[PATCH] migration: Refine the convergence detection

[Hyman Huang]

Currently, the convergence algorithm determines that the migration
cannot converge according to the following principle:
The dirty pages generated in current iteration exceed a specific
percentage (throttle-trigger-threshold, 50 by default) of the number
of transmissions. Let's refer to this criteria as the
"transmission speed," If this criteria is met more than or equal to
twice (dirty_rate_high_cnt >= 2), the throttle percentage needs to
be increased.

In most cases, above implementation is appropriate. However, for a
VM with a huge memory and high memory overload, each iteration is
time-consuming. The VM's computing performance may be throttled at
a high percentage and last for a long time due to the repeated
confirmation behavior. Which may be intolerable for some
computationally sensitive software in the VM. The refinement is for
this scenario.

As the comment mentioned in the migration_trigger_throttle function,
in order to avoid erroneous detection, the original algorithm confirms
the criteria repeatedly. Put differently, once the detection become
more reliable, it does not need to be confirmed twice.

In the refinement, in order to make the detection more accurate, we
introduce another criteria, called the "valid transmission ratio"
to determine the migration convergence. The "valid transmission ratio"
is the ratio of bytes_xfer_period and bytes_dirty_period, which
actually describe the migration efficiency.

When the algorithm repeatedly detects that the current iteration
"valid transmission ratio" is lower than the previous iteration,
the algorithm determines that the migration cannot converge.

For the "transmission speed" and "valid transmission ratio", if one
of the two criteria is met, the penalty percentage would be increased.
This saves the time of the entire iteration and therefore reduces
the time of VM performance degradation.

In conclusion, this refinement significantly reduces the processing
time required for the throttle percentage step to its maximum while
the VM is under a high memory load.

The following are test 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: 10; Memory: 28GB

c. Average bandwidth between source and destination for migration:
1.53 Gbits/sec
----------------------------------------------------------------------
All the supplementary test data shown as follows are basing on
above test environment.

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

We run the following command to compare our refined QEMU with the
original QEMU:

$ ./migration/guestperf.py --cpus 10 --mem 28 --max-iters 40 \
--binary $(path_to_qemu-kvm) \
--dst-host $(destination_ip) \
--transport tcp --kernel $(path_to_vmlinuz) \
--initrd $(path_to_initrd-stress.img) \
--auto-converge \
--auto-converge-step 10 \
--max-iters 40

The following data shows the migration test results with an increase in
stress, ignoring the title row, the odd rows show the unrefined QEMU
test data, and the even rows show the refined QEMU test data:

|---------+--------+-----------+--------------+------------+------------|
| ramsize | sucess | totaltime | downtime(ms) | iterations | switchover |
| (MB)    |        | (ms)      | (ms)         |            | throttle   |
|         |        |           |              |            | percent    |
|---------+--------+-----------+--------------+------------+------------|
|     350 |    yes |    170285 |          399 |         23 |         99 |
|     350 |    yes |     85962 |          412 |         24 |         70 |
|     350 |    yes |    176305 |          199 |         20 |         99 |
|     350 |    yes |     66729 |          321 |         11 |         40 |
|     400 |    yes |    183042 |          469 |         21 |         99 |
|     400 |    yes |     77225 |          421 |         10 |         30 |
|     400 |    yes |    183641 |          866 |         27 |         99 |
|     400 |    yes |     59796 |          479 |         15 |         50 |
|     450 |    yes |    165881 |          820 |         21 |         99 |
|     450 |    yes |     87930 |          368 |         20 |         90 |
|     450 |    yes |    195448 |          452 |         23 |         99 |
|     450 |    yes |     70394 |          295 |          6 |         20 |
|     500 |    yes |    112587 |          471 |         19 |         99 |
|     500 |    yes |     57401 |          299 |          5 |         30 |
|     500 |    yes |    110683 |          657 |         21 |         99 |
|     500 |    yes |     69949 |          649 |          8 |         40 |
|     600 |    yes |    111036 |          324 |         23 |         99 |
|     600 |    yes |     63455 |          346 |          4 |         20 |
|     600 |    yes |    126667 |          426 |         20 |         99 |
|     600 |    yes |    101024 |          643 |         20 |         99 |
|    1000 |    yes |    296216 |          660 |         23 |         99 |
|    1000 |    yes |    106915 |          468 |         16 |         99 |
|    1000 |     no |    300000 |              |            |            |
|    1000 |    yes |    125819 |          824 |         17 |         99 |
|    1200 |     no |    300000 |              |            |            |
|    1200 |    yes |    127379 |          664 |         14 |         90 |
|    1200 |     no |    300000 |              |            |            |
|    1200 |    yes |     67086 |          793 |         11 |         50 |
|---------+--------+-----------+--------------+------------+------------|

To summarize the data above, any data that implies negative optimization
does not appear, and morover, the throttle algorithm seems to become more
responsive to dirty rate increases due to the refined detection.

