Re: -mm seems significanty slower than mainline on kernbench

[Peter Williams <pwil3058@bigpond.net.au>]

Peter Williams wrote:
> Peter Williams wrote:
> 
>> Peter Williams wrote:
>>
>>> Martin Bligh wrote:
>>>
>>>>
>>>>>>
>>>>>> But I was thinking more about the code that (in the original) 
>>>>>> handled the case where the number of tasks to be moved was less 
>>>>>> than 1 but more than 0 (i.e. the cases where "imbalance" would 
>>>>>> have been reduced to zero when divided by SCHED_LOAD_SCALE).  I 
>>>>>> think that I got that part wrong and you can end up with a bias 
>>>>>> load to be moved which is less than any of the bias_prio values 
>>>>>> for any queued tasks (in circumstances where the original code 
>>>>>> would have rounded up to 1 and caused a move).  I think that the 
>>>>>> way to handle this problem is to replace 1 with "average bias 
>>>>>> prio" within that logic.  This would guarantee at least one task 
>>>>>> with a bias_prio small enough to be moved.
>>>>>>
>>>>>> I think that this analysis is a strong argument for my original 
>>>>>> patch being the cause of the problem so I'll go ahead and generate 
>>>>>> a fix. I'll try to have a patch available later this morning.
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>>
>>>>> Attached is a patch that addresses this problem.  Unlike the 
>>>>> description above it does not use "average bias prio" as that 
>>>>> solution would be very complicated.  Instead it makes the 
>>>>> assumption that NICE_TO_BIAS_PRIO(0) is a "good enough" for this 
>>>>> purpose as this is highly likely to be the median bias prio and the 
>>>>> median is probably better for this purpose than the average.
>>>>>
>>>>> Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> Doesn't fix the perf issue.
>>>
>>>
>>>
>>>
>>> OK, thanks.  I think there's a few more places where SCHED_LOAD_SCALE 
>>> needs to be multiplied by NICE_TO_BIAS_PRIO(0).  Basically, anywhere 
>>> that it's added to, subtracted from or compared to a load.  In those 
>>> cases it's being used as a scaled version of 1 and we need a scaled 
>>
>>
>>
>> This would have been better said as "the load generated by 1 task" 
>> rather than just "a scaled version of 1".  Numerically, they're the 
>> same but one is clearer than the other and makes it more obvious why 
>> we need NICE_TO_BIAS_PRIO(0) * SCHED_LOAD_SCALE and where we need it.
>>
>>> version of NICE_TO_BIAS_PRIO(0).  I'll have another patch later today.
>>
>>
>>
>> I'm just testing this at the moment.
> 
> 
> Attached is a new patch to fix the excessive idle problem.  This patch 
> takes a new approach to the problem as it was becoming obvious that 
> trying to alter the load balancing code to cope with biased load was 
> harder than it seemed.
> 
> This approach reverts to the old load values but weights them according 
> to tasks' bias_prio values.  This means that any assumptions by the load 
> balancing code that the load generated by a single task is 
> SCHED_LOAD_SCALE will still hold.  Then, in find_busiest_group(), the 
> imbalance is scaled back up to bias_prio scale so that move_tasks() can 
> move biased load rather than tasks.
> 
> One advantage of this is that when there are no non zero niced tasks the 
> processing will be mathematically the same as the original code. 
> Kernbench results from a 2 CPU Celeron 550Mhz system are:
> 
> Average Optimal -j 8 Load Run:
> Elapsed Time 1056.16 (0.831102)
> User Time 1906.54 (1.38447)
> System Time 182.086 (0.973386)
> Percent CPU 197 (0)
> Context Switches 48727.2 (249.351)
> Sleeps 27623.4 (413.913)
> 
> This indicates that, on average, 98.9% of the total available CPU was 
> used by the build.

Here's the numbers for the same machine with the "improved smp nice 
handling" completely removed i.e. back to 2.6.15 version.

Average Optimal -j 8 Load Run:
Elapsed Time 1059.95 (1.19324)
User Time 1914.94 (1.11102)
System Time 181.846 (0.916695)
Percent CPU 197.4 (0.547723)
Context Switches 40917.4 (469.184)
Sleeps 26654 (320.824)

> 
> Signed-off-by: Peter Williams <pwil3058@bigpond.com.au>
> 
> BTW I think that we need to think about a slightly more complex nice to 
> bias mapping function.  The current one gives a nice==19 1/20 of the 
> bias of a nice=0 task but only gives nice=-20 tasks twice the bias of a 
> nice=0 task.  I don't think this is a big problem as the majority of non 
> nice==0 tasks will have positive nice but should be looked at for a 
> future enhancement.
> 
> Peter
> 
> 
> ------------------------------------------------------------------------
> 
> Index: MM-2.6.X/kernel/sched.c
> ===================================================================
> --- MM-2.6.X.orig/kernel/sched.c	2006-01-13 14:53:34.000000000 +1100
> +++ MM-2.6.X/kernel/sched.c	2006-01-13 15:11:19.000000000 +1100
> @@ -1042,7 +1042,8 @@ void kick_process(task_t *p)
>  static unsigned long source_load(int cpu, int type)
>  {
>  	runqueue_t *rq = cpu_rq(cpu);
> -	unsigned long load_now = rq->prio_bias * SCHED_LOAD_SCALE;
> +	unsigned long load_now = (rq->prio_bias * SCHED_LOAD_SCALE) /
> +		NICE_TO_BIAS_PRIO(0);
>  
>  	if (type == 0)
>  		return load_now;
> @@ -1056,7 +1057,8 @@ static unsigned long source_load(int cpu
>  static inline unsigned long target_load(int cpu, int type)
>  {
>  	runqueue_t *rq = cpu_rq(cpu);
> -	unsigned long load_now = rq->prio_bias * SCHED_LOAD_SCALE;
> +	unsigned long load_now = (rq->prio_bias * SCHED_LOAD_SCALE) /
> +		NICE_TO_BIAS_PRIO(0);
>  
>  	if (type == 0)
>  		return load_now;
> @@ -1322,7 +1324,8 @@ static int try_to_wake_up(task_t *p, uns
>  			 * of the current CPU:
>  			 */
>  			if (sync)
> -				tl -= p->bias_prio * SCHED_LOAD_SCALE;
> +				tl -= (p->bias_prio * SCHED_LOAD_SCALE) /
> +					NICE_TO_BIAS_PRIO(0);
>  
>  			if ((tl <= load &&
>  				tl + target_load(cpu, idx) <= SCHED_LOAD_SCALE) ||
> @@ -2159,7 +2162,7 @@ find_busiest_group(struct sched_domain *
>  	}
>  
>  	/* Get rid of the scaling factor, rounding down as we divide */
> -	*imbalance = *imbalance / SCHED_LOAD_SCALE;
> +	*imbalance = (*imbalance * NICE_TO_BIAS_PRIO(0)) / SCHED_LOAD_SCALE;
>  	return busiest;
>  
>  out_balanced:
> @@ -2472,7 +2475,8 @@ static void rebalance_tick(int this_cpu,
>  	struct sched_domain *sd;
>  	int i;
>  
> -	this_load = this_rq->prio_bias * SCHED_LOAD_SCALE;
> +	this_load = (this_rq->prio_bias * SCHED_LOAD_SCALE) /
> +		NICE_TO_BIAS_PRIO(0);
>  	/* Update our load */
>  	for (i = 0; i < 3; i++) {
>  		unsigned long new_load = this_load;


-- 
Peter Williams                                   pwil3058@bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce