On Wed, Jun 04, 2014 at 09:47:26AM +0200, Vincent Guittot wrote: > On 3 June 2014 17:50, Peter Zijlstra <pet...@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.
pgpukos7kzkq6.pgp
Description: PGP signature
