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

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

* Michael S. Tsirkin (mst@redhat.com) wrote:
> On Mon, Apr 18, 2016 at 11:08:31AM +0000, Li, Liang Z wrote:
> > Hi Dave,
> > 
> > I am now working on how to benefit post-copy by skipping the free pages, 
> > and I remember you have said we should let the destination know the info
> > of free pages so as to avoid request the free pages from the source. 
> > 
> > We have two solutions:
> > 
> > a. send the migration dirty page bitmap to destination before post
> > copy start, so the destination can decide whether to request the pages or 
> > place zero pages by checking the migration dirty page bitmap. The advantage
> > is that we can avoid sending the free pages. the disadvantage is that we have 
> > to send extra data to destination.
> > 
> > b. Check the page request on the source side, if it's not a dirty page, send a zero
> > page header to the destination.
> > 
> > What's your opinion about them?
> > 
> > Liang
> > 
> 
> Both are ad-hoc solutions imho.
> 
> c. put the bitmap in a ramblock, check it on destination before
>    requesting pages.
> 
> This way it's migrated on-demand.

I can see where you're coming from, but I don't like this idea, because
sending data controlling the RAM migration process in RAM blocks controlled
by the same data just sounds too recursive to ever debug.

Dave

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