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Re: [PATCH 5/5] multifd: Only sync once each full round of memory
From: |
Dr. David Alan Gilbert |
Subject: |
Re: [PATCH 5/5] multifd: Only sync once each full round of memory |
Date: |
Tue, 5 Jul 2022 18:13:40 +0100 |
User-agent: |
Mutt/2.2.6 (2022-06-05) |
* 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