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

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

[Miklos Szeredi]

On Tue, Mar 15, 2016 at 8:55 AM, Jakob Unterwurzacher
<jakobunt@gmail.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

>
> -------- copy of the email to linux-kernel -------------------
>
> 2016-01-22 21:10:59
>
> I have noticed an annoying regression that was introduced in 4.2 and is
> still there in 4.4. mmap writes to FUSE filesystems are throttled down
> to basically zero.
>
> Reproducer: https://github.com/rfjakob/mmapwrite , testing against encfs:
>
> $ mmapwrite /tmp/encfs-mnt/foo
>   1 .................................................. 107.01 MB/s
>   2 .................................................. 101.98 MB/s
> [...]
>  68 .................................................. 106.79 MB/s
>  69 .................................................. 105.09 MB/s
>  70 ..................................................   2.02 MB/s
>  71 ..................................................   1.77 MB/s
>  72 ..................................................   0.42 MB/s
>  73 .................................... (hangs)
>
> I have tested kernels from 4.0 and this seems to have been introduced in
> 4.2:
>
> 4.0 ....... 140MB/s permanent
> 4.1 ....... 140MB/s permanent
> 4.2 ....... 100MB/s at the start, sudden slowdown to 1MB/s after ~5GB
> 4.3 ....... 100MB/s at the start, sudden slowdown to 1MB/s after ~1.5GB
> 4.4-rc4 ... 100MB/s at the start, slowly ramps down, 0.3MB/s after ~2GB
> 4.4 ....... 100MB/s at the start, sudden slowdown after ~3GB
>
> Is there a way to disable the throttling? Or at least exempt FUSE until
> there is a proper fix?
>
> Thanks,
> Jakob
>
>
>
>
> On 17.12.2015 00:26, Jakob Unterwurzacher wrote:
>> This seems to be fixed in v4.4-rc5-18-gedb42dc. mmap writes now proceed
>> at solid 100MB/s with full CPU saturation.
>>
>> Thanks,
>> Jakob
>>
>> On Mon, Dec 14, 2015 at 9:06 AM, Jakob Unterwurzacher
>> <jakobunt@gmail.com <mailto:jakobunt@gmail.com>> wrote:
>>
>>     I am the developer of https://github.com/rfjakob/gocryptfs (an
>>     encrypted overlay filesystem like EncFS)
>>     and have ported xfstests over for regression testing
>>     ( https://github.com/rfjakob/fuse-xfstests ).
>>
>>     xfstests generic/074 is how I noticed that mmap write performance
>>     plummeted when Fedora upgraded my kernel to 4.2.5.
>>     It used to complete in 10 minutes and now it will probably take days.
>>     I am on kernel 4.4-rc1 now and still seeing the same issue.
>>
>>     It looks like the kernel at some point the kernel writeback
>>     mechanism gets stuck (or throttles?)
>>
>>     Testing against encfs:
>>
>>     ./mmapwrite /tmp/e2/foo          #
>>     https://github.com/rfjakob/mmapwrite .
>>     ....................................................................................................
>>     98.91 MB/s
>>     ....................................................................................................
>>     93.74 MB/s
>>     ....................................................................................................
>>     103.89 MB/s
>>     ....................................................................................................
>>     100.20 MB/s
>>     ....................................................................................................
>>     104.03 MB/s
>>     ....................................................................................................
>>     98.06 MB/s
>>     ....................................................................................................
>>     10.17 MB/s
>>     ....................................................................................................
>>     9.50 MB/s
>>     .................................. (hangs)
>>
>>     At this point no write requests are submitted to encfs and the
>>     mmapwrite process is
>>     stuck in the kernel in balance_dirty_pages.isra.22.
>>
>>     Bisecting this will be a pain, I would appreciate any suggestions.
>>
>>     Note that this seems to affect every FUSE filesystem, also ntfs-3g.
>>
>>     Best regards,
>>     Jakob
>>
>>
>
>
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Re: [fuse-devel] Horrible mmap write performance (kernel writeback issue?)

[Jakob Unterwurzacher]

On 16.03.2016 10:44, Miklos Szeredi wrote:
> On Tue, Mar 15, 2016 at 8:55 AM, Jakob Unterwurzacher
> <jakobunt@gmail.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@kernel.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@kernel.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

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

[Miklos Szeredi]

On Sat, Mar 26, 2016 at 10:42 PM, Jakob Unterwurzacher
<jakobunt@gmail.com> wrote:
> On 16.03.2016 10:44, Miklos Szeredi wrote:
>> On Tue, Mar 15, 2016 at 8:55 AM, Jakob Unterwurzacher
>> <jakobunt@gmail.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@kernel.org>
>> Date:   Fri May 22 18:23:32 2015 -0400
>>
>>     writeback: update wb_over_bg_thresh() to use wb_domain aware operations
>

Tejun,

Any idea why this commit stalls fuse mmap writes?

Here's the start of the this thread:

  http://marc.info/?l=fuse-devel&m=145008058603261&w=2

Thanks,
Miklos


> Note that this commens seems to only activate changes that happened in the
> commit before, aa661bb:
>
>> commit aa661bbe1e61ce80ca4ae98804f673ede94b0827
>> Author: Tejun Heo <tj@kernel.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

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

[Tejun Heo]

Hello,

On Mon, Mar 28, 2016 at 09:45:53PM +0200, Miklos Szeredi wrote:
> >> commit 947e9762a8ddefda38aa21e249e6a4fec215cd12
> >> Author: Tejun Heo <tj@kernel.org>
> >> Date:   Fri May 22 18:23:32 2015 -0400
> >>
> >>     writeback: update wb_over_bg_thresh() to use wb_domain aware operations
> >
> 
> Tejun,
> 
> Any idea why this commit stalls fuse mmap writes?
> 
> Here's the start of the this thread:
> 
>   http://marc.info/?l=fuse-devel&m=145008058603261&w=2

Hmmm... cgroup writeback support shouldn't affect fuse at all as the
backing device doesn't enable cgroup support.  I probably made some
silly mistake.  Is there a simple reproducer I can play with?

Thanks.

-- 
tejun

Re: Horrible mmap write performance (kernel writeback issue?)

[Antonio SJ Musumeci]

[-- Attachment #1.1: Type: text/plain, Size: 1712 bytes --]

I had a user of mergerfs[0] use mmapwrite[1] and reproduce it immediately.

$ mkdir /tmp/test
$ mergerfs -odefaults /var/tmp /tmp/test
$ mmapwrite /tmp/test/foo

Perhaps of interest is that it affected the user's physical machines
running 4.2.0-34 (Ubuntu 15.10) but neither they or I could reproduce it in
a VM (Virtualbox).

[0] https://github.com/trapexit/mergerfs/releases
[1] https://github.com/rfjakob/mmapwrite

On Wed, Mar 30, 2016 at 2:47 PM, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org> wrote:

> Hello,
>
> On Mon, Mar 28, 2016 at 09:45:53PM +0200, Miklos Szeredi wrote:
> > >> commit 947e9762a8ddefda38aa21e249e6a4fec215cd12
> > >> Author: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
> > >> Date:   Fri May 22 18:23:32 2015 -0400
> > >>
> > >>     writeback: update wb_over_bg_thresh() to use wb_domain aware
> operations
> > >
> >
> > Tejun,
> >
> > Any idea why this commit stalls fuse mmap writes?
> >
> > Here's the start of the this thread:
> >
> >   http://marc.info/?l=fuse-devel&m=145008058603261&w=2
>
> Hmmm... cgroup writeback support shouldn't affect fuse at all as the
> backing device doesn't enable cgroup support.  I probably made some
> silly mistake.  Is there a simple reproducer I can play with?
>
> Thanks.
>
> --
> tejun
>
>
> ------------------------------------------------------------------------------
> Transform Data into Opportunity.
> Accelerate data analysis in your applications with
> Intel Data Analytics Acceleration Library.
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> fuse-devel mailing list
> To unsubscribe or subscribe, visit
> https://lists.sourceforge.net/lists/listinfo/fuse-devel
>

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

[Sedat Dilek]

