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

[Juan Quintela]

Daniel P. Berrangé <berrange@redhat.com> wrote:
> On Tue, Jul 05, 2022 at 02:56:35PM +0100, Dr. David Alan Gilbert 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 could do with avoiding using them as flags where we dont need to.
>
> Before it is too late, shouldn't we do
>
>    #define RAM_SAVE_FLAG_BIGGER_FLAGS 0x200  
>
> to indicate that this will be followed by another uint64 value
> giving us another 64 flags to play with ?

Dunno.  We can recover 2 bits already as I told on the previous answer.
And another two/three once that we move to multifd, so we should be ok
(famous last words).

Once told that, putting a comment saying what is the biggest possible
value looks like a good idea.

Later, Juan.