Messages in this thread | | | Date | Fri, 13 Jul 2001 10:05:21 -0700 | From | Mike Kravetz <> | Subject | Re: CPU affinity & IPI latency |
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On Thu, Jul 12, 2001 at 05:36:41PM -0700, Larry McVoy wrote: > Be careful tuning for LMbench (says the author :-) > > Especially this benchmark. It's certainly possible to get dramatically better > SMP numbers by pinning all the lat_ctx processes to a single CPU, because > the benchmark is single threaded. In other words, if we have 5 processes, > call them A, B, C, D, and E, then the benchmark is passing a token from > A to B to C to D to E and around again. > > If the amount of data/instructions needed by all 5 processes fits in the > cache and you pin all the processes to the same CPU you'll get much > better performance than simply letting them float. > > But making the system do that naively is a bad idea.
I agree, and can't imagine the system ever attempting to take this into account and leave these 5 tasks on the same CPU.
At the other extreme is my observation that 2 tasks on an 8 CPU system are 'round robined' among all 8 CPUs. I think having the 2 tasks stay on 2 of the 8 CPUs would be an improvement with respect to CPU affinity. Actually, the scheduler does 'try' to do this.
It is clear that the behavior of lat_ctx bypasses almost all of the scheduler's attempts at CPU affinity. The real question is, "How often in running 'real workloads' are the schduler's attempts at CPU affinity bypassed?".
-- Mike Kravetz mkravetz@sequent.com IBM Linux Technology Center - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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