Subject: Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        wanpeng.li@hotmail.com, pbonzini@redhat.com, tglx@linutronix.de,
        rkrcmar@redhat.com, dmatlack@google.com, agraf@suse.de,
        peterz@infradead.org, linux-doc@vger.kernel.org,
        Quan Xu <quan.xu0@gmail.com>
References: <1504007201-12904-1-git-send-email-yang.zhang.wz@gmail.com>
 <20170829174147-mutt-send-email-mst@kernel.org>
From: Yang Zhang <yang.zhang.wz@gmail.com>
Message-ID: <259c95bc-3641-965b-4054-a233a6ee785c@gmail.com>
Date: Wed, 13 Sep 2017 19:56:23 +0800
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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