[v2,0/3] migration: auto-converge refinements for huge VM

From: Hyman Huang <yong.huang@smartx.com>

v2:
1. background sync - Throw out the idea of "not updating the bitmap"
   when the RAMBlock of the RAM list is iterated during migration;
   re-implement the background RAM dirty sync using Peter's updated
   method.

2. responsive throttle - Rename the "cpu-responsive throttle" to
   "cpu-throttle-responsive," as suggested by Fabiano.

For the background sync refinement, The new method achieves a much
shorter migration time overall and is much simpler than the previous
approach.

The test approach is the same as in version 1, but we use a new test
host with 1.93 Gb bandwidth. Please refer to the information included
in version 1 below for more details.

The new test result for background dirty sync is as follows; we didn't
test the responsive throttle feature because its logic is still the same.

For each scenario, we tested twice and used 3500MB as high memory load.
ramsize: 150MB
|------------+-----------+----------+-----------+--------------|
|            | totaltime | downtime | iteration | max throttle |
|            | (ms)      | (ms)     | count     | percent      |
|------------+-----------+----------+-----------+--------------|
| original   | 57579     | 577      | 107       | 90%          |
|            | 57434     | 466      | 110       | 90%          |
|------------+-----------+----------+-----------+--------------|
| background | 57728     | 488      | 102       | 90%          |
| sync       | 60223     | 447      | 115       | 90%          |
|------------+-----------+----------+-----------+--------------|

ramsize: 3000MB
|------------+-----------+----------+-----------+--------------|
|            | totaltime | downtime | iteration | max throttle |
|            | (ms)      | (ms)     | count     | percent      |
|------------+-----------+----------+-----------+--------------|
| original   | 291275    | 416      | 26        | 99%          |
|            | 290907    | 567      | 27        | 99%          |
|------------+-----------+----------+-----------+--------------|
| background | 210912    | 465      | 30        | 99%          |
| sync       | 203890    | 442      | 28        | 99%          |
|------------+-----------+----------+-----------+--------------|

ramsize: 3500MB
|------------+-----------+----------+-----------+--------------|
|            | totaltime | downtime | iteration | max throttle |
|            | (ms)      | (ms)     | count     | percent      |
|------------+-----------+----------+-----------+--------------|
| original   | 338445    | 491      | 28        | 99%          |
|            | 341168    | 496      | 28        | 99%          |
|------------+-----------+----------+-----------+--------------|
| background | 209099    | 440      | 27        | 99%          |
| sync       | 206241    | 467      | 30        | 99%          |
|------------+-----------+----------+-----------+--------------|

v1:
This is the first version for auto-converge refinements; refer to the
following link for details about the RFC version:
https://patchew.org/QEMU/cover.1725891841.git.yong.huang@smartx.com/

This series introduces two refinements called "background sync" and
"responsive throttle," respectively.

1. background sync:

   The original auto-converge throttle logic doesn't look like it will
   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 background sync would fix this issue by implementing the background
   dirty bitmap sync and throttle automatically once detect that
   the iteration lasts a long time during the migration.

   The background sync is implemented transparently, and there is no
   new-added interface for upper apps.

2. responsive throttle:

   The original auto-converge throttle logic determines if the migration
   is convergent by one criteria, and if the iteration fits twice, then
   launch the CPU throttle or increase the throttle percentage. This
   results in that the migration_trigger_throttle() won't be invoked for
   a long time if one iteration can take a long time too.

   The responsive throttle introduce one more criteria to assist detecting
   the convergence of the migration, if either of the two criteria is
   met, migration_trigger_throttle() would be called. This also makes it
   more likely that the CPU throttle will be activated, thereby
   accelerating the migration process.

   The responsive throttle provides the 'cpu-responsive-throttle' option
   to enable this feature.

We test this two features with the following environment:

a. Test tool:
guestperf

Refer to the following link to see details:
https://github.com/qemu/qemu/tree/master/tests/migration/guestperf

b. Test VM scale:
CPU: 16; Memory: 100GB

c. Average bandwidth between source and destination for migration:
1.59 Gbits/sec

We use stress tool contained in the initrd-stress.img to update
ramsize MB on every CPU in guest, refer to the following link to
see the source code:
https://github.com/qemu/qemu/blob/master/tests/migration/stress.c

The following command is executed to compare our refined QEMU with the
original QEMU:
# python3.6 guestperf.py --binary /path/to/qemu-kvm --cpus 16 \
--mem 100 --max-iters 200 --max-time 1200  --dst-host {dst_ip} \
--kernel /path/to/vmlinuz --initrd /path/to/initrd-stress.img \
--transport tcp --downtime 500 --auto-converge --auto-converge-step 10 \
--verbose --stress-mem {ramsize}

We set ramsize to 150MB to simulate the light load, 3000MB as moderate
load and 5000MB as heavy load. Test cases were executed three times in
each scenario.

