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From:   Amir Goldstein <amir73il@gmail.com>
Date:   Sat, 12 Sep 2020 10:28:29 +0300
Message-ID: <CAOQ4uxhz8prfD5K7dU68yHdz=iBndCXTg5w4BrF-35B+4ziOwA@mail.gmail.com>
Subject: Re: Kernel Benchmarking
To:     Michael Larabel <Michael@michaellarabel.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        "Ted Ts'o" <tytso@google.com>,
        Andreas Dilger <adilger.kernel@dilger.ca>,
        Ext4 Developers List <linux-ext4@vger.kernel.org>,
        Jan Kara <jack@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>
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On Sat, Sep 12, 2020 at 1:40 AM Michael Larabel
<Michael@michaellarabel.com> wrote:
>
> On 9/11/20 5:07 PM, Linus Torvalds wrote:
> > On Fri, Sep 11, 2020 at 9:19 AM Linus Torvalds
> > <torvalds@linux-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
>
> I've been running your EXT4 patch on more systems and with some
> additional workloads today. While not the original problem, the patch
> does seem to help a fair amount for the MariaDB database sever. This
> wasn't one of the workloads regressing on 5.9 but at least with the
> systems tried so far the patch does make a meaningful improvement to the
> performance. I haven't run into any apparent issues with that patch so
> continuing to try it out on more systems and other database/server
> workloads.
>

Michael,

Can you please add a reference to the original problem report and
to the offending commit? This conversation appeared on the list without
this information.

Are filesystems other than ext4 also affected by this performance
regression?

Thanks,
Amir.