Re: [Qemu-devel] [PATCH v5 3/5] migration: Dynamic cpu throttling for auto-converge

["Jason J. Herne" <jjherne@linux.vnet.ibm.com>]

On 09/01/2015 05:30 PM, Eric Blake wrote:
> On 09/01/2015 08:46 AM, Jason J. Herne wrote:
>> Remove traditional auto-converge static 30ms throttling code and replace it
>> with a dynamic throttling algorithm.
>>
>> Additionally, be more aggressive when deciding when to start throttling.
>> Previously we waited until four unproductive memory passes. Now we begin
>> throttling after only two unproductive memory passes. Four seemed quite
>> arbitrary and only waiting for two passes allows us to complete the migration
>> faster.
>>
>> Signed-off-by: Jason J. Herne <jjherne@linux.vnet.ibm.com>
>> Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
>> ---
>>   arch_init.c           | 88 ++++++++++++++++++---------------------------------
>>   migration/migration.c |  4 +++
>>   2 files changed, 34 insertions(+), 58 deletions(-)
>
>> +static void mig_throttle_guest_down(void)
>> +{
>> +    MigrationState *s = migrate_get_current();
>> +    uint64_t pct_initial =
>> +            s->parameters[MIGRATION_PARAMETER_X_CPU_THROTTLE_INITIAL];
>> +    uint64_t pct_icrement =
>> +            s->parameters[MIGRATION_PARAMETER_X_CPU_THROTTLE_INCREMENT];
>> +
>> +    /* 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(cpu_throttle_get_percentage() + pct_icrement);
>
> What happens if the addition results in a percentage larger than 99?
>

cpu_throttle_set takes responsibility for upper/lower bounds capping.

+void cpu_throttle_set(int new_throttle_pct)
+{
+    /* Ensure throttle percentage is within valid range */
+    new_throttle_pct = MIN(new_throttle_pct, CPU_THROTTLE_PCT_MAX);
+    new_throttle_pct = MAX(new_throttle_pct, CPU_THROTTLE_PCT_MIN);
+
+    atomic_set(&throttle_percentage, new_throttle_pct);
+
+    timer_mod(throttle_timer, qemu_clock_get_ms(QEMU_CLOCK_VIRTUAL_RT) +
+                                       CPU_THROTTLE_TIMESLICE);
+}
+

-- 
-- Jason J. Herne (jjherne@linux.vnet.ibm.com)