[Qemu-devel] [RFC v2 4/4] Example use of rolling statistics in migration

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

This is an example use of the rolling statistics to
watch the 'expected downtime' in a bit more detail than
the current summary figure.

Example outputs from a simple run:

HMP:
expected downtime stats: Min/Max: 222, 1634 Mean: 983.8 (Weighted:
1005.1253) Count: 141 Values: 1248@39356292, 981@39356395, 774@39356497,
876@39356599, 1040@39356702

QMP:
        "expected-downtime-stats": {
            "min": 222,
            "count": 378,
            "mean": 1100.2,
            "max": 1942,
            "weighted-mean": 1115.710848,
            "values": [
                {
                    "tag": 39380740,
                    "value": 1118
                },
                {
                    "tag": 39380842,
                    "value": 953
                },
                {
                    "tag": 39380945,
                    "value": 1017
                },
                {
                    "tag": 39381048,
                    "value": 1336
                },
                {
                    "tag": 39381150,
                    "value": 1077
                }
            ]
        }

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 hmp.c                         |  5 ++++-
 include/migration/migration.h |  1 +
 migration/migration.c         | 15 +++++++++++++++
 qapi-schema.json              |  6 +++++-
 4 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/hmp.c b/hmp.c
index 20241d8..6bd19b2 100644
--- a/hmp.c
+++ b/hmp.c
@@ -138,7 +138,6 @@ void hmp_info_mice(Monitor *mon, const QDict *qdict)
     qapi_free_MouseInfoList(mice_list);
 }
 
-__attribute__ (( unused )) /* Until later in patch series */
 static void monitor_printf_RollingStats(Monitor *mon, const char *title,
                                         RollingStats *r)
 {
@@ -186,6 +185,10 @@ void hmp_info_migrate(Monitor *mon, const QDict *qdict)
             monitor_printf(mon, "expected downtime: %" PRIu64 " milliseconds\n",
                            info->expected_downtime);
         }
+        if (info->has_expected_downtime_stats) {
+            monitor_printf_RollingStats(mon, "expected downtime stats",
+                           info->expected_downtime_stats);
+        }
         if (info->has_downtime) {
             monitor_printf(mon, "downtime: %" PRIu64 " milliseconds\n",
                            info->downtime);
diff --git a/include/migration/migration.h b/include/migration/migration.h
index 703b7d7..b37ec0c 100644
--- a/include/migration/migration.h
+++ b/include/migration/migration.h
@@ -63,6 +63,7 @@ struct MigrationState
     int64_t xbzrle_cache_size;
     int64_t setup_time;
     int64_t dirty_sync_count;
+    RStats *expected_downtime_stats;
 };
 
 void process_incoming_migration(QEMUFile *f);
diff --git a/migration/migration.c b/migration/migration.c
index b3adbc6..4041823 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -24,6 +24,7 @@
 #include "migration/block.h"
 #include "qemu/thread.h"
 #include "qmp-commands.h"
+#include "qemu/rolling-stats.h"
 #include "trace.h"
 
 enum {
@@ -201,6 +202,9 @@ MigrationInfo *qmp_query_migrate(Error **errp)
             - s->total_time;
         info->has_expected_downtime = true;
         info->expected_downtime = s->expected_downtime;
+        info->expected_downtime_stats =
+            rstats_as_RollingStats(s->expected_downtime_stats);
+        info->has_expected_downtime_stats = true;
         info->has_setup_time = true;
         info->setup_time = s->setup_time;
 
@@ -238,6 +242,9 @@ MigrationInfo *qmp_query_migrate(Error **errp)
         info->downtime = s->downtime;
         info->has_setup_time = true;
         info->setup_time = s->setup_time;
+        info->expected_downtime_stats =
+            rstats_as_RollingStats(s->expected_downtime_stats);
+        info->has_expected_downtime_stats = true;
 
         info->has_ram = true;
         info->ram = g_malloc0(sizeof(*info->ram));
@@ -389,12 +396,18 @@ static MigrationState *migrate_init(const MigrationParams *params)
     memcpy(enabled_capabilities, s->enabled_capabilities,
            sizeof(enabled_capabilities));
 
+    /* Resets the current state */
     memset(s, 0, sizeof(*s));
     s->params = *params;
     memcpy(s->enabled_capabilities, enabled_capabilities,
            sizeof(enabled_capabilities));
     s->xbzrle_cache_size = xbzrle_cache_size;
 
+    if (!s->expected_downtime_stats) {
+        s->expected_downtime_stats = rstats_init(5, 0.2);
+    } else {
+        rstats_reset(s->expected_downtime_stats);
+    }
     s->bandwidth_limit = bandwidth_limit;
     s->state = MIG_STATE_SETUP;
     trace_migrate_set_state(MIG_STATE_SETUP);
@@ -658,6 +671,8 @@ static void *migration_thread(void *opaque)
                10000 is a small enough number for our purposes */
             if (s->dirty_bytes_rate && transferred_bytes > 10000) {
                 s->expected_downtime = s->dirty_bytes_rate / bandwidth;
+                rstats_add_value(s->expected_downtime_stats,
+                                 s->expected_downtime, current_time);
             }
 
             qemu_file_reset_rate_limit(s->file);
diff --git a/qapi-schema.json b/qapi-schema.json
index 9f5cdce..c43f0e7 100644
--- a/qapi-schema.json
+++ b/qapi-schema.json
@@ -480,6 +480,9 @@
 #        may be expensive, but do not actually occur during the iterative
 #        migration rounds themselves. (since 1.6)
 #
+# @expected-downtime-stats: #optional more detailed statistics from the
+#        downtime estimation.
+#
 # Since: 0.14.0
 ##
 { 'type': 'MigrationInfo',
@@ -489,7 +492,8 @@
            '*total-time': 'int',
            '*expected-downtime': 'int',
            '*downtime': 'int',
-           '*setup-time': 'int'} }
+           '*setup-time': 'int',
+           '*expected-downtime-stats': 'RollingStats' } }
 
 ##
 # @query-migrate
-- 
2.1.0