CFS Scheduler : Period : for NCPUs : Code Suggestion Change

CFS Scheduler : Period : for NCPUs : Code Suggestion Change

[Mitchell Erblich]

        This is NOT A PATCH.

	PLEASE include my email in the reply as I am not currently on
	the linux kernel mail alias.

	This code snap is grabbed from what is believed to be a semi-current
	OS source (fxr.watson.org) comparison webpage.

	Upon a quick CFS scheduler code walk, increasing the period
	should ALSO be dependent on the number of online/active CPUs.

	The period should be adjusted based on the number of
	online CPUs. This change allows NCPUs * tasks without
	changing/increasing the period.

	On first thought NR_CPUS should give the number of cpu on
	the system, however, this may be different from the number of
	CPUs online, thus..

	Change #1: place after line 425
	int cpu, ncpu;


	Change #2: place before line 427
	for_each_online_cpu(cpu) {
		ncpu++;
         }
	nr_running /=  ncpu;
	

	This may not be from from the latest source, but should be accurate.

	sched_fair.c :

   415  * The idea is to set a period in which each task runs once.
   416  *
   417  * When there are too many tasks (sysctl_sched_nr_latency) we  
have to stretch
   418  * this period because otherwise the slices get too small.
   419  *
   420  * p = (nr <= nl) ? l : l*nr/nl
   421  */
   422 static u64 __sched_period(unsigned long nr_running)
   423 {
   424         u64 period = sysctl_sched_latency;
   425         unsigned long nr_latency = sched_nr_latency;
   426

   427         if (unlikely(nr_running > nr_latency)) {
   428                 period = sysctl_sched_min_granularity;
   429                 period *= nr_running;
   430         }
   431
   432         return period;
   433 }


   435 /*
   436  * We calculate the wall-time slice from the period by taking a  
part
   437  * proportional to the weight.
   438  *
   439  * s = p*P[w/rw]
   440  */
   441 static u64 sched_slice(struct cfs_rq *cfs_rq, struct  
sched_entity *se)
   442 {
   443         unsigned long nr_running = cfs_rq->nr_running;
   444
   445         if (unlikely(!se->on_rq))
   446                 nr_running++;
   447
   448         return calc_delta_weight(__sched_period(nr_running), se);
   449 }
   450


		Thank you, Mitchell Erblich

	

Re: CFS Scheduler : Period : for NCPUs : Code Suggestion Change

[Peter Zijlstra]

On Tue, 2009-06-30 at 18:36 -0700, Mitchell Erblich wrote:
> This is NOT A PATCH.
> 
> 	PLEASE include my email in the reply as I am not currently on
> 	the linux kernel mail alias.
> 
> 	This code snap is grabbed from what is believed to be a semi-current
> 	OS source (fxr.watson.org) comparison webpage.
> 
> 	Upon a quick CFS scheduler code walk, increasing the period
> 	should ALSO be dependent on the number of online/active CPUs.
> 
> 	The period should be adjusted based on the number of
> 	online CPUs. This change allows NCPUs * tasks without
> 	changing/increasing the period.
> 
> 	On first thought NR_CPUS should give the number of cpu on
> 	the system, however, this may be different from the number of
> 	CPUs online, thus..
> 
> 	Change #1: place after line 425
> 	int cpu, ncpu;
> 
> 
> 	Change #2: place before line 427
> 	for_each_online_cpu(cpu) {
> 		ncpu++;
>          }
> 	nr_running /=  ncpu;
> 	

Ah, but the nr_running number used is _per_ cpu already, so would that
address your concern?