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

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Date:   Tue, 29 Aug 2017 22:02:15 +0800
From:   Wanpeng Li <kernellwp@...il.com>
To:     Yang Zhang <yang.zhang.wz@...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

> 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
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.

Regards,
Wanpeng Li