Re: [PATCH 7/8] wbt: add general throttling mechanism

[Jan Kara]

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:
...

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.

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...

								Honza
-- 
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR