linux-kernel - Re: [fuse-devel] Horrible mmap write performance (kernel writeback issue?)

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Date:	Sat, 26 Mar 2016 22:42:45 +0100
From:	Jakob Unterwurzacher <jakobunt@...il.com>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [fuse-devel] Horrible mmap write performance (kernel writeback
 issue?)

On 16.03.2016 10:44, Miklos Szeredi wrote:
> On Tue, Mar 15, 2016 at 8:55 AM, Jakob Unterwurzacher
> <jakobunt@...il.com> wrote:
>> Just for anybody finding this thread: This still happens in v4.4, it
>> just took longer to trigger.
>>
>> I have posted more details to linux-kernel (copy-pasted below),
>> http://thread.gmane.org/gmane.linux.kernel/2132944
> 
> Okay, so you can reproduce this relatively quickly.   Can you try "git
> bisect" to find exactly which commit is responsible?
> 
> Thanks,
> Miklos

That took a while, but it looks like it got it:

> commit 947e9762a8ddefda38aa21e249e6a4fec215cd12
> Author: Tejun Heo <tj@...nel.org>
> Date:   Fri May 22 18:23:32 2015 -0400
> 
>     writeback: update wb_over_bg_thresh() to use wb_domain aware operations

Note that this commens seems to only activate changes that happened in the
commit before, aa661bb:

> commit aa661bbe1e61ce80ca4ae98804f673ede94b0827
> Author: Tejun Heo <tj@...nel.org>
> Date:   Fri May 22 18:23:31 2015 -0400
> 
>     writeback: move over_bground_thresh() to mm/page-writeback.c


Anyway, I can reliably reboot between aa661bb and 947e976 and always get the
same results:

aa661bb passes
947e976 fails

I you want to reproduce, clone https://github.com/rfjakob/mmapwrite.git and
run ./encfs-test.sh (needs encfs installed). On the bad kernel, it will hang
within a few seconds.

Thanks,
Jakob