Re: [RFC PATCH v3 19/30] migration/ram: Ignore multifd flush when doing fixed-ram migration

[Peter Xu <peterx@redhat.com>]

On Wed, Jan 17, 2024 at 03:13:20PM -0300, Fabiano Rosas wrote:
> >> @@ -3242,8 +3243,11 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
> >>  out:
> >>      if (ret >= 0
> >>          && migration_is_setup_or_active(migrate_get_current()->state)) {
> >> -        if (migrate_multifd() && migrate_multifd_flush_after_each_section()) {
> >> -            ret = multifd_send_sync_main(rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
> >> +        if (migrate_multifd() &&
> >> +            (migrate_multifd_flush_after_each_section() ||
> >> +             migrate_fixed_ram())) {
> >> +            ret = multifd_send_sync_main(
> >> +                rs->pss[RAM_CHANNEL_PRECOPY].pss_channel);
> >
> > Why you want this one?  ram_save_iterate() can be called tens for each
> > second iiuc.
> >
> 
> AIUI, this is a requirement for live migration, so that we're not
> sending the new version of the page while the old version is still in
> transit.
> 
> > There's one more?  ram_save_complete():
> >
> >     if (migrate_multifd() && !migrate_multifd_flush_after_each_section()) {
> >         qemu_put_be64(f, RAM_SAVE_FLAG_MULTIFD_FLUSH);
> >     }
> >
> > IIUC that's the one you referred to at [1] above, not sure why you modified
> > the code in ram_save_iterate() instead.
> >
> 
> I mentioned it in the commit message as well:
> 
> " - between iterations, to avoid a slow channel being overrun by a fast
>  channel in the subsequent iteration;"

IMHO you only need to flush all threads in find_dirty_block(). That's when
the "real iteration" happens (IOW, when the "real iteration" is defined to
be a full walk across all guest memories, rather than one single call to
ram_save_iterate()).

Multifd used to do too many flushes, and that's why we had the new
migrate_multifd_flush_after_each_section() and it's a bit of a mess if you
see..

To be super safe, you can also flush at ram_save_complete(), but I doubt
its necessity, and this same question applies to generic multifd: the
multifd_send_sync_main() in the ram_save_complete() can be redundant,
afaiu.  However we can leave that for later to not add even more dependency
to fixed-ram.  If that is justified, we can remove the sync_main for both
socket / file then.

-- 
Peter Xu