Re: [PATCH v2 08/11] sched: get CPU's activity statistic

[Vincent Guittot]

On 4 June 2014 10:08, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote:
>> On 3 June 2014 17:50, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>> > On Wed, May 28, 2014 at 04:47:03PM +0100, Morten Rasmussen wrote:
>> >> Since we may do periodic load-balance every 10 ms or so, we will perform
>> >> a number of load-balances where runnable_avg_sum will mostly be
>> >> reflecting the state of the world before a change (new task queued or
>> >> moved a task to a different cpu). If you had have two tasks continuously
>> >> on one cpu and your other cpu is idle, and you move one of the tasks to
>> >> the other cpu, runnable_avg_sum will remain unchanged, 47742, on the
>> >> first cpu while it starts from 0 on the other one. 10 ms later it will
>> >> have increased a bit, 32 ms later it will be 47742/2, and 345 ms later
>> >> it reaches 47742. In the mean time the cpu doesn't appear fully utilized
>> >> and we might decide to put more tasks on it because we don't know if
>> >> runnable_avg_sum represents a partially utilized cpu (for example a 50%
>> >> task) or if it will continue to rise and eventually get to 47742.
>> >
>> > Ah, no, since we track per task, and update the per-cpu ones when we
>> > migrate tasks, the per-cpu values should be instantly updated.
>> >
>> > If we were to increase per task storage, we might as well also track
>> > running_avg not only runnable_avg.
>>
>> I agree that the removed running_avg should give more useful
>> information about the the load of a CPU.
>>
>> The main issue with running_avg is that it's disturbed by other tasks
>> (as point out previously). As a typical example,  if we have 2 tasks
>> with a load of 25% on 1 CPU, the unweighted runnable_load_avg will be
>> in the range of [100% - 50%] depending of the parallelism of the
>> runtime of the tasks whereas the reality is 50% and the use of
>> running_avg will return this value
>
> I'm not sure I see how 100% is possible, but yes I agree that runnable
> can indeed be inflated due to this queueing effect.

In fact, it can be even worse than that because i forgot to take into
account the geometric series effect which implies that it depends of
the runtime (idletime) of the task

Take 3 examples:

2 tasks that need to run 10ms  simultaneously each 40ms. If they share
the same CPU, they will be on the runqueue 20ms (in fact a bit less
for one of them), Their load (runnable_avg_sum/runnable_avg_period)
will be 33% each so the unweighted runnable_load_avg of the CPU will
be 66%

2 tasks that need to run 25ms simultaneously each 100ms. If they share
the same CPU, they will be on the runqueue 50ms (in fact a bit less
for one of them), Their load (runnable_avg_sum/runnable_avg_period)
will be 74% each so the unweighted runnable_load_avg of the CPU will
be 148%

2 tasks that need to run 50ms  simultaneously each 200ms. If they
share the same CPU, they will be on the runqueue 100ms (in fact a bit
less for one of them), Their load
(runnable_avg_sum/runnable_avg_period) will be 89% each so the
unweighted runnable_load_avg of the CPU will be 180%

Vincent
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