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Message-ID: <25e566b9-a8f4-2d90-0ba3-725f1a215c1f@gmail.com>
Date: Thu, 14 Sep 2017 16:36:00 +0800
From: Quan Xu <quan.xu0@...il.com>
To: Yang Zhang <yang.zhang.wz@...il.com>,
"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
Subject: Re: [RFC PATCH v2 0/7] x86/idle: add halt poll support
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
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
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