Bug#972709: Wishlist/RFC: Change to CONFIG_PREEMPT_NONE in linux-image-cloud-*
On Sun, Nov 22, 2020 at 03:53:32PM -0800, Flavio Veloso Soares wrote:
> Unfortunately, I couldn't find many comprehensive benchmarks of kernel
> CONFIG_PREEMPT* options. The one at
> [1]https://www.codeblueprint.co.uk/2019/12/23/linux-preemption-latency-throughput.html
> seems to be very thorough,
>
> [...]
>
> Not particularly. I'm used to latency benchmarks showing e.g. average,
> 90th percentile, 99th percentile, as well as worst.
I don't think Ben was talking about specific benchmarks. The web page
you cites lacks basic measurements one would expect to see from *any*
meaningful performance benchmark. Comparing maximum latency is fine,
but it's not really relevant by itself. If a configuration change
improves the worst case (100th percentile) but negatively impacts the
50th percentile, is that a change worth making? Maybe. But without
having that data at all, the benchmark really isn't worth much at all.
It's totally reasonable for us to consider making this change, but we
should have comprehensive data about the impact of doing so. What
impact does the change have on different classes of workloads? e.g.
high tps, CPU-bound, IO-bound, etc. It's entirely possible that the
proposed change improves performance under certain workloads, but
negatively impacts others. Without knowing the impact in more in more
detail, which would allow us to evaluate the tradeoffs, I don't think
there's a compelling reason to make a change.
noah
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