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

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Date:   Fri, 1 Sep 2017 14:32:51 +0800
From:   Yang Zhang <yang.zhang.wz@...il.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Wanpeng Li <wanpeng.li@...mail.com>,
        "Michael S. Tsirkin" <mst@...hat.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 2017/8/29 22:02, Wanpeng Li wrote:
>> Here is the data we get when running benchmark netperf:
>>
>>     2. w/ patch:
>>        halt_poll_threshold=10000 -- 15803.89 bits/s -- 159.5 %CPU
>>        halt_poll_threshold=20000 -- 15899.04 bits/s -- 161.5 %CPU
>>        halt_poll_threshold=30000 -- 15642.38 bits/s -- 161.8 %CPU
>>        halt_poll_threshold=40000 -- 18040.76 bits/s -- 184.0 %CPU
>>        halt_poll_threshold=50000 -- 18877.61 bits/s -- 197.3 %CPU
>>
>>     3. kvm dynamic poll
>>        halt_poll_ns=10000 -- 15876.00 bits/s -- 172.2 %CPU
>>        halt_poll_ns=20000 -- 15602.58 bits/s -- 185.4 %CPU
>>        halt_poll_ns=30000 -- 15930.69 bits/s -- 194.4 %CPU
>>        halt_poll_ns=40000 -- 16413.09 bits/s -- 195.3 %CPU
>>        halt_poll_ns=50000 -- 16417.42 bits/s -- 196.3 %CPU
>>
> 
> Actually I'm not sure how much sense it makes to introduce this pv
> stuff and the duplicate adaptive halt-polling logic as what has
> already been done in kvm w/o obvious benefit for real workload like > netperf. In addition, as you mentioned offline to me, enable both the
> patchset and the adaptive halt-polling logic in kvm simultaneously can
> result in more cpu power consumption. I remembered that David from

If we use poll in KVM side, it will consume more cpu than in guest side. 
If use both two, then we can get the same performance as only enable 
guest side poll but it will cost more cpu because of poll KVM side. It 
means we should disable KVM side poll since it cannot give much 
improvement than  guest side except consume more cpu and large latency.

> Google mentioned that Windows Event Objects can get 2x latency
> improvement in KVM FORUM, which means that the adaptive halt-polling
> in kvm should be enabled by default. So if the windows guests and
> linux guests are mixed on the same host, then this patchset will
> result in more cpu power consumption if the customer enable the
> polling in the linux guest. Anyway, if the patchset is finally
> acceptable by maintainer, I will introduce the generic adaptive
> halt-polling framework in kvm to avoid the duplicate logic.

We will add more conditions than the current algorithm in future. But 
it's ok to use the one copy currently, we will do it in next version.


-- 
Yang
Alibaba Cloud Computing