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

[Jan Kara]

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

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