migration/colo.c: Use event instead of semaphore

If multiple packets miscompare in a short timeframe, the semaphore
value will be increased multiple times. This causes multiple
checkpoints even if one would be sufficient.

Fix this by using a event instead of a semaphore for triggering
checkpoints. Now, checkpoint requests will be ignored until the
checkpoint event is sent to colo-compare (which releases the
miscompared packets).

Benchmark results (iperf3):
Client-to-server tcp:
without patch: ~66 Mbit/s
with patch: ~61 Mbit/s
Server-to-client tcp:
without patch: ~702 Kbit/s
with patch: ~16 Mbit/s

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Message-Id: <fd601ba1beb524aada54ba66e87ebfc12cf4574b.1589193382.git.lukasstraub2@web.de>
Reviewed-by: zhanghailiang <zhang.zhanghailiang@huawei.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/migration/migration.h b/migration/migration.h
index 507284e563..f617960522 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -215,8 +215,8 @@ struct MigrationState
     /* The semaphore is used to notify COLO thread that failover is finished */
     QemuSemaphore colo_exit_sem;
 
-    /* The semaphore is used to notify COLO thread to do checkpoint */
-    QemuSemaphore colo_checkpoint_sem;
+    /* The event is used to notify COLO thread to do checkpoint */
+    QemuEvent colo_checkpoint_event;
     int64_t colo_checkpoint_time;
     QEMUTimer *colo_delay_timer;