Signed-off-by: Hyman Huang <yong.huang@smartx.com>
---
 migration/ram.c        | 48 +++++++++++++++++++++++++++++++++++++++---
 migration/trace-events |  1 +
 2 files changed, 46 insertions(+), 3 deletions(-)

diff --git a/migration/ram.c b/migration/ram.c
index edec1a2d07..18b2d422b5 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -414,6 +414,17 @@ struct RAMState {
      * RAM migration.
      */
     unsigned int postcopy_bmap_sync_requested;
+
+    /*
+     * Ratio of bytes_dirty_period and bytes_xfer_period in the last
+     * iteration
+     */
+    uint64_t dirty_ratio_pct;
+    /*
+     * How many times is the most recent iteration dirty ratio is
+     * higher than previous one
+     */
+    int dirty_ratio_high_cnt;
 };
 typedef struct RAMState RAMState;
 
@@ -1013,6 +1024,32 @@ static void migration_dirty_limit_guest(void)
     trace_migration_dirty_limit_guest(quota_dirtyrate);
 }
 
+static bool migration_dirty_ratio_unexpected(RAMState *rs)
+{
+    uint64_t threshold = migrate_throttle_trigger_threshold();
+    uint64_t bytes_xfer_period =
+        migration_transferred_bytes() - rs->bytes_xfer_prev;
+    uint64_t bytes_dirty_period = rs->num_dirty_pages_period * 
TARGET_PAGE_SIZE;
+    uint64_t prev, curr;
+
+    /* Skip the first iterations since it isn't helpful */
+    if (stat64_get(&mig_stats.dirty_sync_count) == 1 || !bytes_xfer_period) {
+        return false;
+    }
+
+    curr = 100 * (bytes_dirty_period * 1.0 / bytes_xfer_period);
+
+    prev = rs->dirty_ratio_pct;
+    rs->dirty_ratio_pct = curr;
+
+    if (prev == 0 || curr <= threshold) {
+        return false;
+    }
+
+    trace_dirty_ratio_pct(curr, prev);
+    return curr >= prev;
+}
+
 static void migration_trigger_throttle(RAMState *rs)
 {
     uint64_t threshold = migrate_throttle_trigger_threshold();
@@ -1028,9 +1065,14 @@ static void migration_trigger_throttle(RAMState *rs)
      * we were in this routine reaches the threshold. If that happens
      * twice, start or increase throttling.
      */
-    if ((bytes_dirty_period > bytes_dirty_threshold) &&
-        (++rs->dirty_rate_high_cnt >= 2)) {
-        rs->dirty_rate_high_cnt = 0;
+    if ((migration_dirty_ratio_unexpected(rs) &&
+         (++rs->dirty_ratio_high_cnt >= 2)) ||
+        ((bytes_dirty_period > bytes_dirty_threshold) &&
+         (++rs->dirty_rate_high_cnt >= 2))) {
+        rs->dirty_rate_high_cnt =
+            rs->dirty_rate_high_cnt >= 2 ? 0 : rs->dirty_rate_high_cnt;
+        rs->dirty_ratio_high_cnt =
+            rs->dirty_ratio_high_cnt >= 2 ? 0 : rs->dirty_ratio_high_cnt;
         if (migrate_auto_converge()) {
             trace_migration_throttle();
             mig_throttle_guest_down(bytes_dirty_period,
diff --git a/migration/trace-events b/migration/trace-events
index 0b7c3324fb..654c52c5e4 100644
--- a/migration/trace-events
+++ b/migration/trace-events
@@ -90,6 +90,7 @@ put_qlist_end(const char *field_name, const char *vmsd_name) 
"%s(%s)"
 qemu_file_fclose(void) ""
 
 # ram.c
+dirty_ratio_pct(uint64_t cur, uint64_t prev) "current ratio: %" PRIu64 " 
previous ratio: %" PRIu64
 get_queued_page(const char *block_name, uint64_t tmp_offset, unsigned long 
page_abs) "%s/0x%" PRIx64 " page_abs=0x%lx"
 get_queued_page_not_dirty(const char *block_name, uint64_t tmp_offset, 
unsigned long page_abs) "%s/0x%" PRIx64 " page_abs=0x%lx"
 migration_bitmap_sync_start(void) ""
-- 
2.39.1