2016-05-25 8:47 GMT+08:00 Wanpeng Li <kernellwp@xxxxxxxxx>:
> 2016-05-25 7:37 GMT+08:00 David Matlack <dmatlack@xxxxxxxxxx>:
>> On Tue, May 24, 2016 at 4:11 PM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>>> 2016-05-25 6:38 GMT+08:00 David Matlack <dmatlack@xxxxxxxxxx>:
>>>> On Tue, May 24, 2016 at 12:57 AM, Wanpeng Li <kernellwp@xxxxxxxxx> wrote:
>>>>> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
>>>>>
>>>>> If an emulated lapic timer will fire soon(in the scope of 10us the
>>>>> base of dynamic halt-polling, lower-end of message passing workload
>>>>> latency TCP_RR's poll time < 10us) we can treat it as a short halt,
>>>>> and poll to wait it fire, the fire callback apic_timer_fn() will set
>>>>> KVM_REQ_PENDING_TIMER, and this flag will be check during busy poll.
>>>>> This can avoid context switch overhead and the latency which we wake
>>>>> up vCPU.
>>>>>
>>>>> This feature is slightly different from current advance expiration
>>>>> way. Advance expiration rely on the vCPU is running(do polling before
>>>>> vmentry). But in some cases, the timer interrupt may be blocked by
>>>>> other thread(i.e., IF bit is clear) and vCPU cannot be scheduled to
>>>>> run immediately. So even advance the timer early, vCPU may still see
>>>>> the latency. But polling is different, it ensures the vCPU to aware
>>>>> the timer expiration before schedule out.
>>>>>
>>>>> echo HRTICK > /sys/kernel/debug/sched_features in dynticks guests.
>
>
> ^^^^^^^^^^^^^^^^^
>
>>>>>
>>>>> Context switching - times in microseconds - smaller is better
>>>>> -------------------------------------------------------------------------
>>>>> Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
>>>>> ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw
>>>>> --------- ------------- ------ ------ ------ ------ ------ ------- -------
>>>>> kernel Linux 4.6.0+ 7.9800 11.0 10.8 14.6 9.4300 13.0 10.2 vanilla
>>>>> kernel Linux 4.6.0+ 15.3 13.6 10.7 12.5 9.0000 12.8 7.38000 poll
>>>>
>>>> These results aren't very compelling. Sometimes polling is faster,
>>>> sometimes vanilla is faster, sometimes they are about the same.
Pin vCPUs get more difference.
Context switching - times in microseconds - smaller is better
-------------------------------------------------------------------------
Host OS 2p/0K 2p/16K 2p/64K 8p/16K 8p/64K 16p/16K 16p/64K
ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw ctxsw
--------- ------------- ------ ------ ------ ------ ------ ------- -------
kernel Linux 4.6.0+ 11.6 14.0 11.8 53.1 12.5 8.16000
11.4 vanilla
kernel Linux 4.6.0+ 45.8 15.1 2.3000 12.9 1.4200 14.6 4.52000 poll
Regards,
Wanpeng Li
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