Messages in this thread | | | Subject | Re: [fuse-devel] Horrible mmap write performance (kernel writeback issue?) | From | Jakob Unterwurzacher <> | Date | Mon, 11 Apr 2016 10:04:42 +0200 |
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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
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