On Wed, Mar 30, 2016 at 8:47 PM, Tejun Heo <tj@kernel.org> wrote:
> Hello,
>
> On Mon, Mar 28, 2016 at 09:45:53PM +0200, Miklos Szeredi wrote:
>> >> commit 947e9762a8ddefda38aa21e249e6a4fec215cd12
>> >> Author: Tejun Heo <tj@kernel.org>
>> >> Date:   Fri May 22 18:23:32 2015 -0400
>> >>
>> >>     writeback: update wb_over_bg_thresh() to use wb_domain aware operations
>> >
>>
>> Tejun,
>>
>> Any idea why this commit stalls fuse mmap writes?
>>
>> Here's the start of the this thread:
>>
>>   http://marc.info/?l=fuse-devel&m=145008058603261&w=2
>
> Hmmm... cgroup writeback support shouldn't affect fuse at all as the
> backing device doesn't enable cgroup support.  I probably made some
> silly mistake.  Is there a simple reproducer I can play with?
>
> Thanks.
>

To quote Jakob from a previous email:

"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."

Hope this helps.

- Sedat -

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

[Jakob Unterwurzacher]

On 30.03.2016 20:47, Tejun Heo wrote:
> Hmmm... cgroup writeback support shouldn't affect fuse at all as the
> backing device doesn't enable cgroup support.  I probably made some
> silly mistake.  Is there a simple reproducer I can play with?

Hi Tejun! A simple reproducer is at https://github.com/rfjakob/mmapwrite .

What seems to be happening in the kernel is that the estimated device bandwith
drops to zero. I'm not even sure how this works for FUSE, but that's what I
gathered from some printk debugging.

What I also found is that once mmapwrite is hung, you can unblock it for some
time by running something like

    cat /dev/zero > /var/tmp/foo

mmapwrite will then steam ahead as long as cat is writing, even though encfs
writes to /tmp (tmpfs) and /var is on the ext4 disk.

Note that the hang happens regardless of the backing device, on both tmpfs and
ext4.

Best regards,
Jakob

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

[Tejun Heo]

Hello,

On Mon, Apr 11, 2016 at 10:04:42AM +0200, Jakob Unterwurzacher wrote:
> What seems to be happening in the kernel is that the estimated device bandwith
> drops to zero. I'm not even sure how this works for FUSE, but that's what I
> gathered from some printk debugging.

Yeah, writeback bw getting messed up is the most likely cause.  Prolly
some silly bug.  I can reproduce the problem.  Looking into it.

Thanks.

-- 
tejun

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

[Ashish Sangwan]

On Tue, Apr 12, 2016 at 5:54 AM, Tejun Heo <tj@kernel.org> wrote:
> Hello,
>
> On Mon, Apr 11, 2016 at 10:04:42AM +0200, Jakob Unterwurzacher wrote:
>> What seems to be happening in the kernel is that the estimated device bandwith
>> drops to zero. I'm not even sure how this works for FUSE, but that's what I
>> gathered from some printk debugging.
>
> Yeah, writeback bw getting messed up is the most likely cause.  Prolly
> some silly bug.  I can reproduce the problem.  Looking into it.

Probably you want to look into:
https://lkml.org/lkml/2016/3/10/21

The patch mentioned above solves the issue for me.

Thanks,
Ashish
>
> Thanks.
>
> --
> tejun

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

[Tejun Heo]

Hello,

On Tue, Apr 12, 2016 at 02:54:08PM +0530, Ashish Sangwan wrote:
> On Tue, Apr 12, 2016 at 5:54 AM, Tejun Heo <tj@kernel.org> wrote:
> > Hello,
> >
> > On Mon, Apr 11, 2016 at 10:04:42AM +0200, Jakob Unterwurzacher wrote:
> >> What seems to be happening in the kernel is that the estimated device bandwith
> >> drops to zero. I'm not even sure how this works for FUSE, but that's what I
> >> gathered from some printk debugging.
> >
> > Yeah, writeback bw getting messed up is the most likely cause.  Prolly
> > some silly bug.  I can reproduce the problem.  Looking into it.
> 
> Probably you want to look into:
> https://lkml.org/lkml/2016/3/10/21
> 
> The patch mentioned above solves the issue for me.

Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
that is the right fix.

Thanks.

