Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization

From: Roman Kagan
Date: Wed Mar 09 2016 - 09:29:37 EST

On Mon, Mar 07, 2016 at 01:40:06PM +0200, Michael S. Tsirkin wrote:
> On Mon, Mar 07, 2016 at 06:49:19AM +0000, Li, Liang Z wrote:
> > > > No. And it's exactly what I mean. The ballooned memory is still
> > > > processed during live migration without skipping. The live migration code is
> > > in migration/ram.c.
> > >
> > > So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST, we can
> > > teach qemu to skip these pages.
> > > Want to write a patch to do this?
> > >
> >
> > Yes, we really can teach qemu to skip these pages and it's not hard.
> > The problem is the poor performance, this PV solution
> Balloon is always PV. And do not call patches solutions please.
> > is aimed to make it more
> > efficient and reduce the performance impact on guest.
> We need to get a bit beyond this. You are making multiple
> changes, it seems to make sense to split it all up, and analyse each
> change separately.

Couldn't agree more.

There are three stages in this optimization:

1) choosing which pages to skip

2) communicating them from guest to host

3) skip transferring uninteresting pages to the remote side on migration

For (3) there seems to be a low-hanging fruit to amend
migration/ram.c:iz_zero_range() to consult /proc/self/pagemap. This
would work for guest RAM that hasn't been touched yet or which has been
ballooned out.

For (1) I've been trying to make a point that skipping clean pages is
much more likely to result in noticable benefit than free pages only.

As for (2), we do seem to have a problem with the existing balloon:
according to your measurements it's very slow; besides, I guess it plays
badly with transparent huge pages (as both the guest and the host work
with one 4k page at a time). This is a problem for other use cases of
balloon (e.g. as a facility for resource management); tackling that
appears a more natural application for optimization efforts.