Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages

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

On Thu, Mar 24, 2016 at 01:19:40AM +0000, Li, Liang Z wrote:
> > > > > 2. Why not use virtio-balloon
> > > > > Actually, the virtio-balloon can do the similar thing by inflating
> > > > > the balloon before live migration, but its performance is no good,
> > > > > for an 8GB idle guest just boots, it takes about 5.7 Sec to
> > > > > inflate the balloon to 7GB, but it only takes 25ms to get a valid
> > > > > free page bitmap from the guest.  There are some of reasons for
> > > > > the bad performance of
> > > > > vitio-balloon:
> > > > > a. allocating pages (5%, 304ms)
> > > >
> > > > Interesting. This is definitely worth improving in guest kernel.
> > > > Also, will it be faster if we allocate and pass to guest huge pages instead?
> > > > Might speed up madvise as well.
> > >
> > > Maybe.
> > >
> > > > > b. sending PFNs to host (71%, 4194ms)
> > > >
> > > > OK, so we probably should teach balloon to pass huge lists in bitmaps.
> > > > Will be benefitial for regular balloon operation, as well.
> > > >
> > >
> > > Agree. Current balloon just send 256 PFNs a time, that's too few and
> > > lead to too many times of virtio transmission, that's the main reason for the
> > bad performance.
> > > Change the VIRTIO_BALLOON_ARRAY_PFNS_MAX to a large value can
> > improve
> > > the performance significant. Maybe we should increase it before doing
> > > the further optimization, do you think so ?
> > 
> > We could push it up a bit higher: 256 is 1kbyte in size, so we can make it 3x
> > bigger and still fit struct virtio_balloon is a single page. But if we are going to
> > add the bitmap variant anyway, we probably shouldn't bother.
> > 
> > > > > c. address translation and madvise() operation (24%, 1423ms)
> > > >
> > > > How is this split between translation and madvise?  I suspect it's
> > > > mostly madvise since you need translation when using bitmap as well.
> > > > Correct? Could you measure this please?  Also, what if we use the
> > > > new MADV_FREE instead?  By how much would this help?
> > > >
> > > For the current balloon, address translation is needed.
> > > But for live migration, there is no need to do address translation.
> > 
> > Well you need ram address in order to clear the dirty bit.
> > How would you get it without translation?
> > 
> 
> If you means that kind of address translation, yes, it need.
> What I want to say is, filter out the free page can be done by bitmap operation.
> 
> Liang

OK so I see that your patches use block->offset in struct RAMBlock
to look up bits in guest-supplied bitmap.
I don't think that's guaranteed to work.

-- 
MST