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

[Rik van Riel]

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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