The following data shows the migration test results with an increase in
stress.

ramsize: 150MB
|------------+-----------+----------+-----------+--------------|
|            | totaltime | downtime | iteration | max throttle |
|            | (ms)      | (ms)     | count     | percent      |
|------------+-----------+----------+-----------+--------------|
| original   | 123685    | 490      | 87        | 99%          |
|            | 116249    | 542      | 45        | 60%          |
|            | 107772    | 587      | 8         | 0%           |
|------------+-----------+----------+-----------+--------------|
| background | 113744    | 1654     | 16        | 20%          |
| sync       | 122623    | 758      | 60        | 80%          |
|            | 112668    | 547      | 23        | 20%          |
|------------+-----------+----------+-----------+--------------|
| background | 113660    | 573      | 5         | 0%           |
| sync +     | 109357    | 576      | 6         | 0%           |
| responsive | 126792    | 494      | 37        | 99%          |
| throttle   |           |          |           |              |
|------------+-----------+----------+-----------+--------------|

ramsize: 3000MB
|------------+-----------+----------+-----------+--------------|
|            | totaltime | downtime | iteration | max throttle |
|            | (ms)      | (ms)     | count     | percent      |
|------------+-----------+----------+-----------+--------------|
| original   | 404398    | 515      | 26        | 99%          |
|            | 392552    | 528      | 25        | 99%          |
|            | 400113    | 447      | 24        | 99%          |
|------------+-----------+----------+-----------+--------------|
| background | 239151    | 681      | 25        | 99%          |
| sync       | 295047    | 587      | 41        | 99%          |
|            | 289936    | 681      | 34        | 99%          |
|------------+-----------+----------+-----------+--------------|
| background | 212786    | 487      | 22        | 99%          |
| sync +     | 225246    | 666      | 23        | 99%          |
| responsive | 244053    | 572      | 27        | 99%          |
| throttle   |           |          |           |              |
|------------+-----------+----------+-----------+--------------|

ramsize: 5000MB
|------------+-----------+----------+-----------+--------------|
|            | totaltime | downtime | iteration | max throttle |
|            | (ms)      | (ms)     | count     | percent      |
|------------+-----------+----------+-----------+--------------|
| original   | 566357    | 644      | 22        | 99%          |
|            | 607471    | 320      | 23        | 99%          |
|            | 603136    | 417      | 22        | 99%          |
|------------+-----------+----------+-----------+--------------|
| background | 284605    | 793      | 27        | 99%          |
| sync       | 272270    | 668      | 28        | 99%          |
|            | 267543    | 545      | 28        | 99%          |
|------------+-----------+----------+-----------+--------------|
| background | 226446    | 413      | 22        | 99%          |
| sync +     | 232082    | 494      | 23        | 99%          |
| responsive | 269863    | 533      | 23        | 99%          |
| throttle   |           |          |           |              |
|------------+-----------+----------+-----------+--------------|

To summarize the data above, any data that implies negative optimization
does not appear, the refinement saves the total time significantly and,
therefore, shortens the duration of the guest performance degradation.

Additionally, we examined the memory performance curves generated from
the test case results above; while no negative optimization is there,
but the performance degradation occurs more quickly. Since it is
inconvenient to display the graphic data, one can independently
verify it.

Hyman Huang (3):
  migration: Support background ramblock dirty sync
  qapi/migration: Introduce cpu-throttle-responsive parameter
  migration: Support responsive CPU throttle

 include/migration/misc.h       |   3 +
 migration/migration-hmp-cmds.c |   8 +++
 migration/migration.c          |  11 +++
 migration/options.c            |  20 ++++++
 migration/options.h            |   1 +
 migration/ram.c                | 119 ++++++++++++++++++++++++++++++++-
 migration/ram.h                |   3 +
 migration/trace-events         |   2 +
 qapi/migration.json            |  16 ++++-
 system/cpu-timers.c            |   2 +
 tests/qtest/migration-test.c   |  30 +++++++++
 11 files changed, 212 insertions(+), 3 deletions(-)