When VM is configured with huge memory, the current throttle logic
doesn't look like to scale, because migration_trigger_throttle()
is only called for each iteration, so it won't be invoked for a long
time if one iteration can take a long time.

The periodic sync and throttle aims to fix the above issue by
synchronizing the remote dirty bitmap and triggering the throttle
periodically. This is a trade-off between synchronization overhead
and CPU throttle impact.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
---
 migration/migration.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index 055d527ff6..fefd93b683 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1420,6 +1420,9 @@ static void migrate_fd_cleanup(MigrationState *s)
             qemu_thread_join(&s->thread);
             s->migration_thread_running = false;
         }
+        if (migrate_periodic_throttle()) {
+            periodic_throttle_stop();
+        }
         bql_lock();
 
         multifd_send_shutdown();
@@ -3263,6 +3266,9 @@ static MigIterateState migration_iteration_run(MigrationState *s)
 
     if ((!pending_size || pending_size < s->threshold_size) && can_switchover) {
         trace_migration_thread_low_pending(pending_size);
+        if (migrate_periodic_throttle()) {
+            periodic_throttle_stop();
+        }
         migration_completion(s);
         return MIG_ITERATE_BREAK;
     }
@@ -3508,6 +3514,11 @@ static void *migration_thread(void *opaque)
     ret = qemu_savevm_state_setup(s->to_dst_file, &local_err);
     bql_unlock();
 
+    if (migrate_periodic_throttle()) {
+        periodic_throttle_setup(true);
+        periodic_throttle_start();
+    }
+
     qemu_savevm_wait_unplug(s, MIGRATION_STATUS_SETUP,
                                MIGRATION_STATUS_ACTIVE);
 
-- 
2.39.1