Re: [PATCH] sched: move enough load to balance average load per task

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

Siddha, Suresh B wrote:
> On Mon, Apr 10, 2006 at 04:45:32PM +1000, Peter Williams wrote:
>> Problem:
>>
>> The current implementation of find_busiest_group() recognizes that 
>> approximately equal average loads per task for each group/queue are 
>> desirable (e.g. this condition will increase the probability that the 
>> top N highest priority tasks on an N CPU system will be on different 
>> CPUs) by being slightly more aggressive when *imbalance is small but the 
>> average load per task in "busiest" group is more than that in "this" 
>> group.  Unfortunately, the amount moved from "busiest" to "this" is too 
>> small to reduce the average load per task on "busiest" (at best there 
>> will be no change and at worst it will get bigger).
> 
> Peter, We don't need to reduce the average load per task on "busiest"
> always. By moving a "busiest_load_per_task", we will increase the 
> average load per task of lesser busy cpu (there by trying to achieve
> the equality with busiest...)

Well, first off, we don't always move busiest_load_per_task we move UP 
TO busiest_load_per_task so there is no way you can make definitive 
statements about what will happen to the value "this_load_per_task" as a 
result of setting *imbalance to busiest_load_per_task.  Load balancing 
is a probabilistic endeavour and we need to take steps that increase the 
probability that we get the desired result.

Without this patch there is no chance that busiest_load_per_task will 
get smaller and whether this_load_per_task will get bigger is 
indeterminate.  With this patch there IS a chance that 
busiest_load_per_task will decrease and an INCREASED chance that 
this_load_per_task will get bigger.  Ergo we have increased the 
probability that the (absolute) difference between this_load_per_task 
and busiest_load_per_task will decrease.  This is a desirable outcome.

Peter
-- 
Peter Williams                                   pwil3058@bigpond.net.au

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