RE: [RFT][PATCH v5 0/7] sched/cpuidle: Idle loop rework

[Doug Smythies]

On 2018.03.20 23:33 Rafael J. Wysocki wrote:
> On Tuesday, March 20, 2018 10:03:50 PM CET Doug Smythies wrote:
>> Summary: My results with kernel 4.16-rc6 and V8 of the patch set
>> are completely different, and now show no clear difference
>> (a longer test might reveal something). 
>
> Does this mean that you see the "powernightmares" pattern with the v8
> again or are you referring to something else?

Sorry for not being clear.
I do not see any "powernightmares" at all with V8.

After this e-mail I did a 3 hour trace and saw none.

>> On 2018.03.20 10:16 Doug Smythies wrote:
>>> On 2018.03.20 03:02 Thomas Ilsche wrote:
>>>
>>>...[snip]...
>>>
>>>> Consider the Skylake server system which has residencies in C1E of
>>>> 20 us and C6 of 800 us. I use a small while(1) {usleep(300);}
>>>> unsynchronized pinned to each core. While this is an artificial
>>>> case, it is a very innocent one - easy to predict and regular. Between
>>>> vanilla 4.16.0-rc5 and idle-loop/v6, the power consumption increases
>>>> from 149.7 W to 158.1 W. On 4.16.0-rc5, the cores sleep almost
>>>> entirely in C1E. With the patches applied, the cores spend ~75% of
>>>> their sleep time in C6, ~25% in C1E. The average time/usage for C1E is
>>>> also lower with v6 at ~350 us rather than the ~550 us in C6 (and in
>>>> C1E with the baseline). Generally the new menu governor seems to chose
>>>> C1E if the next timer is an enabled sched timer - which occasionally
>>>> interrupts the sleep-interval into two C1E sleeps rather than one C6.
>>>>
>>>> Manually disabling C6, reduces power consumption back to 149.5 W.
>>>
>>> ...[snip]...
>>>
>>> Note that one of the tests that I normally do is a work/sleep
>>> frequency sweep from 100 to 2100 Hz, typically at a lowish
>>> workload. I didn't notice anything odd with this test:
>>> 
>>> http://fast.smythies.com/rjw_freq_sweep.png
>
> Would it be possible to produce this graph with the v8 of the
> patchset?

Yes, sure.

>>> However, your test is at 3333 Hz (well, minus overheads).
>>> I did the same as you. And was surprised to confirm
>>> your power findings. In my case package power goes from
>>> ~8.6 watts to ~7.3 watts with idle state 4 (C6) disabled.
>>>
>>> I am getting different residency times than you though.
>>> I also observe different overheads between idle state 4
>>> being disabled or not. i.e. my actual loop frequency
>>> drops from ~2801 Hz to ~2754 Hz.
>>>
>>> Example residencies over the previous minute:
>>>
>>> Idle state 4 (C6) disabled (seconds):
>>>
>>> Idle state 0: 0.001119
>>> Idle state 1: 0.056638
>>> Idle state 2: 13.100550
>>> Idle state 3: 446.266744
>>> Idle state 4: 0.000000
>>>
>>> Idle state 4 (C6) enabled (seconds):
>>>
>>> Idle state 0: 0.034502
>>> Idle state 1: 1.949595
>>> Idle state 2: 78.291793
>>> Idle state 3: 96.467974
>>> Idle state 4: 286.247524
>>
>> Now, with kernel 4.16-rc6 and V8 of the patch set and the poll fix
>> I am unable to measure the processor package power difference
>> between idle state 0 enabled or disabled (i.e. it is in the noise).
>> also the loop time changes (overhead changes) are minimal. However,
>> the overall loop time has dropped to ~2730 Hz, so there seems to be
>> a little more overhead in general.
>>
>> I increased my loop frequency to ~3316 Hz. Similar.
>> 
>> I increased my loop frequency to ~15474 Hz. Similar.
>> Compared to a stock 4.16-rc6 kernel: The loop rate dropped
>> to 15,209 Hz and it (the stock kernel) used about 0.3 more
>> watts (out of 10.97, or ~3% more).
>
> So do you prefer v6 or v8?  I guess the former?

Again sorry for not being clear.
I was saying that V8 is great.

I did more tests after the original e-mail was sent,
and the noted slight overhead drop was not always there
(i.e. it was inconsistent).

... Doug