Messages in this thread | | | Date | Wed, 4 Jun 2014 12:17:24 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 08/11] sched: get CPU's activity statistic |
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On Wed, Jun 04, 2014 at 11:32:10AM +0200, Vincent Guittot wrote: > On 4 June 2014 10:08, Peter Zijlstra <peterz@infradead.org> wrote: > > On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote: > >> On 3 June 2014 17:50, Peter Zijlstra <peterz@infradead.org> 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. [unhandled content-type:application/pgp-signature] | |