Hi all,
I've run into a problem with Cgroup v2 where it doesn't seem to correctly limit
I/O operations when I set both wbps and wiops for a device. However, if I only
set wbps, then everything works as expected.
To reproduce the problem, we can follow these command-based steps:
1. **System Information:**
- Kernel Version and OS Release:
```
$ uname -r
6.10.0-rc5+
$ cat /etc/os-release
PRETTY_NAME="Ubuntu 24.04 LTS"
NAME="Ubuntu"
VERSION_ID="24.04"
VERSION="24.04 LTS (Noble Numbat)"
VERSION_CODENAME=noble
ID=ubuntu
ID_LIKE=debian
HOME_URL="https://www.ubuntu.com/";
SUPPORT_URL="https://help.ubuntu.com/";
BUG_REPORT_URL="https://bugs.launchpad.net/ubuntu/";
PRIVACY_POLICY_URL="https://www.ubuntu.com/legal/terms-and-policies/privacy-policy";
UBUNTU_CODENAME=noble
LOGO=ubuntu-logo
```
2. **Device Information and Settings:**
- List Block Devices and Scheduler:
```
$ lsblk
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINTS
sda 8:0 0 4.4T 0 disk
└─sda1 8:1 0 4.4T 0 part /data
...
$ cat /sys/block/sda/queue/scheduler
none [mq-deadline] kyber bfq
$ cat /sys/block/sda/queue/rotational
1
```
3. **Reproducing the problem:**
- Navigate to the cgroup v2 filesystem and configure I/O settings:
```
$ cd /sys/fs/cgroup/
$ stat -fc %T /sys/fs/cgroup
cgroup2fs
$ mkdir test
$ echo "8:0 wbps=10485760 wiops=100000" > io.max
```
In this setup:
wbps=10485760 sets the write bytes per second limit to 10 MB/s.
wiops=100000 sets the write I/O operations per second limit to 100,000.
- Add process to the cgroup and verify:
```
$ echo $$ > cgroup.procs
$ cat cgroup.procs
3826771
3828513
$ ps -ef|grep 3826771
root 3826771 3826768 0 22:04 pts/1 00:00:00 -bash
root 3828761 3826771 0 22:06 pts/1 00:00:00 ps -ef
root 3828762 3826771 0 22:06 pts/1 00:00:00 grep --color=auto 3826771
```
- Observe I/O performance using `dd` commands and `iostat`:
```
$ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
$ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
```
```
$ iostat -d 1 -h -y -p sda
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
7.00 0.0k 1.3M 0.0k 0.0k 1.3M 0.0k sda
7.00 0.0k 1.3M 0.0k 0.0k 1.3M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
5.00 0.0k 1.2M 0.0k 0.0k 1.2M 0.0k sda
5.00 0.0k 1.2M 0.0k 0.0k 1.2M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
21.00 0.0k 1.4M 0.0k 0.0k 1.4M 0.0k sda
21.00 0.0k 1.4M 0.0k 0.0k 1.4M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
5.00 0.0k 1.2M 0.0k 0.0k 1.2M 0.0k sda
5.00 0.0k 1.2M 0.0k 0.0k 1.2M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
5.00 0.0k 1.2M 0.0k 0.0k 1.2M 0.0k sda
5.00 0.0k 1.2M 0.0k 0.0k 1.2M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
1848.00 0.0k 448.1M 0.0k 0.0k 448.1M 0.0k sda
1848.00 0.0k 448.1M 0.0k 0.0k 448.1M 0.0k sda1
```
Initially, the write speed is slow (<2MB/s) then suddenly bursts to several
hundreds of MB/s.
- Testing with wiops set to max:
```
echo "8:0 wbps=10485760 wiops=max" > io.max
$ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
$ dd if=/dev/zero of=/data/file1 bs=512M count=1 &
```
```
$ iostat -d 1 -h -y -p sda
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
48.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda
48.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
40.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda
40.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
41.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda
41.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
46.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda
46.00 0.0k 10.0M 0.0k 0.0k 10.0M 0.0k sda1
tps kB_read/s kB_wrtn/s kB_dscd/s kB_read kB_wrtn kB_dscd Device
55.00 0.0k 10.2M 0.0k 0.0k 10.2M 0.0k sda
55.00 0.0k 10.2M 0.0k 0.0k 10.2M 0.0k sda1
```
The iostat output shows the write operations as stabilizing at around 10 MB/s,
which aligns with the defined limit of 10 MB/s. After setting wiops to max, the
I/O limits appear to work as expected.
Thanks,
Lance
