Re: [PATCH 0/7] introduce cpu.headroom knob to cpu controller

[Song Liu <songliubraving@fb.com>]

> On Apr 29, 2019, at 8:24 AM, Vincent Guittot <vincent.guittot@linaro.org> wrote:
> 
> Hi Song,
> 
> On Sun, 28 Apr 2019 at 21:47, Song Liu <songliubraving@fb.com> wrote:
>> 
>> Hi Morten and Vincent,
>> 
>>> On Apr 22, 2019, at 6:22 PM, Song Liu <songliubraving@fb.com> wrote:
>>> 
>>> Hi Vincent,
>>> 
>>>> On Apr 17, 2019, at 5:56 AM, Vincent Guittot <vincent.guittot@linaro.org> wrote:
>>>> 
>>>> On Wed, 10 Apr 2019 at 21:43, Song Liu <songliubraving@fb.com> wrote:
>>>>> 
>>>>> Hi Morten,
>>>>> 
>>>>>> On Apr 10, 2019, at 4:59 AM, Morten Rasmussen <morten.rasmussen@arm.com> wrote:
>>>>>> 
>>>> 
>>>>>> 
>>>>>> The bit that isn't clear to me, is _why_ adding idle cycles helps your
>>>>>> workload. I'm not convinced that adding headroom gives any latency
>>>>>> improvements beyond watering down the impact of your side jobs. AFAIK,
>>>>> 
>>>>> We think the latency improvements actually come from watering down the
>>>>> impact of side jobs. It is not just statistically improving average
>>>>> latency numbers, but also reduces resource contention caused by the side
>>>>> workload. I don't know whether it is from reducing contention of ALUs,
>>>>> memory bandwidth, CPU caches, or something else, but we saw reduced
>>>>> latencies when headroom is used.
>>>>> 
>>>>>> the throttling mechanism effectively removes the throttled tasks from
>>>>>> the schedule according to a specific duty cycle. When the side job is
>>>>>> not throttled the main workload is experiencing the same latency issues
>>>>>> as before, but by dynamically tuning the side job throttling you can
>>>>>> achieve a better average latency. Am I missing something?
>>>>>> 
>>>>>> Have you looked at your distribution of main job latency and tried to
>>>>>> compare with when throttling is active/not active?
>>>>> 
>>>>> cfs_bandwidth adjusts allowed runtime for each task_group each period
>>>>> (configurable, 100ms by default). cpu.headroom logic applies gentle
>>>>> throttling, so that the side workload gets some runtime in every period.
>>>>> Therefore, if we look at time window equal to or bigger than 100ms, we
>>>>> don't really see "throttling active time" vs. "throttling inactive time".
>>>>> 
>>>>>> 
>>>>>> I'm wondering if the headroom solution is really the right solution for
>>>>>> your use-case or if what you are really after is something which is
>>>>>> lower priority than just setting the weight to 1. Something that
>>>>> 
>>>>> The experiments show that, cpu.weight does proper work for priority: the
>>>>> main workload gets priority to use the CPU; while the side workload only
>>>>> fill the idle CPU. However, this is not sufficient, as the side workload
>>>>> creates big enough contention to impact the main workload.
>>>>> 
>>>>>> (nearly) always gets pre-empted by your main job (SCHED_BATCH and
>>>>>> SCHED_IDLE might not be enough). If your main job consist
>>>>>> of lots of relatively short wake-ups things like the min_granularity
>>>>>> could have significant latency impact.
>>>>> 
>>>>> cpu.headroom gives benefits in addition to optimizations in pre-empt
>>>>> side. By maintaining some idle time, fewer pre-empt actions are
>>>>> necessary, thus the main workload will get better latency.
>>>> 
>>>> I agree with Morten's proposal, SCHED_IDLE should help your latency
>>>> problem because side job will be directly preempted unlike normal cfs
>>>> task even lowest priority.
>>>> In addition to min_granularity, sched_period also has an impact on the
>>>> time that a task has to wait before preempting the running task. Also,
>>>> some sched_feature like GENTLE_FAIR_SLEEPERS can also impact the
>>>> latency of a task.
>>>> 
>>>> It would be nice to know if the latency problem comes from contention
>>>> on cache resources or if it's mainly because you main load waits
>>>> before running on a CPU
>>>> 
>>>> Regards,
>>>> Vincent
>>> 
>>> Thanks for these suggestions. Here are some more tests to show the impact
>>> of scheduler knobs and cpu.headroom.
>>> 
>>> side-load | cpu.headroom | side/cpu.weight | min_gran | cpu-idle | main/latency
>>> --------------------------------------------------------------------------------
>>> none    |      0       |     n/a         |    1 ms  |  45.20%  |   1.00
>>> ffmpeg   |      0       |      1          |   10 ms  |   3.38%  |   1.46
>>> ffmpeg   |      0       |   SCHED_IDLE    |    1 ms  |   5.69%  |   1.42
>>> ffmpeg   |    20%       |   SCHED_IDLE    |    1 ms  |  19.00%  |   1.13
>>> ffmpeg   |    30%       |   SCHED_IDLE    |    1 ms  |  27.60%  |   1.08
>>> 
>>> In all these cases, the main workload is loaded with same level of
>>> traffic (request per second). Main workload latency numbers are normalized
>>> based on the baseline (first row).
>>> 
>>> For the baseline, the main workload runs without any side workload, the
>>> system has about 45.20% idle CPU.
>>> 
>>> The next two rows compare the impact of scheduling knobs cpu.weight and
>>> sched_min_granularity. With cpu.weight of 1 and min_granularity of 10ms,
>>> we see a latency of 1.46; with SCHED_IDLE and min_granularity of 1ms, we
>>> see a latency of 1.42. So SCHED_IDLE and min_granularity help protecting
>>> the main workload. However, it is not sufficient, as the latency overhead
>>> is high (>40%).
>>> 
>>> The last two rows show the benefit of cpu.headroom. With 20% headroom,
>>> the latency is 1.13; while with 30% headroom, the latency is 1.08.
>>> 
>>> We can also see a clear correlation between latency and global idle CPU:
>>> more idle CPU yields better lower latency.
>>> 
>>> Over all, these results show that cpu.headroom provides effective
>>> mechanism to control the latency impact of side workloads. Other knobs
>>> could also help the latency, but they are not as effective and flexible
>>> as cpu.headroom.
>>> 
>>> Does this analysis address your concern?
> 
> So, you results show that sched_idle class doesn't provide the
> intended behavior because it still delay the scheduling of sched_other
> tasks. In fact, the wakeup path of the scheduler doesn't make any
> difference between a cpu running a sched_other and a cpu running a
> sched_idle when looking for the idlest cpu and it can create some
> contentions between sched_other tasks whereas a cpu runs sched_idle
> task.

