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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 09, 2016 at 05:28:54PM +0300, Roman Kagan wrote:
> 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.

I guess when you say clean you mean zero?

Yea. In fact, one can zero out any number of pages
quickly by putting them in balloon and immediately
taking them out.

Access will fault a zero page in, then COW kicks in.

We could have a new zero VQ (or some other option)
to pass these pages guest to host, but this only
works well if page size matches the host page size.




> 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.
> 
> Thanks,
> Roman.