diff mbox

[v5,38/45] Don't sync dirty bitmaps in postcopy

Message ID 1424883128-9841-39-git-send-email-dgilbert@redhat.com
State New
Headers show

Commit Message

Dr. David Alan Gilbert Feb. 25, 2015, 4:52 p.m. UTC 
From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Once we're in postcopy the source processors are stopped and memory
shouldn't change any more, so there's no need to look at the dirty
map.

There are two notes to this:
  1) If we do resync and a page had changed then the page would get
     sent again, which the destination wouldn't allow (since it might
     have also modified the page)
  2) Before disabling this I'd seen very rare cases where a page had been
     marked dirtied although the memory contents are apparently identical

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 arch_init.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

Comments

David Gibson March 24, 2015, 4:58 a.m. UTC | #1 
On Wed, Feb 25, 2015 at 04:52:01PM +0000, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Once we're in postcopy the source processors are stopped and memory
> shouldn't change any more, so there's no need to look at the dirty
> map.
> 
> There are two notes to this:
>   1) If we do resync and a page had changed then the page would get
>      sent again, which the destination wouldn't allow (since it might
>      have also modified the page)

I'm not quite sure what you mean by "resync" in this context.

>   2) Before disabling this I'd seen very rare cases where a page had been
>      marked dirtied although the memory contents are apparently identical
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Dr. David Alan Gilbert March 24, 2015, 9:05 a.m. UTC | #2 
* David Gibson (david@gibson.dropbear.id.au) wrote:
> On Wed, Feb 25, 2015 at 04:52:01PM +0000, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Once we're in postcopy the source processors are stopped and memory
> > shouldn't change any more, so there's no need to look at the dirty
> > map.
> > 
> > There are two notes to this:
> >   1) If we do resync and a page had changed then the page would get
> >      sent again, which the destination wouldn't allow (since it might
> >      have also modified the page)
> 
> I'm not quite sure what you mean by "resync" in this context.

I mean synchronise the raw dirty bitmap with the migration bitmap,
which we normally do at every cycle through migration.

> >   2) Before disabling this I'd seen very rare cases where a page had been
> >      marked dirtied although the memory contents are apparently identical
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

Thanks,

Dave

> 
> -- 
> David Gibson			| I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
> 				| _way_ _around_!
> http://www.ozlabs.org/~dgibson


--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
diff mbox

Patch

diff --git a/arch_init.c b/arch_init.c
index 7a1c9ea..9d8ca95 100644
--- a/arch_init.c
+++ b/arch_init.c
@@ -1398,7 +1398,10 @@  static int ram_save_iterate(QEMUFile *f, void *opaque)
 static int ram_save_complete(QEMUFile *f, void *opaque)
 {
     qemu_mutex_lock_ramlist();
-    migration_bitmap_sync();
+
+    if (!migration_postcopy_phase(migrate_get_current())) {
+        migration_bitmap_sync();
+    }
 
     ram_control_before_iterate(f, RAM_CONTROL_FINISH);
 
@@ -1433,7 +1436,8 @@  static void ram_save_pending(QEMUFile *f, void *opaque, uint64_t max_size,
 
     remaining_size = ram_save_remaining() * TARGET_PAGE_SIZE;
 
-    if (remaining_size < max_size) {
+    if (!migration_postcopy_phase(migrate_get_current()) &&
+        remaining_size < max_size) {
         qemu_mutex_lock_iothread();
         migration_bitmap_sync();
         qemu_mutex_unlock_iothread();