Re: [PATCH v3 17/22] sched: packing small tasks in wake/exec balancing

[Alex Shi]

On 01/15/2013 01:00 AM, Morten Rasmussen wrote:
>>> Why multiply rq->util by nr_running?
>>> > > 
>>> > > Let's take an example where rq->util = 50, nr_running = 2, and putil =
>>> > > 10. In this case the value of putil doesn't really matter as vacancy
>>> > > would be negative anyway since FULL_UTIL - rq->util * nr_running is -1.
>>> > > However, with rq->util = 50 there should be plenty of spare cpu time to
>>> > > take another task.
>> > 
>> > for this example, the util is not full maybe due to it was just wake up,
>> > it still is possible like to run full time. So, I try to give it the
>> > large guess load.
> I don't see why rq->util should be treated different depending on the
> number of tasks causing the load. rq->util = 50 means that the cpu is
> busy about 50% of the time no matter how many tasks contibute to that
> load.
> 
> If nr_running = 1 instead in my example, you would consider the cpu
> vacant if putil = 6, but if nr_running > 1 you would not. Why should the
> two scenarios be treated differently?
> 
>>> > > 
>>> > > Also, why multiply putil by 8? rq->util must be very close to 0 for
>>> > > vacancy to be positive if putil is close to 12 (12.5%).
>> > 
>> > just want to pack small util tasks, since packing is possible to hurt
>> > performance.
> I agree that packing may affect performance. But why don't you reduce
> FULL_UTIL instead of multiplying by 8? With current expression you will
> not pack a 10% task if rq->util = 20 and nr_running = 1, but you would
> pack a 6% task even if rq->util = 50 and the resulting cpu load is much
> higher.
> 

Yes, the threshold has no strong theory or experiment support. I had
tried cyclitest which Vicent used, the case's load avg is too small to
be caught. so just use half of Vicent value as 12.5%. If you has more
reasonable value, let me know.

As to nr_running engaged as multiple mode. it's base on 2 reasons.
1, load avg/util need 345ms to accumulate as 100%. so, if a tasks is
cost full cpu time, it still has 345ms with rq->util < 1.
2, if there are more tasks, like 2 tasks running on one cpu, it's
possible to has capacity to burn 200% cpu time, while the biggest
rq->util is still 100%.

Consider to figure out precise utils is complicate and cost much. I do
this simple calculation. It is not very precise, but it is efficient and
more bias toward performance.

-- 
Thanks Alex
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