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

[Daniel P . Berrangé]

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.

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.

With regards,
Daniel
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