On 28.09.2015 22:22, Dr. David Alan Gilbert wrote:
* Denis V. Lunev (d...@openvz.org) wrote:
From: Igor Redko <red...@virtuozzo.com>
To get this estimation we must divide pending_size by bandwidth
according to description of expected-downtime ("qmp-commands.hx:3246"):
"expected-downtime": only present while migration is active
total amount in ms for downtime that was calculated on
the last bitmap round (json-int)
Previous version was just wrong because dirty_bytes_rate and bandwidth
are measured in Bytes/ms, so dividing first by second we get some
dimensionless quantity.
As it said in description above this value is showed during active
migration phase and recalculated only after transferring all memory
and if this process took more than 1 sec. So maybe just nobody noticed
that bug.
While I agree the existing code looks wrong, I don't see how this is
any more correct.
This patch is aimed to fix units of expected_downtime. It is reasonable
that expected_downtime should be measured in milliseconds. While the
existing implementation lacks of any units.
I think 'pending_size' is an estimate of the number of bytes left
to transfer, the intention being that most of those are transferred
prior to pausing the machine, if those are transferred before pausing
then they aren't part of the downtime.
Yes, 'pending_size' is an estimate of the number of bytes left to
transfer, indeed.
But the condition:
> if (s->dirty_bytes_rate && transferred_bytes > 10000) {
slightly modifies the meaning of pending_size correspondingly.
dirty_bytes_rate is set in migration_sync() that is called when
pending_size < max_downtime * bandwidth. This estimation is higher than
max_downtime by design
It feels that:
* If the guest wasn't dirtying pages, then you wouldn't have to
pause the guest; if it was just dirtying them a little then you
wouldn't have much to transfer after the pages you'd already
sent; so if the guest dirty pages fast then the estimate should be
larger; so 'dirty_bytes_rate' being on top of the fraction feels right.
* If the bandwidth is higher then the estimate should be smaller; so
'bandwidth' being on the bottom of the fraction feels right.
Dave
The 'expected_downtime' in the existing code takes two types of values:
* positive - dirty_bytes_rate is higher than bandwidth. In this
case migration doesn't complete.
* zero - bandwidth is higher than dirty_bytes_rate. In this case
migration is possible, but we don’t have the downtime value.
This patch has some imperfections. But if we would look back into
history, it seems that this patch just restores the broken logic.
The existing code is introduced by commit
https://github.com/qemu/qemu/commit/90f8ae724a575861f093fbdbfd49a925bcfec327
which claims, that it just restores the mistakenly deleted estimation
(commit
https://github.com/qemu/qemu/commit/e4ed1541ac9413eac494a03532e34beaf8a7d1c5)
Meanwhile, the estimation has changed during this restore operation. The
estimation before the removal (before
e4ed1541ac9413eac494a03532e34beaf8a7d1c5) was just like the one in my patch.
So maybe we should think about improvement of this estimation.
I'm suggest using something like:
expected_downtime = migrate_max_downtime * dirty_bytes_rate /
bandwidth
In my opinion this is more correct than the existing approach since the
last step of the migration process (before pause) is transferring of
max_size bytes (max_size = bandwidth * migrate_max_downtime() /
1000000). So the bytes that were dirtied at this step will be
transferred during downtime. The transferred bytes count is
dirty_bytes_rate * max_size/bandwidth or migrate_max_downtime *
dirty_bytes_rate and division by the bandwidth results in a formula:
expected_downtime = migrate_max_downtime * dirty_bytes_rate / bandwidth
Igor
Signed-off-by: Igor Redko <red...@virtuozzo.com>
Reviewed-by: Anna Melekhova <an...@virtuozzo.com>
Signed-off-by: Denis V. Lunev <d...@openvz.org>
CC: Juan Quintela <quint...@redhat.com>
CC: Amit Shah <amit.s...@redhat.com>
---
migration/migration.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/migration/migration.c b/migration/migration.c
index 662e77e..d55d545 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -994,7 +994,7 @@ static void *migration_thread(void *opaque)
/* if we haven't sent anything, we don't want to recalculate
10000 is a small enough number for our purposes */
if (s->dirty_bytes_rate && transferred_bytes > 10000) {
- s->expected_downtime = s->dirty_bytes_rate / bandwidth;
+ s->expected_downtime = pending_size / bandwidth;
}
qemu_file_reset_rate_limit(s->file);
--
2.1.4
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
