On 08/26/2012 03:58 AM, Michael S. Tsirkin wrote:

On Sat, Aug 25, 2012 at 02:24:55AM -0300, Rafael Aquini wrote:

quoted

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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