:
> On 07/03/2013 01:34 AM, Joonsoo Kim wrote:
> > -		if (page)
> > +		do {
> > +			page = buffered_rmqueue(preferred_zone, zone, order,
> > +							gfp_mask, migratetype);
> > +			if (!page)
> > +				break;
> > +
> > +			if (!nr_pages) {
> > +				count++;
> > +				break;
> > +			}
> > +
> > +			pages[count++] = page;
> > +			if (count >= *nr_pages)
> > +				break;
> > +
> > +			mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
> > +			if (!zone_watermark_ok(zone, order, mark,
> > +					classzone_idx, alloc_flags))
> > +				break;
> > +		} while (1);
> 
> I'm really surprised this works as well as it does.  Calling
> buffered_rmqueue() a bunch of times enables/disables interrupts a bunch
> of times, and mucks with the percpu pages lists a whole bunch.
> buffered_rmqueue() is really meant for _single_ pages, not to be called
> a bunch of times in a row.
> 
> Why not just do a single rmqueue_bulk() call?

Hello, Dave.

There are some reasons why I implement the feature in this way.

rmqueue_bulk() needs a zone lock. If we allocate not so many pages,
for example, 2 or 3 pages, it can have much more overhead rather than
allocationg 1 page multiple times. So, IMHO, it is better that
multiple pages allocation is supported on top of percpu pages list.

And I think that enables/disables interrupts a bunch of times help
to avoid a latency problem. If we disable interrupts until the whole works
is finished, interrupts can be handled too lately.
free_hot_cold_page_list() already do enables/disalbed interrupts a bunch of
times.

Thanks for helpful comment!

> 
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