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

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Date:	Mon, 11 Apr 2016 10:04:42 +0200
From:	Jakob Unterwurzacher <jakobunt@...il.com>
To:	Tejun Heo <tj@...nel.org>, Miklos Szeredi <miklos@...redi.hu>
Cc:	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [fuse-devel] Horrible mmap write performance (kernel writeback
 issue?)

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