Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 6 Nov 2023 11:32:02 -0500 | Subject | Re: [RFC PATCH v2 0/2] sched/fair migration reduction features | From | Mathieu Desnoyers <> |
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On 2023-10-26 23:27, K Prateek Nayak wrote: [...] > -- > It is a mixed bag of results, as expected. I would love to hear your > thoughts on the results. Meanwhile, I'll try to get some more data > from other benchmarks.
I suspect that workloads that exhibit a client-server (1:1) pairing pattern are hurt by the bias towards leaving tasks on their prev runqueue: they benefit from moving both client/server tasks as close as possible so they share either the same core or a common cache.
The hackbench workload is also client-server, but there are N-client and N-server threads, creating a N:N relationship which really does not work well when trying to pull tasks on sync wakeup: tasks then bounce all over the place.
It's tricky though. If we try to fix the "1:1" client-server pattern with a heuristic, we may miss scenarios which are close to 1:1 but don't exactly match.
I'm working on a rewrite of select_task_rq_fair, with the aim to tackle the more general task placement problem taking into account the following:
- We want to converge towards a task placement that moves tasks with most waker/wakee interactions as close as possible in the cache topology, - We can use the core util_est/capacity metrics to calculate whether we have capacity left to enqueue a task in a core's runqueue. - The underlying assumption is that work conserving [1] is not a good characteristic to aim for, because it does not take into account the overhead associated with migrations, and thus lack of cache locality.
Thanks,
Mathieu
[1] I use the definition of "work conserving" found here: https://people.ece.ubc.ca/sasha/papers/eurosys16-final29.pdf
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
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