On 26 April 2013 12:30, Peter Zijlstra peterz@infradead.org wrote:
On Wed, Mar 27, 2013 at 12:00:40PM +0100, Vincent Guittot wrote:
On 27 March 2013 11:21, Preeti U Murthy preeti@linux.vnet.ibm.com wrote:
Hi,
On 03/26/2013 05:56 PM, Peter Zijlstra wrote:
On Fri, 2013-03-22 at 13:25 +0100, Vincent Guittot wrote:
+static bool is_buddy_busy(int cpu) +{
struct rq *rq = cpu_rq(cpu);/** A busy buddy is a CPU with a high load or a small load witha lot of
* running tasks.*/return (rq->avg.runnable_avg_sum >(rq->avg.runnable_avg_period / (rq->nr_running- 2)));
+}
Why does the comment talk about load but we don't see it in the equation. Also, why does nr_running matter at all? I thought we'd simply bother with utilization, if fully utilized we're done etc..
By load, I mean : 100 * avg.runnable_avg_sum / avg.runnable_avg_period In addition, i take into account the number of tasks already in the runqueue in order to define the business of a CPU. A CPU with a load of 50% without any tasks in the runqeue in not busy at this time and we can migrate tasks on it but if the CPU already has 2 tasks in its runqueue, it means that newly wake up task will have to share the CPU with other tasks so we consider that the CPU is already busy and we will fall back to default behavior. The equation considers that a CPU is not busy if 100 * avg.runnable_avg_sum / avg.runnable_avg_period < 100 / (nr_running + 2)
I'm still somewhat confused by all this. So raising nr_running will lower the required utilization to be considered busy. Suppose we have 50 tasks running, all at 1% utilization (bulk wakeup) we'd consider the cpu busy, even though its picking its nose for half the time.
I'm assuming it's mean to limit process latency or so? Why are you concerned with that? This seems like an arbitrary power vs performance tweak without solid effidence its needed or even wanted.
Yes the goal was to limit the wake up latency because this version was only trying to modify the scheduler behavior when the system was not busy in order to pack the small tasks like background activities but without decreasing the performance so we were concerned by wakeup latency.
The new version proposes a more aggressive mode that packs all tasks until CPUs becomes full.
Vincent