On 8/3/2023 23:53, Peter Xu wrote:
> Migration bandwidth is a very important value to live migration. It's
> because it's one of the major factors that we'll make decision on when to
> switchover to destination in a precopy process.
>
> This value is currently estimated by QEMU during the whole live migration
> process by monitoring how fast we were sending the data. This can be the
> most accurate bandwidth if in the ideal world, where we're always feeding
> unlimited data to the migration channel, and then it'll be limited to the
> bandwidth that is available.
>
> However in reality it may be very different, e.g., over a 10Gbps network we
> can see query-migrate showing migration bandwidth of only a few tens of
> MB/s just because there are plenty of other things the migration thread
> might be doing. For example, the migration thread can be busy scanning
> zero pages, or it can be fetching dirty bitmap from other external dirty
> sources (like vhost or KVM). It means we may not be pushing data as much
> as possible to migration channel, so the bandwidth estimated from "how many
> data we sent in the channel" can be dramatically inaccurate sometimes,
> e.g., that a few tens of MB/s even if 10Gbps available, and then the
> decision to switchover will be further affected by this.
>
> The migration may not even converge at all with the downtime specified,
> with that wrong estimation of bandwidth.
>
> The issue is QEMU itself may not be able to avoid those uncertainties on
> measuing the real "available migration bandwidth". At least not something
> I can think of so far.
>
> One way to fix this is when the user is fully aware of the available
> bandwidth, then we can allow the user to help providing an accurate value.
>
> For example, if the user has a dedicated channel of 10Gbps for migration
> for this specific VM, the user can specify this bandwidth so QEMU can
> always do the calculation based on this fact, trusting the user as long as
> specified.
>
> A new parameter "max-switchover-bandwidth" is introduced just for this. So
> when the user specified this parameter, instead of trusting the estimated
> value from QEMU itself (based on the QEMUFile send speed), let's trust the
> user more by using this value to decide when to switchover, assuming that
> we'll have such bandwidth available then.
>
> When the user wants to have migration only use 5Gbps out of that 10Gbps,
> one can set max-bandwidth to 5Gbps, along with max-switchover-bandwidth to
> 5Gbps so it'll never use over 5Gbps too (so the user can have the rest
Hi Peter. I'm curious if we specify max-switchover-bandwidth to 5Gbps over a
10Gbps network, in the completion stage will it send the remaining data in 5Gbps
using downtime_limit time or in 10Gbps (saturate the network) using the
downtime_limit / 2 time? Seems this parameter won't rate limit the final stage:)
> 5Gbps for other things). So it can be useful even if the network is not
> dedicated, but as long as the user can know a solid value.
>
> This can resolve issues like "unconvergence migration" which is caused by
> hilarious low "migration bandwidth" detected for whatever reason.
>
> Reported-by: Zhiyi Guo <zh...@redhat.com>
> Signed-off-by: Peter Xu <pet...@redhat.com>
> ---
> qapi/migration.json | 14 +++++++++++++-
> migration/migration.h | 2 +-
> migration/options.h | 1 +
> migration/migration-hmp-cmds.c | 14 ++++++++++++++
> migration/migration.c | 19 +++++++++++++++----
> migration/options.c | 28 ++++++++++++++++++++++++++++
> migration/trace-events | 2 +-
> 7 files changed, 73 insertions(+), 7 deletions(-)
>
> diff --git a/qapi/migration.json b/qapi/migration.json
> index bb798f87a5..6a04fb7d36 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -759,6 +759,16 @@
> # @max-bandwidth: to set maximum speed for migration. maximum speed
> # in bytes per second. (Since 2.8)
> #
> +# @max-switchover-bandwidth: to set available bandwidth for migration.
> +# By default, this value is zero, means the user is not aware of
> +# the available bandwidth that can be used by QEMU migration, so
> +# QEMU will estimate the bandwidth automatically. This can be set
> +# when the estimated value is not accurate, while the user is able
> +# to guarantee such bandwidth is available for migration purpose
> +# during the migration procedure. When specified correctly, this
> +# can make the switchover decision much more accurate, which will
> +# also be based on the max downtime specified. (Since 8.2)
> +#
> # @downtime-limit: set maximum tolerated downtime for migration.
> # maximum downtime in milliseconds (Since 2.8)
> #
> @@ -840,7 +850,7 @@
> 'cpu-throttle-initial', 'cpu-throttle-increment',
> 'cpu-throttle-tailslow',
> 'tls-creds', 'tls-hostname', 'tls-authz', 'max-bandwidth',
> - 'downtime-limit',
> + 'max-switchover-bandwidth', 'downtime-limit',
> { 'name': 'x-checkpoint-delay', 'features': [ 'unstable' ] },
> 'block-incremental',
> 'multifd-channels',
> @@ -885,6 +895,7 @@
> '*tls-hostname': 'StrOrNull',
> '*tls-authz': 'StrOrNull',
> '*max-bandwidth': 'size',
> + '*max-switchover-bandwidth': 'size',
> '*downtime-limit': 'uint64',
> '*x-checkpoint-delay': { 'type': 'uint32',
> 'features': [ 'unstable' ] },
> @@ -949,6 +960,7 @@
> '*tls-hostname': 'str',
> '*tls-authz': 'str',
> '*max-bandwidth': 'size',
> + '*max-switchover-bandwidth': 'size',
> '*downtime-limit': 'uint64',
> '*x-checkpoint-delay': { 'type': 'uint32',
> 'features': [ 'unstable' ] },
> diff --git a/migration/migration.h b/migration/migration.h
> index 6eea18db36..f18cee27f7 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -283,7 +283,7 @@ struct MigrationState {
> /*
> * The final stage happens when the remaining data is smaller than
> * this threshold; it's calculated from the requested downtime and
> - * measured bandwidth
> + * measured bandwidth, or max-switchover-bandwidth if specified.
