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SubjectRe: Runtime overhead of PREEMPT_DYNAMIC
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On Fri, 2023-10-20 at 11:14 +0200, Emanuele Rocca wrote:
>
> Results of the tbench benchmark:
>
> - PREEMPT_DYNAMIC=n Throughput 3831.82 MB/sec
> - PREEMPT_DYNAMIC=y Throughput 3006.54 MB/sec
> - 20% performance degradation with PREEMPT_DYNAMIC
>
> All the tests above were performed using Linux 6.5.6 on a AMD Ryzen 7 3700X
> 8-Core CPU.
>
> Could someone please confirm whether the approach above seems sane and perhaps
> try and reproduce the results?

FWIW I can't reproduce anything like those crazy tbench numbers. It's
neither free nor crazy expensive here. It's kinda hard to imagine the
author's mailbox surviving submission day were it _that_ horrid, surely
something is amiss.

i7-4790 (quad+smt) avg cmdline
6.5.8-voluntary 3685.08 3679.93 3704.98 3689.99 1.000
6.5.8-dynamic 3571.62 3568.61 3550.55 3563.59 .965
3651.37 3599.87 3615.18 3622.14 .981 preempt=none
3459.58 3514.09 3539.88 3504.51 .949 preempt=full

voluntary (my usual)
CONFIG_PREEMPT_VOLUNTARY_BUILD=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
# CONFIG_PREEMPT_DYNAMIC is not set

dynamic (same config, just flip dynamic switch)
CONFIG_PREEMPT_BUILD=y
# CONFIG_PREEMPT_NONE is not set
CONFIG_PREEMPT_VOLUNTARY=y
# CONFIG_PREEMPT is not set
CONFIG_PREEMPT_COUNT=y
CONFIG_PREEMPTION=y
CONFIG_PREEMPT_DYNAMIC=y



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