Re: [PATCH 5/5] multifd: Only sync once each full round of memory

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Tue, Jul 05, 2022 at 05:11:46PM +0200, Juan Quintela wrote:
> > "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
> > > * Juan Quintela (quintela@redhat.com) wrote:
> > >> We need to add a new flag to mean to sync at that point.
> > >> Notice that we still synchronize at the end of setup and at the end of
> > >> complete stages.
> > >> 
> > >> Signed-off-by: Juan Quintela <quintela@redhat.com>
> > >> ---
> > >>  migration/migration.c |  2 +-
> > >>  migration/ram.c       | 42 ++++++++++++++++++++++++++++++------------
> > >>  2 files changed, 31 insertions(+), 13 deletions(-)
> > >> 
> > >> diff --git a/migration/migration.c b/migration/migration.c
> > >> index 3f79df0b70..6627787fc2 100644
> > >> --- a/migration/migration.c
> > >> +++ b/migration/migration.c
> > >> @@ -4283,7 +4283,7 @@ static Property migration_properties[] = {
> > >>                        DEFAULT_MIGRATE_ANNOUNCE_STEP),
> > >>      /* We will change to false when we introduce the new mechanism */
> > >>      DEFINE_PROP_BOOL("multifd-sync-each-iteration", MigrationState,
> > >> -                      multifd_sync_each_iteration, true),
> > >> +                      multifd_sync_each_iteration, false),
> > >>  
> > >>      /* Migration capabilities */
> > >>      DEFINE_PROP_MIG_CAP("x-xbzrle", MIGRATION_CAPABILITY_XBZRLE),
> > >> diff --git a/migration/ram.c b/migration/ram.c
> > >> index 2c7289edad..6792986565 100644
> > >> --- a/migration/ram.c
> > >> +++ b/migration/ram.c
> > >> @@ -81,6 +81,7 @@
> > >>  #define RAM_SAVE_FLAG_XBZRLE   0x40
> > >>  /* 0x80 is reserved in migration.h start with 0x100 next */
> > >>  #define RAM_SAVE_FLAG_COMPRESS_PAGE    0x100
> > >> +#define RAM_SAVE_FLAG_MULTIFD_SYNC     0x200
> > >
> > > Note this is the very last usable flag!
> > 
> > We can recover two flags right now:
> > 
> > RAM_SAVE_FLAG_FULL is not used anymore.
> > 0x80 is free since years ago.
> > 
> > Once multifd is default, there are some other that could go.

I have suggested that a few times in the past.

> Non-multifd migration isn't likely to go away any time soon, given
> distros desire to support migration between QEMU's with quite
> significantly different versions. So feels like quite a long time
> before we might reclaim more flags.
> 
> > > We could do with avoiding using them as flags where we dont need to.
> > 
> > I can't really think on another way to do it.  The other thing that I
> > can do is just reuse one of the flags that don't make sense for multifd
> > (RAM_SAVE_FLAG_ZERO after zero pages patch,
> > RAM_SAVE_FLAG_XBZRLE/COMPRESS_PAGE).
> 
> Re-using flags based on use context differences feels like a recipe
> to confuse people.

Note that most of these things aren't really 'flags'; in the sense that
only a few of them are actually combinable; so we should start using
combinations to mean things new.

Dave

> With regards,
> Daniel
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK