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
