Re: [patch v5 09/15] sched: add power aware scheduling in fork/exec/wake

From: Alex Shi
Date: Sun Feb 24 2013 - 21:23:39 EST

On 02/25/2013 01:51 AM, Preeti U Murthy wrote:
> Hi,
> On 02/24/2013 02:57 PM, Alex Shi wrote:
>> On 02/22/2013 04:54 PM, Peter Zijlstra wrote:
>>> On Thu, 2013-02-21 at 22:40 +0800, Alex Shi wrote:
>>>>> The name is a secondary issue, first you need to explain why you
>>>> think
>>>>> nr_running is a useful metric at all.
>>>>> You can have a high nr_running and a low utilization (a burst of
>>>>> wakeups, each waking a process that'll instantly go to sleep again),
>>>> or
>>>>> low nr_running and high utilization (a single process cpu bound
>>>>> process).
>>>> It is true in periodic balance. But in fork/exec/waking timing, the
>>>> incoming processes usually need to do something before sleep again.
>>> You'd be surprised, there's a fair number of workloads that have
>>> negligible runtime on wakeup.
>> will appreciate if you like introduce some workload. :)
>> BTW, do you has some idea to handle them?
>> Actually, if tasks is just like transitory, it is also hard to catch
>> them in balance, like 'cyclitest -t 100' on my 4 LCPU laptop, vmstat
>> just can catch 1 or 2 tasks very second.
>>>> I use nr_running to measure how the group busy, due to 3 reasons:
>>>> 1, the current performance policy doesn't use utilization too.
>>> We were planning to fix that now that its available.
>> I had tried, but failed on aim9 benchmark. As a result I give up to use
>> utilization in performance balance.
>> Some trying and talking in the thread.
>>>> 2, the power policy don't care load weight.
>>> Then its broken, it should very much still care about weight.
>> Here power policy just use nr_running as the criteria to check if it's
>> eligible for power aware balance. when do balancing the load weight is
>> still the key judgment.
>>>> 3, I tested some benchmarks, kbuild/tbench/hackbench/aim7 etc, some
>>>> benchmark results looks clear bad when use utilization. if my memory
>>>> right, the hackbench/aim7 both looks bad. I had tried many ways to
>>>> engage utilization into this balance, like use utilization only, or
>>>> use
>>>> utilization * nr_running etc. but still can not find a way to recover
>>>> the lose. But with nr_running, the performance seems doesn't lose much
>>>> with power policy.
>>> You're failing to explain why utilization performs bad and you don't
>>> explain why nr_running is better. That things work simply isn't good
>> Um, let me try to explain again, The utilisation need much time to
>> accumulate itself(345ms). Whenever with or without load weight, many
>> bursting tasks just give a minimum weight to the carrier CPU at the
>> first few ms. So, it is too easy to do a incorrect distribution here and
>> need migration on later periodic balancing.
> Why can't this be attacked in *either* of the following ways:
> 1.Attack this problem at the source, by ensuring that the utilisation is
> accumulated faster by making the update window smaller.

It is a double blade sword. Small period will response quickly, but
loses lots of history record. A extreme short period is just same as
current instant utilization.
> 2.Balance on nr->running only if you detect burst wakeups.
> Alex, you had released a patch earlier which could detect this right?

Yes, the patch is here:

One of problem is the how to decide the criteria of the burst? If we set
5 waking up/ms is burst, we will lose 4 waking up/ms.
another problem is the burst detection cost, we need tracking a period
history info of the waking up, better on whole group. but that give the
extra cost in burst.

solution candidates:
After talk with MikeG, I remove the runnable load avg in performance
load balance.

Using nr_running as instant utilization may narrow the power policy
suitable situation. -- consider for power consumption, a light but cpu
intensive task will cost much more power than a heavy load but run
occasionally task. And it fit all my benchmarks
aim7/hackbench/kbuild/cyclitest/netperf etc.

> Instead of balancing on nr_running all the time, why not balance on it
> only if burst wakeups are detected. By doing so you ensure that
> nr_running as a metric for load balancing is used when it is right to do
> so and the reason to use it also gets well documented.
> Regards
> Preeti U Murthy

Thanks Alex
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