Re: [PATCH 0/5] writeback: Fix bandwidth estimates

[Michael Stapelberg <stapelberg+linux@google.com>]

Thanks for sending this patch series!

I have used the mmap.c reproducer as before, with the following parameters:
* mkdir /tmp/mnt
* fusermount -u /tmp/mnt; /root/fuse-2.9.9/example/fusexmp_fh -f /tmp/mnt
* dd if=/dev/urandom of=/tmp/was bs=1M count=99
* while :; do grep ^Bdi /sys/kernel/debug/bdi/0:44/stats; sleep 0.1; done
* while :; do time WORKAROUND=1 ~/mmap /tmp/was
/tmp/mnt/tmp/stapelberg.1; sleep 5; done

Previously, after a few iterations, the BdiWriteBandwidth measure
would gradually approach 0.

With your patch series applied, the BdiWriteBandwidth is updated much
more quickly, and converges to ≈16000 kBps.
When I start copying more quickly, the bandwidth measure rises quickly.

As far as I understand, this should fix the problem (provided 16000
kBps is an okay value).
Certainly, I don’t see the downward spiral either with your patches :)


On Mon, 5 Jul 2021 at 18:23, Jan Kara <jack@suse.cz> wrote:
>
> Hello,
>
> this patch series fixes estimate of writeback throughput when device is not
> fully busy doing writeback. Michael Stapelberg has reported that such
> workload (e.g. generated by linking) tends to push estimated throughput down
> to 0 and as a result writeback on the device is practically stalled.
>
> The first three patches fix the reported issue, the remaining two patches
> are unrelated cleanups of problems I've noticed when reading the code.
>
> Michael, can you try whether these patches fix the problems you've observed?
> I cannot trigger the "bandwidth going down" spiral with them anymore.
>
>                                                                 Honza