Re: [PATCH] migration/throttle: Make throttle slower at tail stage

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

* Keqian Zhu (zhukeqian1@huawei.com) wrote:
> At the tail stage of throttle, VM is very sensitive to
> CPU percentage. We just throttle 30% of remaining CPU
> when throttle is more than 80 percentage.

This is a bit unusual;  all of the rest of the throttling has no
fixed constants; all values are set by parameters.

You've got the two, the '80' and the '0.3'

I thinkt he easy solution is to replace your parameter 'tailslow' by two
new parameters; 'tailstart' and 'tailrate';  both defaulting to 100%.

Then you make it:

        if (cpu_throttle_now >= pct_tailstart) {
            /* Eat some scale of CPU from remaining */
            cpu_throttle_inc = ceil((100 - cpu_throttle_now) * pct_tailrate);

(with percentage scaling added).

Then setting 'tailstart' to 80 and 'tailrate' to 30 is equivalent to
what you have, but means we have no magical constants in the code.

Dave


> This doesn't conflict with cpu_throttle_increment.
> 
> This may make migration time longer, and is disabled
> by default.
> 
> Signed-off-by: Keqian Zhu <zhukeqian1@huawei.com>
> ---
> Cc: Juan Quintela <quintela@redhat.com>
> Cc: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Markus Armbruster <armbru@redhat.com>
> ---
>  migration/migration.c | 13 +++++++++++++
>  migration/ram.c       | 18 ++++++++++++++++--
>  monitor/hmp-cmds.c    |  4 ++++
>  qapi/migration.json   | 21 +++++++++++++++++++++
>  4 files changed, 54 insertions(+), 2 deletions(-)
> 
> diff --git a/migration/migration.c b/migration/migration.c
> index 3a21a4686c..37b569cee9 100644
> --- a/migration/migration.c
> +++ b/migration/migration.c
> @@ -782,6 +782,8 @@ MigrationParameters *qmp_query_migrate_parameters(Error **errp)
>      params->cpu_throttle_initial = s->parameters.cpu_throttle_initial;
>      params->has_cpu_throttle_increment = true;
>      params->cpu_throttle_increment = s->parameters.cpu_throttle_increment;
> +    params->has_cpu_throttle_tailslow = true;
> +    params->cpu_throttle_tailslow = s->parameters.cpu_throttle_tailslow;
>      params->has_tls_creds = true;
>      params->tls_creds = g_strdup(s->parameters.tls_creds);
>      params->has_tls_hostname = true;
> @@ -1287,6 +1289,10 @@ static void migrate_params_test_apply(MigrateSetParameters *params,
>          dest->cpu_throttle_increment = params->cpu_throttle_increment;
>      }
>  
> +    if (params->has_cpu_throttle_tailslow) {
> +        dest->cpu_throttle_tailslow = params->cpu_throttle_tailslow;
> +    }
> +
>      if (params->has_tls_creds) {
>          assert(params->tls_creds->type == QTYPE_QSTRING);
>          dest->tls_creds = g_strdup(params->tls_creds->u.s);
> @@ -1368,6 +1374,10 @@ static void migrate_params_apply(MigrateSetParameters *params, Error **errp)
>          s->parameters.cpu_throttle_increment = params->cpu_throttle_increment;
>      }
>  
> +    if (params->has_cpu_throttle_tailslow) {
> +        s->parameters.cpu_throttle_tailslow = params->cpu_throttle_tailslow;
> +    }
> +
>      if (params->has_tls_creds) {
>          g_free(s->parameters.tls_creds);
>          assert(params->tls_creds->type == QTYPE_QSTRING);
> @@ -3504,6 +3514,8 @@ static Property migration_properties[] = {
>      DEFINE_PROP_UINT8("x-cpu-throttle-increment", MigrationState,
>                        parameters.cpu_throttle_increment,
>                        DEFAULT_MIGRATE_CPU_THROTTLE_INCREMENT),
> +    DEFINE_PROP_BOOL("x-cpu-throttle-tailslow", MigrationState,
> +                      parameters.cpu_throttle_tailslow, false),
>      DEFINE_PROP_SIZE("x-max-bandwidth", MigrationState,
>                        parameters.max_bandwidth, MAX_THROTTLE),
>      DEFINE_PROP_UINT64("x-downtime-limit", MigrationState,
> @@ -3600,6 +3612,7 @@ static void migration_instance_init(Object *obj)
>      params->has_decompress_threads = true;
>      params->has_cpu_throttle_initial = true;
>      params->has_cpu_throttle_increment = true;
> +    params->has_cpu_throttle_tailslow = true;
>      params->has_max_bandwidth = true;
>      params->has_downtime_limit = true;
>      params->has_x_checkpoint_delay = true;
> diff --git a/migration/ram.c b/migration/ram.c
> index ed23ed1c7c..d86413a5d1 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -29,6 +29,7 @@
>  #include "qemu/osdep.h"
>  #include "cpu.h"
>  #include <zlib.h>
> +#include <math.h>
>  #include "qemu/cutils.h"
>  #include "qemu/bitops.h"
>  #include "qemu/bitmap.h"
> @@ -620,15 +621,28 @@ static void mig_throttle_guest_down(void)
>  {
>      MigrationState *s = migrate_get_current();
>      uint64_t pct_initial = s->parameters.cpu_throttle_initial;
> -    uint64_t pct_icrement = s->parameters.cpu_throttle_increment;
> +    uint64_t pct_increment = s->parameters.cpu_throttle_increment;
> +    bool pct_tailslow = s->parameters.cpu_throttle_tailslow;
>      int pct_max = s->parameters.max_cpu_throttle;
>  
> +    const uint64_t cpu_throttle_now = cpu_throttle_get_percentage();
> +    uint64_t cpu_throttle_inc;
> +
>      /* We have not started throttling yet. Let's start it. */
>      if (!cpu_throttle_active()) {
