Re: [Qemu-devel] migration: broken ram_save_pending

[Paolo Bonzini <pbonzini@redhat.com>]

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?

Paolo