linux-kernel - Re: [PATCHSET v3][RFC] Make background writeback not suck

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Message-ID: <56FEA898.7070108@googlemail.com>
Date:	Fri, 1 Apr 2016 18:58:00 +0200
From:	Holger Hoffstätte 
	<holger.hoffstaette@...glemail.com>
To:	Dave Chinner <david@...morbit.com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCHSET v3][RFC] Make background writeback not suck

On 04/01/16 03:01, Dave Chinner wrote:
> Can you go back to your original kernel, and lower nr_requests to 8?

Sure, did that and as expected it didn't help much. Under prolonged stress
it was actually even a bit worse than writeback throttling. IMHO that's not
really surprising either, since small queues now punish everyone and
in interactive mode I really want to e.g. start loading hundreds of small
thumbnails at once, or du a directory.

Instead of randomized aka manual/interactive testing I created a simple
stress tester:

#!/bin/sh
while [[ true ]]
do
    cp bigfile bigfile.out
done

and running that in the background turns the system into a tar pit,
which is laughable when you consider that I have 24G and 8 cores.

With the writeback patchset and wb_percent=1 (yes, really!) it is almost
unnoticeable, but according to nmon still writes ~250-280 MB/s.
This is with deadline on ext4 on an older SATA-3 SSD that can still
do peak ~465 MB/s (with dd).

cheers,
Holger