RE: Performance of low-cpu utilisation benchmark regressed severely since 4.6

[Doug Smythies]

On 2017.04.22 14:08 Rafael wrote:
> On Friday, April 21, 2017 11:29:06 PM Doug Smythies wrote:
>> On 2017.04.20 18:18 Rafael wrote:
>>> On Thursday, April 20, 2017 07:55:57 AM Doug Smythies wrote:
>>>> On 2017.04.19 01:16 Mel Gorman wrote:
>>>>> On Fri, Apr 14, 2017 at 04:01:40PM -0700, Doug Smythies wrote:
>>>>>> Hi Mel,
>>>
>>> [cut]
>>>
>>>>> And the revert does help albeit not being an option for reasons Rafael
>>>>> covered.
>>>> 
>>>> New data point: Kernel 4.11-rc7  intel_pstate, powersave forcing the
>>>> load based algorithm: Elapsed 3178 seconds.
>>>> 
>>>> If I understand your data correctly, my load based results are the opposite of yours.
>>>> 
>>>> Mel: 4.11-rc5 vanilla: Elapsed mean: 3750.20 Seconds
>>>> Mel: 4.11-rc5 load based: Elapsed mean: 2503.27 Seconds
>>>> Or: 33.25%
>>>> 
>>>> Doug: 4.11-rc6 stock: Elapsed total (5 runs): 2364.45 Seconds
>>>> Doug: 4.11-rc7 force load based: Elapsed total (5 runs): 3178 Seconds
>>>> Or: -34.4%
>>>
>>> I wonder if you can do the same thing I've just advised Mel to do.  That is,
>>> take my linux-next branch:
>>>
>>> git://git.kernel.org/pub/scm/linux/kernel/git/rafael/linux-pm.git linux-next
>>>
>>> (which is new material for 4.12 on top of 4.11-rc7) and reduce
>>> INTEL_PSTATE_DEFAULT_SAMPLING_INTERVAL (in intel_pstate.c) in it by 1/2
>>> (force load-based if need be, I'm not sure what PM profile of your test system
>>> is).
>> 
>> I did not need to force load-based. I do not know how to figure it out from
>> an acpidump the way Srinivas does. I did a trace and figured out what algorithm
>> it was using from the data.
>> 
>> Reference test, before changing INTEL_PSTATE_DEFAULT_SAMPLING_INTERVAL:
>> 3239.4 seconds.
>> 
>> Test after changing INTEL_PSTATE_DEFAULT_SAMPLING_INTERVAL:
>> 3195.5 seconds.
>
> So it does have an effect, but relatively small.

I don't know how repeatable the tests results are.
i.e. I don't know if the 1.36% change is within experimental
error or not. That being said, the trend does seem consistent.

> I wonder if further reducing INTEL_PSTATE_DEFAULT_SAMPLING_INTERVAL to 2 ms
> will make any difference.

I went all the way to 1 ms, just for the test:
3123.9 Seconds

>> By far, and with any code, I get the fastest elapsed time, of course next
>> to performance mode, but not by much, by limiting the test to only use
>> just 1 cpu: 1814.2 Seconds.
>
> Interesting.
>
> It looks like the cost is mostly related to moving the load from one CPU to
> another and waiting for the new one to ramp up then.
>
> I guess the workload consists of many small tasks that each start on new CPUs
> and cause that ping-pong to happen.

Yes, and (from trace data) many tasks are very very very small. Also the test
appears to take a few holidays, of up to 1 second, during execution.

>> (performance governor, restated from a previous e-mail: 1776.05 seconds)
>
> But that causes the processor to stay in the maximum sustainable P-state all
> the time, which on Sandy Bridge is quite costly energetically.

Agreed. I only provide these data points as a reference and so that we know
what the boundary conditions (limits) are.

> We can do one more trick I forgot about.  Namely, if we are about to increase
> the P-state, we can jump to the average between the target and the max
> instead of just the target, like in the appended patch (on top of linux-next).
>
> That will make the P-state selection really aggressive, so costly energetically,
> but it shoud small jumps of the average load above 0 to case big jumps of
> the target P-state.

I'm already seeing the energy costs of some of this stuff.
3050.2 Seconds.
Idle power 4.06 Watts.

Idle power for kernel 4.11-rc7 (performance-based): 3.89 Watts.
Idle power for kernel 4.11-rc7, using load-based: 4.01 watts
Idle power for kernel 4.11-rc7 next linux-pm: 3.91 watts