linux-ext4 - Re: Kernel Benchmarking

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Message-ID: <CAHk-=wjfw3U5eTGWLaisPHg1+jXsCX=xLZgqPx4KJeHhEqRnEQ@mail.gmail.com>
Date:   Fri, 11 Sep 2020 15:07:55 -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 9:19 AM Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> 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.

Ahh. It turns out that I should have looked more at the fault path
after all. It was higher up in the profile, but I ignored it because I
found that lock-unlock-lock pattern lower down.

The main contention point is actually filemap_fault(). Your apache
test accesses the 'test.html' file that is mmap'ed into memory, and
all the threads hammer on that one single file concurrently and that
seems to be the main page lock contention.

Which is really sad - the page lock there isn't really all that
interesting, and the normal "read()" path doesn't even take it. But
faulting the page in does so because the page will have a long-term
existence in the page tables, and so there's a worry about racing with
truncate.

Interesting, but also very annoying.

Anyway, I don't have a solution for it, but thought I'd let you know
that I'm still looking at this.

                Linus