linux-kernel - Re: [PATCH v9 0/5] make balloon pages movable by compaction

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Date:	Sun, 26 Aug 2012 10:40:37 -0400
From:	Rik van Riel <riel@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mel Gorman <mel@....ul.ie>, Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v9 0/5] make balloon pages movable by compaction

On 08/26/2012 03:58 AM, Michael S. Tsirkin wrote:
> On Sat, Aug 25, 2012 at 02:24:55AM -0300, Rafael Aquini wrote:
>> Memory fragmentation introduced by ballooning might reduce significantly
>> the number of 2MB contiguous memory blocks that can be used within a guest,
>> thus imposing performance penalties associated with the reduced number of
>> transparent huge pages that could be used by the guest workload.
>>
>> This patch-set follows the main idea discussed at 2012 LSFMMS session:
>> "Ballooning for transparent huge pages" -- http://lwn.net/Articles/490114/
>> to introduce the required changes to the virtio_balloon driver, as well as
>> the changes to the core compaction & migration bits, in order to make those
>> subsystems aware of ballooned pages and allow memory balloon pages become
>> movable within a guest, thus avoiding the aforementioned fragmentation issue
>
> Meta-question: are there any numbers showing gain from this patchset?
>
> The reason I ask, on migration we notify host about each page
> individually.  If this is rare maybe the patchset does not help much.
> If this is common we would be better off building up a list of multiple
> pages and passing them in one go.

The gain is in getting a better THP allocation rate inside the
guest, allowing applications to run faster.

The rarer it is for this code to run, the better - it means we
are getting the benefits without the overhead :)

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