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Message-ID: <zjbn575huc6pk7jpv2ipoayfk4bvfu5z5imb5muk5drksa7p3q@xcr5imtt4zro>
Date: Mon, 12 Aug 2024 17:43:04 +0200
From: Michal Koutný <mkoutny@...e.com>
To: Lance Yang <ioworker0@...il.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
linux-block@...r.kernel.org, cgroups@...r.kernel.org, josef@...icpanda.com, tj@...nel.org,
fujita.tomonori@....ntt.co.jp, boqun.feng@...il.com, a.hindborg@...sung.com,
paolo.valente@...more.it, axboe@...nel.dk, vbabka@...nel.org, david@...hat.com,
21cnbao@...il.com, baolin.wang@...ux.alibaba.com, libang.li@...group.com,
Yu Kuai <yukuai3@...wei.com>
Subject: Re: [BUG] cgroupv2/blk: inconsistent I/O behavior in Cgroup v2 with
set device wbps and wiops
+Cc Kuai
On Mon, Aug 12, 2024 at 11:00:30PM GMT, Lance Yang <ioworker0@...il.com> wrote:
> 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.
What it would be on average?
IOW how long would the whole operation in throttled cgroup take?
>
> - 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
Thanks for the report Lance. Is this something you started seeing after
a kernel update or switch to cgroup v2? (Or you simply noticed with this
setup only?)
Michal
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