[Qemu-devel] [PATCH v3 1/1] migration: calculate expected_downtime with ram_bytes_remaining()

[Balamuruhan S <bala24@linux.vnet.ibm.com>]

expected_downtime value is not accurate with dirty_pages_rate * page_size,
using ram_bytes_remaining would yeild it correct. It will initially be a
gross over-estimate, but for for non-converging migrations it should
approach a reasonable estimate later on.

currently bandwidth and expected_downtime value are calculated in
migration_update_counters() during each iteration from
migration_thread(), where as remaining ram is calculated in
qmp_query_migrate() when we actually call "info migrate". Due to this
there is some difference in expected_downtime value being calculated.

with this patch bandwidth, expected_downtime and remaining ram are
calculated in migration_update_counters(), retrieve the same value during
"info migrate". By this approach we get almost close enough value.

Reported-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Signed-off-by: Balamuruhan S <bala24@linux.vnet.ibm.com>
---
 migration/migration.c | 11 ++++++++---
 migration/migration.h |  1 +
 2 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/migration/migration.c b/migration/migration.c
index 52a5092add..5d721ee481 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -614,7 +614,7 @@ static void populate_ram_info(MigrationInfo *info, MigrationState *s)
     }
 
     if (s->state != MIGRATION_STATUS_COMPLETED) {
-        info->ram->remaining = ram_bytes_remaining();
+        info->ram->remaining = s->ram_bytes_remaining;
         info->ram->dirty_pages_rate = ram_counters.dirty_pages_rate;
     }
 }
@@ -2227,6 +2227,7 @@ static void migration_update_counters(MigrationState *s,
     transferred = qemu_ftell(s->to_dst_file) - s->iteration_initial_bytes;
     time_spent = current_time - s->iteration_start_time;
     bandwidth = (double)transferred / time_spent;
+    s->ram_bytes_remaining = ram_bytes_remaining();
     s->threshold_size = bandwidth * s->parameters.downtime_limit;
 
     s->mbps = (((double) transferred * 8.0) /
@@ -2237,8 +2238,12 @@ static void migration_update_counters(MigrationState *s,
      * recalculate. 10000 is a small enough number for our purposes
      */
     if (ram_counters.dirty_pages_rate && transferred > 10000) {
-        s->expected_downtime = ram_counters.dirty_pages_rate *
-            qemu_target_page_size() / bandwidth;
+        /*
+         * It will initially be a gross over-estimate, but for for
+         * non-converging migrations it should approach a reasonable estimate
+         * later on
+         */
+        s->expected_downtime = s->ram_bytes_remaining / bandwidth;
     }
 
     qemu_file_reset_rate_limit(s->to_dst_file);
diff --git a/migration/migration.h b/migration/migration.h
index 8d2f320c48..8584f8e22e 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -128,6 +128,7 @@ struct MigrationState
     int64_t downtime_start;
     int64_t downtime;
     int64_t expected_downtime;
+    int64_t ram_bytes_remaining;
     bool enabled_capabilities[MIGRATION_CAPABILITY__MAX];
     int64_t setup_time;
     /*
-- 
2.14.3