* Nick Piggin <[email protected]> wrote:
> Ingo, I wasn't aware that tasks are bouncing around wildly; does your
> patch improve things? Then by definition it must penalise workloads
> where the pairings are more predictable?
for TPC, most of the non-to-idle migrations are 'wrong'. So basically
any change that gets rid of extra migrations is a win. This does not
mean that it is all bouncing madly.
> I would prefer to try fixing wake balancing before giving up and
> turning it off for busy CPUs.
agreed, and that was my suggestion: improve the heuristics to not hurt
workloads where there is no natural pairing.
one possible way would be to do a task_hot() check in the passive
balancing code, and only migrate the task when it's been inactive for a
long time: that should be the case for most TPC wakeups. (This assumes
an accurate cache-hot estimator, for which another patch exists.)
> Without any form of wake balancing, then a multiprocessor system will
> tend to have a completely random distribution of tasks over CPUs over
> time. I prefer to add a driver so it is not completely random for
> amenable workloads.
but my patch does not do 'no form of wake balancing'. It will do
non-load-related wake balancing if the target CPU is idle. Arguably,
that can easily be 'never' under common workloads.
Ingo
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