linux-kernel - Re: Runtime overhead of PREEMPT

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Message-ID: <7a818250a8f36476f13b57a172fdb1ab23645edc.camel@gmx.de>
Date:   Fri, 20 Oct 2023 14:42:22 +0200
From:   Mike Galbraith <efault@....de>
To:     ema@...ian.org, linux-kernel@...r.kernel.org
Cc:     Michal Hocko <mhocko@...e.com>
Subject: Re: Runtime overhead of PREEMPT_DYNAMIC

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