-- 
tejun

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

[Sedat Dilek]

On Tue, Apr 12, 2016 at 1:09 PM, Tejun Heo <tj@kernel.org> wrote:
> Hello,
>
> On Tue, Apr 12, 2016 at 02:54:08PM +0530, Ashish Sangwan wrote:
>> On Tue, Apr 12, 2016 at 5:54 AM, Tejun Heo <tj@kernel.org> wrote:
>> > Hello,
>> >
>> > On Mon, Apr 11, 2016 at 10:04:42AM +0200, Jakob Unterwurzacher wrote:
>> >> What seems to be happening in the kernel is that the estimated device bandwith
>> >> drops to zero. I'm not even sure how this works for FUSE, but that's what I
>> >> gathered from some printk debugging.
>> >
>> > Yeah, writeback bw getting messed up is the most likely cause.  Prolly
>> > some silly bug.  I can reproduce the problem.  Looking into it.
>>
>> Probably you want to look into:
>> https://lkml.org/lkml/2016/3/10/21
>>
>> The patch mentioned above solves the issue for me.
>
> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
> that is the right fix.
>

Feel free to add my...

     Tested-by: Sedat Dilek <sedat.dilek@gmail.com>

Patch available from [1].

- sed@ -

[1] https://patchwork.kernel.org/patch/8554181/

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

[Jakob Unterwurzacher]

On 12.04.2016 13:09, Tejun Heo wrote:
>>
>> Probably you want to look into:
>> https://lkml.org/lkml/2016/3/10/21
>>
>> The patch mentioned above solves the issue for me.
> 
> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
> that is the right fix.

Works wonderfully now, thanks to everybody involved. Is it too late for 4.6?

Best regards,
Jakob

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

[Howard Cochran]

On Mon, Apr 18, 2016 at 5:06 PM, Jakob Unterwurzacher
<jakobunt@gmail.com> wrote:
> On 12.04.2016 13:09, Tejun Heo wrote:
>>>
>>> Probably you want to look into:
>>> https://lkml.org/lkml/2016/3/10/21
>>>
>>> The patch mentioned above solves the issue for me.
>>
>> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
>> that is the right fix.
>
> Works wonderfully now, thanks to everybody involved. Is it too late for 4.6?
>
> Best regards,
> Jakob
>
Jakob, et. al.

You're welcome.  That performance problem stumped me for a couple of
weeks until tracked it down and submitted the fix.

Howard

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

[Sedat Dilek]

On Wed, Apr 20, 2016 at 3:35 AM, Howard Cochran
<hcochran@kernelspring.com> wrote:
> On Mon, Apr 18, 2016 at 5:06 PM, Jakob Unterwurzacher
> <jakobunt@gmail.com> wrote:
>> On 12.04.2016 13:09, Tejun Heo wrote:
>>>>
>>>> Probably you want to look into:
>>>> https://lkml.org/lkml/2016/3/10/21
>>>>
>>>> The patch mentioned above solves the issue for me.
>>>
>>> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
>>> that is the right fix.
>>
>> Works wonderfully now, thanks to everybody involved. Is it too late for 4.6?
>>
>> Best regards,
>> Jakob
>>
> Jakob, et. al.
>
> You're welcome.  That performance problem stumped me for a couple of
> weeks until tracked it down and submitted the fix.
>

What has happened to "writeback: Fix performance regression in
wb_over_bg_thresh()"?
I checked Linux v4.6-rc5, it is not included.
Is it in another Git tree? If yes, where?

- sed@ -

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

[Sedat Dilek]

On 4/25/16, Sedat Dilek <sedat.dilek@gmail.com> wrote:
> On Wed, Apr 20, 2016 at 3:35 AM, Howard Cochran
> <hcochran@kernelspring.com> wrote:
>> On Mon, Apr 18, 2016 at 5:06 PM, Jakob Unterwurzacher
>> <jakobunt@gmail.com> wrote:
>>> On 12.04.2016 13:09, Tejun Heo wrote:
>>>>>
>>>>> Probably you want to look into:
>>>>> https://lkml.org/lkml/2016/3/10/21
>>>>>
>>>>> The patch mentioned above solves the issue for me.
>>>>
>>>> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
>>>> that is the right fix.
>>>
>>> Works wonderfully now, thanks to everybody involved. Is it too late for
>>> 4.6?
>>>
>>> Best regards,
>>> Jakob
>>>
>> Jakob, et. al.
>>
>> You're welcome.  That performance problem stumped me for a couple of
>> weeks until tracked it down and submitted the fix.
>>
>
> What has happened to "writeback: Fix performance regression in
> wb_over_bg_thresh()"?
> I checked Linux v4.6-rc5, it is not included.
> Is it in another Git tree? If yes, where?
>

Ping!

