Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support

Quan Xu
2017-09-14T09:41:10Z

On 2017/9/14 17:19, Wanpeng Li wrote:
> 2017-09-14 16:36 GMT+08:00 Quan Xu <quan.xu0@gmail.com>:
>>
>> on 2017/9/13 19:56, Yang Zhang wrote:
>>> On 2017/8/29 22:56, Michael S. Tsirkin wrote:
>>>> On Tue, Aug 29, 2017 at 11:46:34AM +0000, Yang Zhang wrote:
>>>>> Some latency-intensive workload will see obviously performance
>>>>> drop when running inside VM.
>>>>
>>>> But are we trading a lot of CPU for a bit of lower latency?
>>>>
>>>>> The main reason is that the overhead
>>>>> is amplified when running inside VM. The most cost i have seen is
>>>>> inside idle path.
>>>>>
>>>>> This patch introduces a new mechanism to poll for a while before
>>>>> entering idle state. If schedule is needed during poll, then we
>>>>> don't need to goes through the heavy overhead path.
>>>>
>>>> Isn't it the job of an idle driver to find the best way to
>>>> halt the CPU?
>>>>
>>>> It looks like just by adding a cstate we can make it
>>>> halt at higher latencies only. And at lower latencies,
>>>> if it's doing a good job we can hopefully use mwait to
>>>> stop the CPU.
>>>>
>>>> In fact I have been experimenting with exactly that.
>>>> Some initial results are encouraging but I could use help
>>>> with testing and especially tuning. If you can help
>>>> pls let me know!
>>>
>>> Quan, Can you help to test it and give result? Thanks.
>>>
>> Hi, MST
>>
>> I have tested the patch "intel_idle: add pv cstates when running on kvm" on
>> a recent host that allows guests
>> to execute mwait without an exit. also I have tested our patch "[RFC PATCH
>> v2 0/7] x86/idle: add halt poll support",
>> upstream linux, and idle=poll.
>>
>> the following is the result (which seems better than ever berfore, as I ran
>> test case on a more powerful machine):
>>
>> for __netperf__, the first column is trans. rate per sec, the second column
>> is CPU utilzation.
>>
>> 1. upstream linux
> This "upstream linux" means that disables the kvm adaptive
> halt-polling after confirm with Xu Quan.

upstream linux -- the source code is just from upstream linux, without
our patch or MST's patch..
yes, we disable kvm halt-polling(halt_poll_ns=0) for _all_of_ following
cases.

Quan

> Regards,
> Wanpeng Li
>
>> 28371.7 bits/s -- 76.6 %CPU
>>
>> 2. idle=poll
>>
>> 34372 bit/s -- 999.3 %CPU
>>
>> 3. "[RFC PATCH v2 0/7] x86/idle: add halt poll support", with different
>> values of parameter 'halt_poll_threshold':
>>
>> 28362.7 bits/s -- 74.7 %CPU (halt_poll_threshold=10000)
>> 32949.5 bits/s -- 82.5 %CPU (halt_poll_threshold=20000)
>> 39717.9 bits/s -- 104.1 %CPU (halt_poll_threshold=30000)
>> 40137.9 bits/s -- 104.4 %CPU (halt_poll_threshold=40000)
>> 40079.8 bits/s -- 105.6 %CPU (halt_poll_threshold=50000)
>>
>>
>> 4. "intel_idle: add pv cstates when running on kvm"
>>
>> 33041.8 bits/s -- 999.4 %CPU
>>
>>
>>
>>
>>
>> for __ctxsw__, the first column is the time per process context switches,
>> the second column is CPU utilzation..
>>
>> 1. upstream linux
>>
>> 3624.19 ns/ctxsw -- 191.9 %CPU
>>
>> 2. idle=poll
>>
>> 3419.66 ns/ctxsw -- 999.2 %CPU
>>
>> 3. "[RFC PATCH v2 0/7] x86/idle: add halt poll support", with different
>> values of parameter 'halt_poll_threshold':
>>
>> 1123.40 ns/ctxsw -- 199.6 %CPU (halt_poll_threshold=10000)
>> 1127.38 ns/ctxsw -- 199.7 %CPU (halt_poll_threshold=20000)
>> 1113.58 ns/ctxsw -- 199.6 %CPU (halt_poll_threshold=30000)
>> 1117.12 ns/ctxsw -- 199.6 %CPU (halt_poll_threshold=40000)
>> 1121.62 ns/ctxsw -- 199.6 %CPU (halt_poll_threshold=50000)
>>
>> 4. "intel_idle: add pv cstates when running on kvm"
>>
>> 3427.59 ns/ctxsw -- 999.4 %CPU
>>
>> -Quan