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 [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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