From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
To: Song Liu <song@kernel.org>
Cc: Yajun Deng <yajun.deng@linux.dev>,
masahiroy@kernel.org, williams@redhat.com,
Paul Menzel <pmenzel@molgen.mpg.de>,
open list <linux-kernel@vger.kernel.org>,
linux-rt-users@vger.kernel.org,
linux-raid <linux-raid@vger.kernel.org>,
stable@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH v3] lib/raid6: Reduce high latency by using migrate instead of preempt
Date: Fri, 17 Dec 2021 19:17:58 +0100 [thread overview]
Message-ID: <YbzUVgPH9KDAMpA1@linutronix.de> (raw)
In-Reply-To: <CAPhsuW7GhYyfNOQg3VovU7cqC0nnRTbm1B7bFkWWa75k8YgHew@mail.gmail.com>
On 2021-12-17 09:25:25 [-0800], Song Liu wrote:
> > The delay is a jiffy so it depends on CONFIG_HZ. You do benchmark for
> > the best algorithm and if you get preempted during that period then your
> > results may be wrong and you make a bad selection.
>
> With current code, the delay _should be_ 16 jiffies. However, the experiment
> hits way longer latencies. I agree this may cause inaccurate benchmark results
> and thus suboptimal RAID algorithm.
Everything less than CONFIG_PREEMPT does not have an explicit
requirement for preemption so higher latencies are not unusual. *If*
this is a problem on <= PREEMPT_VOLUNTARY then a cond_resched() between
loops would be the usual thing to do. But only *if* it is a real problem
which I doubt. It is not a preemtible kernel after all…
> I guess the key question is whether long latency at module loading time matters.
> If that doesn't matter, we should just drop this.
Correct. And should this be problematic on PREEMPT_RT then I would
restrict CONFIG_RAID6_PQ_BENCHMARK to !PREEMPT_RT.
> Thanks,
> Song
Sebastian
next prev parent reply other threads:[~2021-12-17 18:18 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-17 2:16 [PATCH v3] lib/raid6: Reduce high latency by using migrate instead of preempt Yajun Deng
2021-12-17 13:42 ` Sebastian Andrzej Siewior
2021-12-17 17:25 ` Song Liu
2021-12-17 18:17 ` Sebastian Andrzej Siewior [this message]
2021-12-17 22:01 ` Daniel Vacek
2021-12-17 21:58 ` Daniel Vacek
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