Patch from Howard ist still not in Linus upstream aka Linux v4.6-rc6.
Comments, please.

- sed@ -

Re: Horrible mmap write performance (kernel writeback issue?)

[Jakob Unterwurzacher]

[-- Attachment #1.1: Type: text/plain, Size: 1497 bytes --]

Landed in 4.6-rc7:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=74d369443325063a5f0260e63971decb950fd8fa

On Mon, May 2, 2016 at 11:39 AM, Sedat Dilek <sedat.dilek-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

> On 4/25/16, Sedat Dilek <sedat.dilek-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> > On Wed, Apr 20, 2016 at 3:35 AM, Howard Cochran
> > <hcochran-UM4yVSfgXfh5+dc36SJeWg@public.gmane.org> wrote:
> >> On Mon, Apr 18, 2016 at 5:06 PM, Jakob Unterwurzacher
> >> <jakobunt-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> >>> On 12.04.2016 13:09, Tejun Heo wrote:
> >>>>>
> >>>>> Probably you want to look into:
> >>>>> https://lkml.org/lkml/2016/3/10/21
> >>>>>
> >>>>> The patch mentioned above solves the issue for me.
> >>>>
> >>>> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.  Yeah,
> >>>> that is the right fix.
> >>>
> >>> Works wonderfully now, thanks to everybody involved. Is it too late for
> >>> 4.6?
> >>>
> >>> Best regards,
> >>> Jakob
> >>>
> >> Jakob, et. al.
> >>
> >> You're welcome.  That performance problem stumped me for a couple of
> >> weeks until tracked it down and submitted the fix.
> >>
> >
> > What has happened to "writeback: Fix performance regression in
> > wb_over_bg_thresh()"?
> > I checked Linux v4.6-rc5, it is not included.
> > Is it in another Git tree? If yes, where?
> >
>
> Ping!
>
> Patch from Howard ist still not in Linus upstream aka Linux v4.6-rc6.
> Comments, please.
>
> - sed@ -
>

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

[Sedat Dilek]

On 5/11/16, Jakob Unterwurzacher <jakobunt@gmail.com> wrote:
> Landed in 4.6-rc7:
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=74d369443325063a5f0260e63971decb950fd8fa
>

Top-posting :-(.

Linux v4.4.10 and v4.5.4 final (currently in review as
4.4.10-rc1/4.5.4-rc1) will have it, too.

- Sedat -

> On Mon, May 2, 2016 at 11:39 AM, Sedat Dilek <sedat.dilek@gmail.com> wrote:
>
>> On 4/25/16, Sedat Dilek <sedat.dilek@gmail.com> wrote:
>> > On Wed, Apr 20, 2016 at 3:35 AM, Howard Cochran
>> > <hcochran@kernelspring.com> wrote:
>> >> On Mon, Apr 18, 2016 at 5:06 PM, Jakob Unterwurzacher
>> >> <jakobunt@gmail.com> wrote:
>> >>> On 12.04.2016 13:09, Tejun Heo wrote:
>> >>>>>
>> >>>>> Probably you want to look into:
>> >>>>> https://lkml.org/lkml/2016/3/10/21
>> >>>>>
>> >>>>> The patch mentioned above solves the issue for me.
>> >>>>
>> >>>> Heh, I tracked it down to wb_over_bg_thresh() and fell asleep.
>> >>>> Yeah,
>> >>>> that is the right fix.
>> >>>
>> >>> Works wonderfully now, thanks to everybody involved. Is it too late
>> >>> for
>> >>> 4.6?
>> >>>
>> >>> Best regards,
>> >>> Jakob
>> >>>
>> >> Jakob, et. al.
>> >>
>> >> You're welcome.  That performance problem stumped me for a couple of
>> >> weeks until tracked it down and submitted the fix.
>> >>
>> >
>> > What has happened to "writeback: Fix performance regression in
>> > wb_over_bg_thresh()"?
>> > I checked Linux v4.6-rc5, it is not included.
>> > Is it in another Git tree? If yes, where?
>> >
>>
>> Ping!
>>
>> Patch from Howard ist still not in Linus upstream aka Linux v4.6-rc6.
>> Comments, please.
>>
>> - sed@ -
>>
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