> */
> int64_t threshold_size;
>
> diff --git a/migration/options.h b/migration/options.h
> index 045e2a41a2..a510ca94c9 100644
> --- a/migration/options.h
> +++ b/migration/options.h
> @@ -80,6 +80,7 @@ int migrate_decompress_threads(void);
> uint64_t migrate_downtime_limit(void);
> uint8_t migrate_max_cpu_throttle(void);
> uint64_t migrate_max_bandwidth(void);
> +uint64_t migrate_max_switchover_bandwidth(void);
> uint64_t migrate_max_postcopy_bandwidth(void);
> int migrate_multifd_channels(void);
> MultiFDCompression migrate_multifd_compression(void);
> diff --git a/migration/migration-hmp-cmds.c b/migration/migration-hmp-cmds.c
> index c115ef2d23..d7572d4c0a 100644
> --- a/migration/migration-hmp-cmds.c
> +++ b/migration/migration-hmp-cmds.c
> @@ -321,6 +321,10 @@ void hmp_info_migrate_parameters(Monitor *mon, const
> QDict *qdict)
> monitor_printf(mon, "%s: %" PRIu64 " bytes/second\n",
> MigrationParameter_str(MIGRATION_PARAMETER_MAX_BANDWIDTH),
> params->max_bandwidth);
> + assert(params->has_max_switchover_bandwidth);
> + monitor_printf(mon, "%s: %" PRIu64 " bytes/second\n",
> +
> MigrationParameter_str(MIGRATION_PARAMETER_MAX_SWITCHOVER_BANDWIDTH),
> + params->max_switchover_bandwidth);
> assert(params->has_downtime_limit);
> monitor_printf(mon, "%s: %" PRIu64 " ms\n",
> MigrationParameter_str(MIGRATION_PARAMETER_DOWNTIME_LIMIT),
> @@ -574,6 +578,16 @@ void hmp_migrate_set_parameter(Monitor *mon, const QDict
> *qdict)
> }
> p->max_bandwidth = valuebw;
> break;
> + case MIGRATION_PARAMETER_MAX_SWITCHOVER_BANDWIDTH:
> + p->has_max_switchover_bandwidth = true;
> + ret = qemu_strtosz_MiB(valuestr, NULL, &valuebw);
> + if (ret < 0 || valuebw > INT64_MAX
> + || (size_t)valuebw != valuebw) {
> + error_setg(&err, "Invalid size %s", valuestr);
> + break;
> + }
> + p->max_switchover_bandwidth = valuebw;
> + break;
> case MIGRATION_PARAMETER_DOWNTIME_LIMIT:
> p->has_downtime_limit = true;
> visit_type_size(v, param, &p->downtime_limit, &err);
> diff --git a/migration/migration.c b/migration/migration.c
> index 5528acb65e..8493e3ca49 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -2684,7 +2684,7 @@ static void migration_update_counters(MigrationState *s,
> {
> uint64_t transferred, transferred_pages, time_spent;
> uint64_t current_bytes; /* bytes transferred since the beginning */
> - double bandwidth;
> + double bandwidth, avail_bw;
>
> if (current_time < s->iteration_start_time + BUFFER_DELAY) {
> return;
> @@ -2694,7 +2694,17 @@ static void migration_update_counters(MigrationState
> *s,
> transferred = current_bytes - s->iteration_initial_bytes;
> time_spent = current_time - s->iteration_start_time;
> bandwidth = (double)transferred / time_spent;
> - s->threshold_size = bandwidth * migrate_downtime_limit();
> + if (migrate_max_switchover_bandwidth()) {
> + /*
> + * If the user specified an available bandwidth, let's trust the
> + * user so that can be more accurate than what we estimated.
