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

[Li, Liang Z]

> > > > 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