I don't think scheduling delay is the only (or dominating) factor of 
extra latency. Here are some data to show it. 

I measured IPC (instructions per cycle) of the main workload under 
different scenarios:

side-load | cpu.headroom | side/cpu.weight  | IPC   
----------------------------------------------------
 none     |     0%       |       N/A        | 0.66 
 ffmpeg   |     0%       |    SCHED_IDLE    | 0.53 
 ffmpeg   |    20%       |    SCHED_IDLE    | 0.58 
 ffmpeg   |    30%       |    SCHED_IDLE    | 0.62 

These data show that the side workload has a negative impact on the 
main workload's IPC. And cpu.headroom could help reduce this impact.  

Therefore, while optimizations in the wakeup path should help the 
latency; cpu.headroom would add _significant_ benefit on top of that. 

Does this assessment make sense? 


> Viresh (cced to this email) is working on improving such behavior at
> wake up and has sent an patch related to the subject:
> https://lkml.org/lkml/2019/4/25/251
> I'm curious if this would improve the results.

I could try it with our workload next week (I am at LSF/MM this 
week). Also, please keep in mind that this test sometimes takes 
multiple days to setup and run. 

Thanks,
Song

> 
> Regards,
> Vincent
> 
>>> 
>>> Thanks,
>>> Song
>>> 
>> 
>> Could you please share your comments and suggestions on this work? Did
>> the results address your questions/concerns?
>> 
>> Thanks again,
>> Song
>> 
>>>> 
>>>>> 
>>>>> Thanks,
>>>>> Song
>>>>> 
>>>>>> 
>>>>>> Morten