Re: [fuse-devel] Horrible mmap write performance (kernel writeback issue?)

[Jakob Unterwurzacher <jakobunt@gmail.com>]

On 30.03.2016 20:47, Tejun Heo wrote:
> Hmmm... cgroup writeback support shouldn't affect fuse at all as the
> backing device doesn't enable cgroup support.  I probably made some
> silly mistake.  Is there a simple reproducer I can play with?

Hi Tejun! A simple reproducer is at https://github.com/rfjakob/mmapwrite .

What seems to be happening in the kernel is that the estimated device bandwith
drops to zero. I'm not even sure how this works for FUSE, but that's what I
gathered from some printk debugging.

What I also found is that once mmapwrite is hung, you can unblock it for some
time by running something like

    cat /dev/zero > /var/tmp/foo

mmapwrite will then steam ahead as long as cat is writing, even though encfs
writes to /tmp (tmpfs) and /var is on the ext4 disk.

Note that the hang happens regardless of the backing device, on both tmpfs and
ext4.

Best regards,
Jakob