On Wed, Jun 04, 2014 at 12:07:29PM +0100, Vincent Guittot wrote:
On 4 June 2014 12:36, Morten Rasmussen morten.rasmussen@arm.com wrote:
On Wed, Jun 04, 2014 at 11:17:24AM +0100, Peter Zijlstra wrote:
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%.
But that is assuming that the first task gets to run to completion of it busy period. If it uses up its sched_slice and we switch to the other tasks, they both get to wait.
For example, if the sched_slice is 5 ms and the busy period is 10 ms, the execution pattern would be: A, B, A, B, idle, ... In that case A is runnable for 15 ms and B is for 20 ms. Assuming that the overall period is 40 ms, the A runnable is 37.5% and B is 50%.
The exact value for your scheduling example above is: A runnable will be 47% and B runnable will be 60% (unless i make a mistake in my computation)
I get:
A: 15/40 ms = 37.5% B: 20/40 ms = 50%
Schedule:
| 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | 5 ms | A: run rq run ----------- sleeping ------------- run B: rq run rq run ---- sleeping ------------- rq
and CPU runnable will be 60% too
rq->avg.runnable_avg_sum should be 50%. You have two tasks running for 20 ms every 40 ms.
Right?
Morten