Re: [PATCH v6 3/3] multifd: Only flush once each full round of memory

[Peter Xu]

On Thu, Feb 16, 2023 at 12:00:55PM +0100, Juan Quintela wrote:
> >> @@ -4169,7 +4190,9 @@ int ram_load_postcopy(QEMUFile *f, int channel)
> >>              }
> >>              decompress_data_with_multi_threads(f, page_buffer, len);
> >>              break;
> >> -
> >> +        case RAM_SAVE_FLAG_MULTIFD_FLUSH:
> >> +            multifd_recv_sync_main();
> >> +            break;
> >>          case RAM_SAVE_FLAG_EOS:
> >>              /* normal exit */
> >>              if (migrate_multifd_flush_after_each_section()) {
> >
> > We could have dropped RAM_SAVE_FLAG_MULTIFD_FLUSH and RAM_SAVE_FLAG_EOS for
> > now until we support postcopy+multifd.
> 
> I don't think so.
> 
> We have this curse of biblic proportions called Backwards compatibility.
> 
> We need to mark the beggining and end of sections.  That is independent
> of multifd.
> And for multifd we have to flush all channels at the end of each
> iteration through RAM.  We could do that without involving the main
> thread, but I don't see the point of doing that.

Oops, sorry I didn't mean to drop the flags RAM_SAVE_FLAG_EOS itself, but
the calls to multifd_recv_sync_main().

RAM_SAVE_FLAG_MULTIFD_FLUSH as a whole can be dropped.

Thanks,

-- 
Peter Xu