linux-ext4 - Re: Kernel Benchmarking

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Message-ID: <CAHk-=whP-7Uw9WgWgjRgF1mCg+NnkOPpWjVw+a9M3F9C52DrVg@mail.gmail.com>
Date:   Fri, 11 Sep 2020 09:19:57 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Michael Larabel <Michael@...haellarabel.com>
Cc:     "Ted Ts'o" <tytso@...gle.com>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: Kernel Benchmarking

On Fri, Sep 11, 2020 at 6:42 AM Michael Larabel
<Michael@...haellarabel.com> wrote:
>
> From preliminary testing of the patch on a Threadripper box, the EXT4 locking patch did help with a small improvement at 10 concurrent users for Apache but all the higher counts didn't end up showing any real change with the patch.

Ok, it's probably simply that fairness is really bad for performance
here in general, and that special case is just that - a special case,
not the main issue.

I'll have to think about it. We've certainly seen this before (the
queued spinlocks brought the same fairness issues), but this is much
worse because of how it affects scheduling on a big level.

Some middle ground hybrid model (unfair in the common case, but with
at least _some_ measure of fairness for the worst-case situation to
avoid the worst-case latency spikes) would be best, but I don't see
how to do it.

              Linus

                 Linus

                  Linus