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

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Date:   Thu, 14 Sep 2017 17:40:56 +0800
From:   Quan Xu <quan.xu0@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     Yang Zhang <yang.zhang.wz@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Radim Krcmar <rkrcmar@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Alexander Graf <agraf@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-doc@...r.kernel.org
Subject: Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support



On 2017/9/14 17:19, Wanpeng Li wrote:
> 2017-09-14 16:36 GMT+08:00 Quan Xu <quan.xu0@...il.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