On 4 June 2014 10:08, Peter Zijlstra <[email protected]> wrote: > On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote: >> On 3 June 2014 17:50, Peter Zijlstra <[email protected]> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
