* Paolo Bonzini ([email protected]) 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 / [email protected] / Manchester, UK
