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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
