[Qemu-devel] [PATCH 1/1] migration: calculate expected_downtime considering redirtied ram

[bala24@linux.vnet.ibm.com]

From: Balamuruhan S <bala24@linux.vnet.ibm.com>

currently we calculate expected_downtime by time taken to transfer
remaining ram, but during the time we had transferred remaining ram
few pages of ram might be redirtied and we need to retransfer it,
so it is better to consider them for calculating expected_downtime
for getting more accurate values.

Total ram to be transferred = remaining ram + (redirtied ram at the
                                               time when the remaining
                                               ram gets transferred)

redirtied ram = dirty_pages_rate * time taken to transfer remaining ram

redirtied ram = dirty_pages_rate * (remaining ram / bandwidth)

expected_downtime = (remaining ram + redirtied ram) / bandwidth

Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Suggested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
---
 migration/migration.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/migration/migration.c b/migration/migration.c
index ffc4d9e556..dc38e9a380 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2903,7 +2903,13 @@ static void migration_update_counters(MigrationState *s,
      * recalculate. 10000 is a small enough number for our purposes
      */
     if (ram_counters.dirty_pages_rate && transferred > 10000) {
-        s->expected_downtime = ram_counters.remaining / bandwidth;
+        /* Time required to transfer remaining ram */
+        remaining_ram_transfer_time = ram_counters.remaining / bandwidth
+
+        /* redirty of ram at the time remaining ram gets transferred*/
+        newly_dirtied_ram = ram_counters.dirty_pages_rate * remaining_ram_transfer_time
+
+        s->expected_downtime = (ram_counters.remaining + newly_dirtied_ram) / bandwidth;
     }
 
     qemu_file_reset_rate_limit(s->to_dst_file);
-- 
2.14.5