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

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Date:   Thu, 14 Sep 2017 17:19:59 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Quan Xu <quan.xu0@...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

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.

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