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

[Michael S. Tsirkin]

On Tue, Apr 19, 2016 at 08:12:02PM +0100, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (address@hidden) 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

Thinking some more about it, we could send the request for page from
desination to guest.  Once we get free pages, we send them to source
with a flag that says "I don't really need these pages".  Could be a new
message with a bitmap, or just multiple existing ones that request
pages.

source marks these pages sent without actually sending them.
if it sees it sent all pages it exits as previously.

> > 
> > -- 
> > MST
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK