linux-kernel - Re: FUSE: write operations trigger balance_dirty

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Message-ID: <CAJfpegt4ymM8Zuto8vDX4djP5S-t3DMaaKn0ntwCsG1JaBpExg@mail.gmail.com>
Date:   Thu, 9 Aug 2018 14:25:47 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     刘硕然 <liushuoran@...com>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        刘海锋 <bjliuhaifeng@...com>,
        郭卫龙 <guoweilong@...com>
Subject: Re: FUSE: write operations trigger balance_dirty_pages when using
 writeback cache

On Thu, Aug 9, 2018 at 2:08 PM, 刘硕然 <liushuoran@...com> wrote:
> 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.

Yes.  So if BDI_CAP_STRICTLIMIT is causing a serious performance
bottleneck, then we need to think about solving this without losing
the benefits.  Simply removing it is definitely not a proper solution.

Thanks,
Miklos