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