RE: [EXT] Re: FYI, fsnotify contention with aio and io_uring.

[Pierre Labat <plabat@micron.com>]

Micron Confidential

Hi Jeff and Jens,

About "FAN_MODIFY fsnotify watch set on /dev".

Was using Fedora34 distro (with 6.3.9 kernel), and fio. Without any particular/specific setting.
I tried to see what could watch /dev but failed at that.
I used the inotify-info tool, but that display watchers using the inotify interface. And nothing was watching /dev via inotify.
Need to figure out how to do the same but for the fanotify interface.
I'll look at it again and let you know.

Regards,

Pierre



Micron Confidential
> -----Original Message-----
> From: Jens Axboe <axboe@kernel.dk>
> Sent: Tuesday, August 8, 2023 2:41 PM
> To: Jeff Moyer <jmoyer@redhat.com>; Pierre Labat <plabat@micron.com>
> Cc: 'io-uring@vger.kernel.org' <io-uring@vger.kernel.org>
> Subject: [EXT] Re: FYI, fsnotify contention with aio and io_uring.
>
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>
> On 8/7/23 2:11?PM, Jeff Moyer wrote:
> > Hi, Pierre,
> >
> > Pierre Labat <plabat@micron.com> writes:
> >
> >> Hi,
> >>
> >> This is FYI, may be you already knows about that, but in case you
> don't....
> >>
> >> I was pushing the limit of the number of nvme read IOPS, the FIO +
> >> the Linux OS can handle. For that, I have something special under the
> >> Linux nvme driver. As a consequence I am not limited by whatever the
> >> NVME SSD max IOPS or IO latency would be.
> >>
> >> As I cranked the number of system cores and FIO jobs doing direct 4k
> >> random read on /dev/nvme0n1, I hit a wall. The IOPS scaling slows
> >> (less than linear) and around 15 FIO jobs on 15 core threads, the
> >> overall IOPS, in fact, goes down as I add more FIO jobs. For example
> >> on a system with 24 cores/48 threads, when I goes beyond 15 FIO jobs,
> >> the overall IOPS starts to go down.
> >>
> >> This happens the same for io_uring and aio. Was using kernel version
> 6.3.9. Using one namespace (/dev/nvme0n1).
> >
> > [snip]
> >
> >> As you can see 76% of the cpu on the box is sucked up by
> >> lockref_get_not_zero() and lockref_put_return().  Looking at the
> >> code, there is contention when IO_uring call fsnotify_access().
> >
> > Is there a FAN_MODIFY fsnotify watch set on /dev?  If so, it might be
> > a good idea to find out what set it and why.
>
> This would be my guess too, some distros do seem to do that. The
> notification bits scale horribly, nobody should use it for anything high
> performance...
>
> --
> Jens Axboe