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

Quan Xu
2017-09-14T08:36:11Z

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