linux-kernel - fuse: max_background and congestion

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Message-ID: <87oa1g90nx.fsf@thinkpad.rath.org>
Date:   Tue, 15 Nov 2016 08:18:42 -0800
From:   Nikolaus Rath <Nikolaus@...h.org>
To:     fuse-devel@...ts.sourceforge.net,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Miklos Szeredi <mszeredi@...hat.com>
Subject: fuse: max_background and congestion_threshold settings

Hello,

Could someone explain to me the meaning of the max_background and
congestion_threshold settings of the fuse module?

At first I assumed that max_background specifies the maximum number of
pending requests (i.e., requests that have been send to userspace but
for which no reply was received yet). But looking at fs/fuse/dev.c, it
looks as if not every request is included in this number. 

I also figured out that if the number of background requests (whatever
they are) exceeds the congestion threshold, fuse calls
set_bdi_congested() for the backing device. But what does this do? And
does this become a no-op if there is no backing device?

Thanks,
-Nikolaus
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