Date:	Tue, 3 May 2016 10:59:59 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Jan Kara <jack@...e.cz>, Jens Axboe <axboe@...com>
Cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-block@...r.kernel.org, dchinner@...hat.com,
	sedat.dilek@...il.com
Subject: Re: [PATCH 7/8] wbt: add general throttling mechanism

On 05/03/2016 09:48 AM, Jan Kara wrote:
> On Tue 03-05-16 17:40:32, Jan Kara wrote:
>> On Tue 03-05-16 11:34:10, Jan Kara wrote:
>>> Yeah, once I'll hunt down that regression with old disk, I can have a look
>>> into how writeback throttling plays together with blkio-controller.
>>
>> So I've tried the following script (note that you need cgroup v2 for
>> writeback IO to be throttled):
>>
>> ---
>> mkdir /sys/fs/cgroup/group1
>> echo 1000 >/sys/fs/cgroup/group1/io.weight
>> dd if=/dev/zero of=/mnt/file1 bs=1M count=10000&
>> DD1=$!
>> echo $DD1 >/sys/fs/cgroup/group1/cgroup.procs
>>
>> mkdir /sys/fs/cgroup/group2
>> echo 100 >/sys/fs/cgroup/group2/io.weight
>> #echo "259:65536 wbps=5000000" >/sys/fs/cgroup/group2/io.max
>> echo "259:65536 wbps=max" >/sys/fs/cgroup/group2/io.max
>> dd if=/dev/zero of=/mnt/file2 bs=1M count=10000&
>> DD2=$!
>> echo $DD2 >/sys/fs/cgroup/group2/cgroup.procs
>>
>> while true; do
>>          sleep 1
>>          kill -USR1 $DD1
>>          kill -USR1 $DD2
>>          echo  '======================================================='
>> done
>> ---
>>
>> and watched the progress of the dd processes in different cgroups. The 1/10
>> weight difference has no effect with your writeback patches - the situation
>> after one minute:
>>
>> 3120+1 records in
>> 3120+1 records out
>> 3272392704 bytes (3.3 GB) copied, 63.7119 s, 51.4 MB/s
>> 3217+1 records in
>> 3217+1 records out
>> 3374010368 bytes (3.4 GB) copied, 63.5819 s, 53.1 MB/s
>>
>> I should add that even without your patches the progress doesn't quite
>> correspond to the weight ratio:
>
> Forgot to fill in corresponding data for unpatched kernel here:
>
> 5962+2 records in
> 5962+2 records out
> 6252281856 bytes (6.3 GB) copied, 64.1719 s, 97.4 MB/s
> 1502+0 records in
> 1502+0 records out
> 1574961152 bytes (1.6 GB) copied, 64.207 s, 24.5 MB/s

Thanks for testing this, I'll see what we can do about that. It stands 
to reason that we'll throttle a heavier writer more, statistically. But 
I'm assuming this above test was run basically with just the writes 
going, so no real competition? And hence we end up throttling them 
equally much, destroying the weighting in the process. But for both 
cases, we basically don't pay any attention to cgroup weights.

>> but still there is noticeable difference to cgroups with different weights.
>>
>> OTOH blk-throttle combines well with your patches: Limiting one cgroup to
>> 5 M/s results in numbers like:
>>
>> 3883+2 records in
>> 3883+2 records out
>> 4072091648 bytes (4.1 GB) copied, 36.6713 s, 111 MB/s
>> 413+0 records in
>> 413+0 records out
>> 433061888 bytes (433 MB) copied, 36.8939 s, 11.7 MB/s
>>
>> which is fine and comparable with unpatched kernel. Higher throughput
>> number is because we do buffered writes and dd reports what it wrote into
>> page cache. And there is no wonder blk-throttle combines fine - it
>> throttles bios which happens before we reach writeback throttling
>> mechanism.

OK, that's good, at least that part works fine. And yes, the throttle 
path is hit before we end up in the make_request_fn, which is where wbt 
drops in.

>> So I belive this demonstrates that your writeback throttling just doesn't
>> work well with selective scheduling policy that happens below it because it
>> can essentially lead to IO priority inversion issues...

It this testing still done on the QD=1 ATA disk? Not too surprising that 
this falls apart, since we have very little room to maneuver. I wonder 
if a normal SATA with NCQ would behave better in this regard. I'll have 
to test a bit and think about how we can best handle this case.

-- 
Jens Axboe

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