> + */
> + avail_bw = migrate_max_switchover_bandwidth();
> + } else {
> + /* If the user doesn't specify bandwidth, we use the estimated */
> + avail_bw = bandwidth;
> + }
> + s->threshold_size = avail_bw * migrate_downtime_limit();
>
> s->mbps = (((double) transferred * 8.0) /
> ((double) time_spent / 1000.0)) / 1000.0 / 1000.0;
> @@ -2711,7 +2721,7 @@ static void migration_update_counters(MigrationState *s,
> if (stat64_get(&mig_stats.dirty_pages_rate) &&
> transferred > 10000) {
> s->expected_downtime =
> - stat64_get(&mig_stats.dirty_bytes_last_sync) / bandwidth;
> + stat64_get(&mig_stats.dirty_bytes_last_sync) / avail_bw;
> }
>
> migration_rate_reset(s->to_dst_file);
> @@ -2719,7 +2729,8 @@ static void migration_update_counters(MigrationState *s,
> update_iteration_initial_status(s);
>
> trace_migrate_transferred(transferred, time_spent,
> - bandwidth, s->threshold_size);
> + bandwidth, migrate_max_switchover_bandwidth(),
> + s->threshold_size);
> }
>
> static bool migration_can_switchover(MigrationState *s)
> diff --git a/migration/options.c b/migration/options.c
> index 1d1e1321b0..19d87ab812 100644
> --- a/migration/options.c
> +++ b/migration/options.c
> @@ -125,6 +125,8 @@ Property migration_properties[] = {
> parameters.cpu_throttle_tailslow, false),
> DEFINE_PROP_SIZE("x-max-bandwidth", MigrationState,
> parameters.max_bandwidth, MAX_THROTTLE),
> + DEFINE_PROP_SIZE("max-switchover-bandwidth", MigrationState,
> + parameters.max_switchover_bandwidth, 0),
> DEFINE_PROP_UINT64("x-downtime-limit", MigrationState,
> parameters.downtime_limit,
> DEFAULT_MIGRATE_SET_DOWNTIME),
> @@ -780,6 +782,13 @@ uint64_t migrate_max_bandwidth(void)
> return s->parameters.max_bandwidth;
> }
>
> +uint64_t migrate_max_switchover_bandwidth(void)
> +{
> + MigrationState *s = migrate_get_current();
> +
> + return s->parameters.max_switchover_bandwidth;
> +}
> +
> uint64_t migrate_max_postcopy_bandwidth(void)
> {
> MigrationState *s = migrate_get_current();
> @@ -917,6 +926,8 @@ MigrationParameters *qmp_query_migrate_parameters(Error
> **errp)
> s->parameters.tls_authz : "");
> params->has_max_bandwidth = true;
> params->max_bandwidth = s->parameters.max_bandwidth;
> + params->has_max_switchover_bandwidth = true;
> + params->max_switchover_bandwidth =
> s->parameters.max_switchover_bandwidth;
> params->has_downtime_limit = true;
> params->downtime_limit = s->parameters.downtime_limit;
> params->has_x_checkpoint_delay = true;
> @@ -1056,6 +1067,15 @@ bool migrate_params_check(MigrationParameters *params,
> Error **errp)
> return false;
> }
>
> + if (params->has_max_switchover_bandwidth &&
> + (params->max_switchover_bandwidth > SIZE_MAX)) {
> + error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
> + "max_switchover_bandwidth",
> + "an integer in the range of 0 to "stringify(SIZE_MAX)
> + " bytes/second");
> + return false;
> + }
> +
> if (params->has_downtime_limit &&
> (params->downtime_limit > MAX_MIGRATE_DOWNTIME)) {
> error_setg(errp, QERR_INVALID_PARAMETER_VALUE,
> @@ -1225,6 +1245,10 @@ static void
> migrate_params_test_apply(MigrateSetParameters *params,
> dest->max_bandwidth = params->max_bandwidth;
> }
>
> + if (params->has_max_switchover_bandwidth) {
> + dest->max_switchover_bandwidth = params->max_switchover_bandwidth;
> + }
> +
> if (params->has_downtime_limit) {
> dest->downtime_limit = params->downtime_limit;
> }
> @@ -1341,6 +1365,10 @@ static void migrate_params_apply(MigrateSetParameters
> *params, Error **errp)
> }
> }
>
> + if (params->has_max_switchover_bandwidth) {
> + s->parameters.max_switchover_bandwidth =
> params->max_switchover_bandwidth;
> + }
> +
> if (params->has_downtime_limit) {
> s->parameters.downtime_limit = params->downtime_limit;
> }
> diff --git a/migration/trace-events b/migration/trace-events
> index 4666f19325..1296b8db5b 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -185,7 +185,7 @@ source_return_path_thread_shut(uint32_t val) "0x%x"
> source_return_path_thread_resume_ack(uint32_t v) "%"PRIu32
> source_return_path_thread_switchover_acked(void) ""
> migration_thread_low_pending(uint64_t pending) "%" PRIu64
> -migrate_transferred(uint64_t transferred, uint64_t time_spent, uint64_t
> bandwidth, uint64_t size) "transferred %" PRIu64 " time_spent %" PRIu64 "
> bandwidth %" PRIu64 " max_size %" PRId64
> +migrate_transferred(uint64_t transferred, uint64_t time_spent, uint64_t
> bandwidth, uint64_t avail_bw, uint64_t size) "transferred %" PRIu64 "
> time_spent %" PRIu64 " bandwidth %" PRIu64 " avail_bw %" PRIu64 " max_size %"
> PRId64
> process_incoming_migration_co_end(int ret, int ps) "ret=%d postcopy-state=%d"
> process_incoming_migration_co_postcopy_end_main(void) ""
> postcopy_preempt_enabled(bool value) "%d"
