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

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Message-ID: <259c95bc-3641-965b-4054-a233a6ee785c@gmail.com>
Date:   Wed, 13 Sep 2017 19:56:23 +0800
From:   Yang Zhang <yang.zhang.wz@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        wanpeng.li@...mail.com, pbonzini@...hat.com, tglx@...utronix.de,
        rkrcmar@...hat.com, dmatlack@...gle.com, agraf@...e.de,
        peterz@...radead.org, linux-doc@...r.kernel.org,
        Quan Xu <quan.xu0@...il.com>
Subject: Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support

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.

-- 
Yang
Alibaba Cloud Computing