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

From: Li, Liang Z
Subject: Re: [Qemu-devel] [RFC Design Doc]Speed up live migration by skipping free pages
Date: Mon, 18 Apr 2016 14:36:31 +0000

> 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 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?


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