On 4 June 2014 11:23, Peter Zijlstra <[email protected]> wrote: > On Wed, Jun 04, 2014 at 09:55:42AM +0100, Morten Rasmussen wrote: >> Both running_avg and runnable_avg are affected by other tasks on the >> same cpus, but in different ways. They are equal if you only have one >> task on a cpu. If you have more, running_avg will give you the true >> requirement of the tasks until the cpu is fully utilized. At which point >> the task running_avg will drop if you add more tasks (the unweighted sum >> of task running_avgs remains constant). >> >> runnable_avg on the other hand, might be affected as soon as you have >> two task running on the same cpu if they are runnable at the same time. >> That isn't necessarily a bad thing for load-balancing purposes, because >> tasks that are runnable at the same time are likely to be run more >> efficiently by placing them on different cpus. You might view as at sort >> of built in concurrency factor, somewhat similar to what Yuyang is >> proposing. runnable_avg increases rapidly when the cpu is over-utilized. > > Agreed. > >> > 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. >> >> You should only be able to get to 75% worst case for runnable_avg for >> that example. The total running_avg is 50% no matter if the tasks >> overlaps or not. > > Yes, 75% is what I ended up with.
Can you explain how you reach 75% as it depends on the runtime and a runtime longer than 345ms will end to a 100% load whatever the idletime was previously ? > >> f you had five tasks on one cpu that each have a 25% requirement you can >> get individual task runnable_avgs of up to 100% (cpu unweighted >> runnable_load_avg can get up 500%, I think), but the task running_avgs >> would be 20% each (total of 100%). > > Yeah, more or less so indeed. I had not considered the queueing effects > on runnable_avg yesterday, so good that that got raised. > > That does indeed invalidate my: runnable - running := extra cpu required > thing. It ends up being the extra cpu required for 0 latency but gobs of > idle time, which is something else entirely. -- 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/