>          cpu_throttle_set(pct_initial);
>      } else {
>          /* Throttling already on, just increase the rate */
> -        cpu_throttle_set(MIN(cpu_throttle_get_percentage() + pct_icrement,
> +        if (cpu_throttle_now >= 80 && pct_tailslow) {
> +            /* Eat some scale of CPU from remaining */
> +            cpu_throttle_inc = ceil((100 - cpu_throttle_now) * 0.3);
> +            if (cpu_throttle_inc > pct_increment) {
> +                cpu_throttle_inc = pct_increment;
> +            }
> +        } else {
> +            cpu_throttle_inc = pct_increment;
> +        }
> +        cpu_throttle_set(MIN(cpu_throttle_now + cpu_throttle_inc,
>                           pct_max));
>      }
>  }
> diff --git a/monitor/hmp-cmds.c b/monitor/hmp-cmds.c
> index 558fe06b8f..b5f5c0b382 100644
> --- a/monitor/hmp-cmds.c
> +++ b/monitor/hmp-cmds.c
> @@ -416,6 +416,10 @@ void hmp_info_migrate_parameters(Monitor *mon, const QDict *qdict)
>          monitor_printf(mon, "%s: %u\n",
>              MigrationParameter_str(MIGRATION_PARAMETER_CPU_THROTTLE_INCREMENT),
>              params->cpu_throttle_increment);
> +        assert(params->has_cpu_throttle_tailslow);
> +        monitor_printf(mon, "%s: %s\n",
> +            MigrationParameter_str(MIGRATION_PARAMETER_CPU_THROTTLE_TAILSLOW),
> +            params->cpu_throttle_tailslow ? "on" : "off");
>          assert(params->has_max_cpu_throttle);
>          monitor_printf(mon, "%s: %u\n",
>              MigrationParameter_str(MIGRATION_PARAMETER_MAX_CPU_THROTTLE),
> diff --git a/qapi/migration.json b/qapi/migration.json
> index b7348d0c8b..0ac94e00f2 100644
> --- a/qapi/migration.json
> +++ b/qapi/migration.json
> @@ -532,6 +532,12 @@
>  #                          auto-converge detects that migration is not making
>  #                          progress. The default value is 10. (Since 2.7)
>  #
> +# @cpu-throttle-tailslow: Make throttle slower at tail stage
> +#                         At the tail stage of throttle, VM is very sensitive to
> +#                         CPU percentage. We just throttle 30% of remaining CPU
> +#                         when throttle is more than 80 percentage. The default
> +#                         value is false. (Since 4.1)
> +#
>  # @tls-creds: ID of the 'tls-creds' object that provides credentials for
>  #             establishing a TLS connection over the migration data channel.
>  #             On the outgoing side of the migration, the credentials must
> @@ -594,6 +600,7 @@
>             'compress-level', 'compress-threads', 'decompress-threads',
>             'compress-wait-thread',
>             'cpu-throttle-initial', 'cpu-throttle-increment',
> +           'cpu-throttle-tailslow',
>             'tls-creds', 'tls-hostname', 'tls-authz', 'max-bandwidth',
>             'downtime-limit', 'x-checkpoint-delay', 'block-incremental',
>             'multifd-channels',
> @@ -634,6 +641,12 @@
>  #                          auto-converge detects that migration is not making
>  #                          progress. The default value is 10. (Since 2.7)
>  #
> +# @cpu-throttle-tailslow: Make throttle slower at tail stage
> +#                         At the tail stage of throttle, VM is very sensitive to
> +#                         CPU percentage. We just throttle 30% of remaining CPU
> +#                         when throttle is more than 80 percentage. The default
> +#                         value is false. (Since 4.1)
> +#
>  # @tls-creds: ID of the 'tls-creds' object that provides credentials
>  #             for establishing a TLS connection over the migration data
>  #             channel. On the outgoing side of the migration, the credentials
> @@ -703,6 +716,7 @@
>              '*decompress-threads': 'int',
>              '*cpu-throttle-initial': 'int',
>              '*cpu-throttle-increment': 'int',
> +            '*cpu-throttle-tailslow': 'bool',
>              '*tls-creds': 'StrOrNull',
>              '*tls-hostname': 'StrOrNull',
>              '*tls-authz': 'StrOrNull',
> @@ -767,6 +781,12 @@
>  #                          auto-converge detects that migration is not making
>  #                          progress. (Since 2.7)
>  #
> +# @cpu-throttle-tailslow: Make throttle slower at tail stage
> +#                         At the tail stage of throttle, VM is very sensitive to
> +#                         CPU percentage. We just throttle 30% of remaining CPU
> +#                         when throttle is more than 80 percentage. The default
> +#                         value is false. (Since 4.1)
> +#
>  # @tls-creds: ID of the 'tls-creds' object that provides credentials
>  #             for establishing a TLS connection over the migration data
>  #             channel. On the outgoing side of the migration, the credentials
> @@ -836,6 +856,7 @@
>              '*decompress-threads': 'uint8',
>              '*cpu-throttle-initial': 'uint8',
>              '*cpu-throttle-increment': 'uint8',
> +            '*cpu-throttle-tailslow': 'bool',
>              '*tls-creds': 'str',
>              '*tls-hostname': 'str',
>              '*tls-authz': 'str',
> -- 
> 2.19.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK