Re: FUSE: write operations trigger balance_dirty_pages when using writeback cache

[åçç]

Thanks for the advice. I tried removing BDI_CAP_STRICTLIMIT, and it works. There is no balance_dirty_pages() triggered, and the performance improves a lot.

Tested by libfuse passthrough_ll example and fio:
./passthrough_ll -o writeback /mnt/fuse/
fio --name=test --ioengine=psync --directory=/mnt/fuse/home/test --bs=4k --direct=0 --size=64M --rw=write --fallocate=0 --numjobs=1

performance with BDI_CAP_STRICTLIMIT:
WRITE: bw=158MiB/s (165MB/s), 158MiB/s-158MiB/s (165MB/s-165MB/s), io=64.0MiB (67.1MB), run=406-406msec

Performance without BDI_CAP_STRICTLIMIT:
WRITE: bw=1561MiB/s (1637MB/s), 1561MiB/s-1561MiB/s (1637MB/s-1637MB/s), io=64.0MiB (67.1MB), run=41-41msec

However, I wonder if there are some side-effects to remove it? Since it seems that the original purpose of this feature is to prevent FUSE from consuming too much memory. Please correct me if I am mistaken. Thanks in advance.


Regards,
Shuoran


-----éäåä-----
åää: Miklos Szeredi [mailto:miklos@xxxxxxxxxx] 
åéæé: 2018å8æ9æ 16:30
æää: åçç <liushuoran@xxxxxx>
æé: linux-fsdevel@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
äé: Re: FUSE: write operations trigger balance_dirty_pages when using writeback cache

On Thu, Aug 9, 2018 at 9:31 AM, åçç <liushuoran@xxxxxx> wrote:
> Thank you for the prompt reply.
>
> I tried this config, but still can get balance_dirty_pages triggered.

I think it may be due to BDI_CAP_STRICTLIMIT used by fuse.  If you remove that setting from fuse in the kernel you should not be getting the balance_dirty_pages() as often.

Not sure if that's the realproblem, though, that depends on how much time is spent in balance_dirty_pages().  You can try profiling the kernel to find that out.

My guess is that the real cause of the slowdown is some other place.
There's for example a known issue with selinux related getxattr thrashing.  Disabling getxattr on your filesystem may significantly improve performance.

Thanks,
Miklos