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

[Preeti U Murthy <preeti@linux.vnet.ibm.com>]

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.
> https://lkml.org/lkml/2013/1/6/96
> https://lkml.org/lkml/2013/1/22/662
>>
>>> 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.

2.Balance on nr->running only if you detect burst wakeups.
Alex, you had released a patch earlier which could detect this right?
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