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

[Li, Liang Z]

> > > > 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.
> > >
> > Hi MST,
> >
> > I think you mean  putting the free page bitmap in a ramblock. Right?
> > If some of the free pages become dirty after updating the free page
> > bitmap, and these pages are discarded by destination, how can we
> > distinguish these discarded pages with the free pages?
> >
> > Could you elaborate how it works?
> >
> > Thanks!
> > Liang
> 
> Maybe I'm confused - IIUC it's postcopy so VM is running on destination, if
> page is dirty it was modified there so we don't need to get it from source.
> 

The current post-copy is implemented as pre-copy plus post-copy, there is a 
stage
 that the source told the destination to discard the newly dirty pages before 
vm start
to run on destination. I don't think the free page bitmap will help, we need a 
dirty page
bitmap. The dirty page bitmap should not be put in a ramblock, it does not 
belong to guest.

> But really I agree with David here - at step 1 just ignore post-copy, don't
> special-case it, even if it becomes slower with your patch.
> Think about it later.
> 

Yes, we can simply ignore the free page if post-copy is enabled.  Think about 
it now
will make my work easier :)

Thanks!

Liang

> >
> >
> > > --
> > > MST
> > > --