Re: [Qemu-devel] migration: broken ram_save_pending

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

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> Il 05/02/2014 10:09, Dr. David Alan Gilbert ha scritto:
> >I think the case Alexey is hitting is:
> >   1 A few dirtied pages
> >   2 but because of the hpratio most of the data is actually zero
> >     - indeed most of the target-page sized chunks are zero
> >   3 Thus the data compresses very heavily
> >   4 When the bandwidth/delay calculation happens it's spent a reasonable
> >     amount of time transferring a reasonable amount of pages but not
> >     actually many bytes on the wire, so the estimate of the available
> >     bandwidth available is lower than reality.
> >   5 The max-downtime calculation is a comparison of pending-dirty uncompressed
> >     bytes with compressed bandwidth
> >
> >(5) is bound to fail if the compression ratio is particularly high, which
> >because of the hpratio it is if we're just dirtying one word in an entire
> >host page.
> 
> So far so good, but why isn't pending-dirty (aka
> migration_dirty_pages in the code) zero?

Because:
    * the code is still running and keeps redirtying a small handful of pages
    * but because we've underestimated our available bandwidth we never stop
      it and just throw those pages across immediately

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