Re: [Qemu-devel] [PATCH] migration: use XBZRLE only after bulk stage

[Peter Lieven]

On 01.03.2013 14:52, Eric Blake wrote:
> On 03/01/2013 06:31 AM, Peter Lieven wrote:
> > at the beginning of migration all pages are marked dirty and
> > in the first round a bulk migration of all pages is performed.
> >
> > currently all these pages are copied to the page cache regardless
> > if there are frequently updated or not. this doesn't make sense
> > since most of these pages are never transferred again.
> >
> > this patch changes the XBZRLE transfer to only be used after
> > the bulk stage has been completed. that means a page is added
> > to the page cache the second time it is transferred and XBZRLE
> > can benefit from the third time of transfer.
> >
> > since the page cache is likely smaller than the number of pages
> > its also likely that in the second round the page is missing in the
> > cache due to collisions in the bulk phase.
> >
> > on the other hand a lot of unneccssary mallocs, memdups and frees
>
> s/unneccssary/unnecessary/
>
> > are saved.
> >
> > Signed-off-by: Peter Lieven <address@hidden>
>
> Do you have any benchmark numbers?  At any rate, the explanation seems
> sound, so a benchmark should show this.

Do you have a particular test pattern in mind? If there is nothing going on
in the VM XBZRLE will not be better than normal copy at all.

Otherwise you will have N xbzrle misses and 0 xbzrle pages without the patch
and 0 xbzrle misses and 0 xbzrle pages with the patch.

Peter

> > ---
> >   arch_init.c |    5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
>
> Reviewed-by: Eric Blake <address@hidden>