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SubjectRe: -mm seems significanty slower than mainline on kernbench
Con Kolivas wrote:
> On Wed, 11 Jan 2006 12:41 pm, Martin Bligh wrote:
>>Seems to have gone wrong between 2.6.14-rc1-mm1 and 2.6.14-rc2-mm1 ?
>>See for clearest effect.
> The only new scheduler patch at that time was this:
> +sched-modified-nice-support-for-smp-load-balancing.patch
> which was Peter's modifications to my smp nice support. cc'ed Peter

This patch will probably have overhead implications as it will skip some
tasks during load balancing looking for ones whose bias_prio is small
enough to fit within the amount of bias to be moved. Because the
candidate tasks are ordered in the array by dynamic priority and not
nice (or bias_prio) the search is exhaustive. I need to think about
whether this can be made a little smarter e.g. skip to the next idx as
soon as a task whose bias_prio is too large is encountered. This would
have the effect of missing some tasks that could have been moved due but
these will generally be tasks with interactive bonuses causing them to
be in a lower slot in the queue than would otherwise be the case and as
they generally do small CPU runs not moving them probably won't make
much difference.

> I guess we need to check whether reversing this patch helps.

It would be interesting to see if it does.

If it does we probably have to wear the cost (and try to reduce it) as
without this change smp nice support is fairly ineffective due to the
fact that it moves exactly the same tasks as would be moved without it.
At the most it changes the frequency at which load balancing occurs.

Peter Williams

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