Date:   Sun, 28 Apr 2019 19:47:24 +0000
From:   Song Liu <songliubraving@...com>
To:     Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>
CC:     linux-kernel <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH 0/7] introduce cpu.headroom knob to cpu controller

Hi Morten and Vincent,

> On Apr 22, 2019, at 6:22 PM, Song Liu <songliubraving@...com> wrote:
> 
> Hi Vincent,
> 
>> On Apr 17, 2019, at 5:56 AM, Vincent Guittot <vincent.guittot@...aro.org> wrote:
>> 
>> On Wed, 10 Apr 2019 at 21:43, Song Liu <songliubraving@...com> wrote:
>>> 
>>> Hi Morten,
>>> 
>>>> On Apr 10, 2019, at 4:59 AM, Morten Rasmussen <morten.rasmussen@....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? 
> 
> 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

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