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

[Peter Zijlstra]

On Wed, Jun 04, 2014 at 11:32:10AM +0200, Vincent Guittot wrote:
> 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.
 
Let me explain the 75%, take any one of the above scenarios. Lets call
the two tasks A and B, and let for a moment assume A always wins and
runs first, and then B.

So A will be runnable for 25%, B otoh will be runnable the entire time A
is actually running plus its own running time, giving 50%. Together that
makes 75%.

If you release the assumption that A runs first, but instead assume they
equally win the first execution, you get them averaging at 37.5% each,
which combined will still give 75%.

> 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%

And this is because the running time is 'large' compared to the decay
and we get hit by the weight of the recent state? Yes, I can see that,
the avg will fluctuate due to the nature of this thing.

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