On Sat, Sep 12, 2020 at 1:40 AM Michael Larabel
[off-list ref] wrote:

On 9/11/20 5:07 PM, Linus Torvalds wrote:

quoted

On Fri, Sep 11, 2020 at 9:19 AM Linus Torvalds
[off-list ref] wrote:

quoted

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.