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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Roman Kagan (rkagan@virtuozzo.com) wrote:
> On Fri, Mar 04, 2016 at 08:23:09AM +0000, Li, Liang Z wrote:
> > > On Thu, Mar 03, 2016 at 05:46:15PM +0000, Dr. David Alan Gilbert wrote:
> > > > * Liang Li (liang.z.li@intel.com) wrote:
> > > > > The current QEMU live migration implementation mark the all the
> > > > > guest's RAM pages as dirtied in the ram bulk stage, all these pages
> > > > > will be processed and that takes quit a lot of CPU cycles.
> > > > >
> > > > > From guest's point of view, it doesn't care about the content in
> > > > > free pages. We can make use of this fact and skip processing the
> > > > > free pages in the ram bulk stage, it can save a lot CPU cycles and
> > > > > reduce the network traffic significantly while speed up the live
> > > > > migration process obviously.
> > > > >
> > > > > This patch set is the QEMU side implementation.
> > > > >
> > > > > The virtio-balloon is extended so that QEMU can get the free pages
> > > > > information from the guest through virtio.
> > > > >
> > > > > After getting the free pages information (a bitmap), QEMU can use it
> > > > > to filter out the guest's free pages in the ram bulk stage. This
> > > > > make the live migration process much more efficient.
> > > >
> > > > Hi,
> > > >   An interesting solution; I know a few different people have been
> > > > looking at how to speed up ballooned VM migration.
> > > >
> > > >   I wonder if it would be possible to avoid the kernel changes by
> > > > parsing /proc/self/pagemap - if that can be used to detect
> > > > unmapped/zero mapped pages in the guest ram, would it achieve the
> > > same result?
> > > 
> > > Yes I was about to suggest the same thing: it's simple and makes use of the
> > > existing infrastructure.  And you wouldn't need to care if the pages were
> > > unmapped by ballooning or anything else (alternative balloon
> > > implementations, not yet touched by the guest, etc.).  Besides, you wouldn't
> > > need to synchronize with the guest.
> > > 
> > > Roman.
> > 
> > The unmapped/zero mapped pages can be detected by parsing /proc/self/pagemap,
> > but the free pages can't be detected by this. Imaging an application allocates a large amount
> > of memory , after using, it frees the memory, then live migration happens. All these free pages
> > will be process and sent to the destination, it's not optimal.
> 
> First, the likelihood of such a situation is marginal, there's no point
> optimizing for it specifically.
> 
> And second, even if that happens, you inflate the balloon right before
> the migration and the free memory will get umapped very quickly, so this
> case is covered nicely by the same technique that works for more
> realistic cases, too.

Although I wonder which is cheaper; that would be fairly expensive for
the guest wouldn't it? And you'd somehow have to kick the guest
before migration to do the ballooning - and how long would you wait
for it to finish?

Dave

> 
> Roman.
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK