diff mbox series

[v8,5/6] migration: Reduce time of loading non-iterable vmstate

Message ID 20230317081904.24389-6-xuchuangxclwt@bytedance.com
State New
Headers show
Series migration: reduce time of loading non-iterable vmstate | expand

Commit Message

Chuang Xu March 17, 2023, 8:19 a.m. UTC 
The duration of loading non-iterable vmstate accounts for a significant
portion of downtime (starting with the timestamp of source qemu stop and
ending with the timestamp of target qemu start). Most of the time is spent
committing memory region changes repeatedly.

This patch packs all the changes to memory region during the period of
loading non-iterable vmstate in a single memory transaction. With the
increase of devices, this patch will greatly improve the performance.

Note that the following test results are based on the application of the
next patch. Without the next patch, the improvement will be reduced.

Here are the test1 results:
test info:
- Host
  - Intel(R) Xeon(R) Platinum 8362 CPU
  - Mellanox Technologies MT28841
- VM
  - 32 CPUs 128GB RAM VM
  - 8 16-queue vhost-net device
  - 16 4-queue vhost-user-blk device.

	time of loading non-iterable vmstate     downtime
before		about 112 ms			  285 ms
after		about 20 ms			  194 ms

In test2, we keep the number of the device the same as test1, reduce the
number of queues per device:

Here are the test2 results:
test info:
- Host
  - Intel(R) Xeon(R) Platinum 8362 CPU
  - Mellanox Technologies MT28841
- VM
  - 32 CPUs 128GB RAM VM
  - 8 1-queue vhost-net device
  - 16 1-queue vhost-user-blk device.

	time of loading non-iterable vmstate     downtime
before		about 65 ms			 about 151 ms

after		about 19 ms			 about 100 ms

In test3, we keep the number of queues per device the same as test1, reduce
the number of devices:

Here are the test3 results:
test info:
- Host
  - Intel(R) Xeon(R) Platinum 8362 CPU
  - Mellanox Technologies MT28841
- VM
  - 32 CPUs 128GB RAM VM
  - 1 16-queue vhost-net device
  - 1 4-queue vhost-user-blk device.

	time of loading non-iterable vmstate     downtime
before		about 24 ms			 about 51 ms
after		about 9 ms			 about 36 ms

As we can see from the test results above, both the number of queues and
the number of devices have a great impact on the time of loading non-iterable
vmstate. The growth of the number of devices and queues will lead to more
mr commits, and the time consumption caused by the flatview reconstruction
will also increase.

Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
---
 migration/savevm.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
diff mbox series

Patch

diff --git a/migration/savevm.c b/migration/savevm.c
index aa54a67fda..ecf7b27000 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2787,7 +2787,25 @@  int qemu_loadvm_state(QEMUFile *f)
 
     cpu_synchronize_all_pre_loadvm();
 
+    /*
+     * Call memory_region_transaction_begin() before loading vmstate.
+     * This call is paired with memory_region_transaction_commit() at
+     * the end of qemu_loadvm_state_main(), in order to pack all the
+     * changes to memory region during the period of loading
+     * non-iterable vmstate in a single memory transaction.
+     * This operation will reduce time of loading non-iterable vmstate
+     */
+    memory_region_transaction_begin();
+
     ret = qemu_loadvm_state_main(f, mis);
+
+    /*
+     * Call memory_region_transaction_commit() after loading vmstate.
+     * At this point, qemu actually completes all the previous memory
+     * region transactions.
+     */
+    memory_region_transaction_commit();
+
     qemu_event_set(&mis->main_thread_load_event);
 
     trace_qemu_loadvm_state_post_main(ret);