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From: Wanpeng Li <kernellwp@gmail.com>
Newsgroups: linux.kernel
Subject: Re: [PATCH v4] KVM: halt-polling: poll for the upcoming fire timers
Date: Wed, 25 May 2016 03:30:02 +0200
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2016-05-25 8:47 GMT+08:00 Wanpeng Li <kernellwp@gmail.com>:
> 2016-05-25 7:37 GMT+08:00 David Matlack <dmatlack@google.com>:
>> On Tue, May 24, 2016 at 4:11 PM, Wanpeng Li <kernellwp@gmail.com> wrote:
>>> 2016-05-25 6:38 GMT+08:00 David Matlack <dmatlack@google.com>:
>>>> On Tue, May 24, 2016 at 12:57 AM, Wanpeng Li <kernellwp@gmail.com> wrote:
>>>>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>>>>>
>>>>> 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