Hi Dr.David,
Sorry for the reply delay, just come back from holiday.
On 2020/4/30 22:12, Dr. David Alan Gilbert wrote:
> * Keqian Zhu (zhukeqi...@huawei.com) wrote:
>> At the tail stage of throttling, the Guest is very sensitive to
>> CPU percentage while the @cpu-throttle-increment is excessive
>> usually at tail stage.
[...]
>> -static void mig_throttle_guest_down(void)
>> +static void mig_throttle_guest_down(uint64_t bytes_dirty_period,
>> + uint64_t bytes_dirty_threshold)
>> {
>> 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;
>>
>> + uint64_t throttle_now = cpu_throttle_get_percentage();
>> + uint64_t cpu_now, cpu_ideal, 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,
>> - pct_max));
>> + if (!pct_tailslow) {
>> + throttle_inc = pct_increment;
>> + } else {
>> + /* Compute the ideal CPU percentage used by Guest, which may
>> + * make the dirty rate match the dirty rate threshold. */
>> + cpu_now = 100 - throttle_now;
>> + cpu_ideal = cpu_now * (bytes_dirty_threshold * 1.0 /
>> + bytes_dirty_period);
>
> I worry if we need a divide-by-0 check; but that seems unlikely.
mig_throttle_guest_down is called when bytes_dirty_period is bigger than
bytes_dirty_threshold, and bytes_dirty_threshold is of unsigned type, so
bytes_dirty_period will not be zero here. I will add an assert check here
to make it clear.
> Now if that worked out as huge, then I think the MIN's guard it even
> with overflow below, so I think we're OK.
Yes, it will not exceed legacy increment.
>
>> + throttle_inc = MIN(cpu_now - cpu_ideal, pct_increment);
>> + }
>> + cpu_throttle_set(MIN(throttle_now + throttle_inc, pct_max));
>> }
>> }
>>
[...]
>> --
>> 2.19.1
>>
> --
> Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
>
Thanks,
Keqian
>
> .
>
