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Paul Draper
Asked: 2020-01-25 09:34:35 +0800 CST

How to get I/O priority to work on Ubuntu?

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Ubuntu has ionice, but as far as I can tell, it does absolutely nothing.

I suspect this is because Ubuntu replaced cfq with deadline and deadline doesn't support priorities.

Is there any possible way to have prioritized I/O on Ubuntu anymore?

EDIT: The context is that I have a database restore that easily consumes all my I/O and renders my system unusable until it has finished. I'd like it to remain usable for other tasks.

performance scheduler io

2 Answers

  1. Best Answer

    You have to change your scheduler from deadlineto cfq.

    You can do that per blockdevice and non permanent via 

    sudo echo cfq > /sys/block/<blockdevice>/queue

    Or for the whole system, permanent via grub, with adding the elevator=cfqparameter to the grub commandline options.

    This is an excellent post explaining how you can do that.

    What the drawbacks of such a change are is better covered elsewhere.

    The TL;DR is

    According to the test results, each scheduler has different advantages over others. CFQ scheduler is suitable for the systems that require balanced I/O access and do not need process prioritization. Deadline scheduler has better performance on read-intensive works. Noop is for the systems on the cloud or hypervisors. BFQ performs better on interactive use-case scenarios. Noop is the simplest scheduler and it is considered to have the potential for optimized new implementations targeting SSD block devices.

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  2. This answer might be helpfull for Ubuntu 20.04, which lacks the cfq scheduler: https://askubuntu.com/a/1350605

    From that answer:

    Ubuntu 20.04 is using Kernel 5.4 (with HWE: 5.8) with only the mq-deadline scheduler compiled in.

    You can check it by viewing /sys/block/sda/queue/scheduler. The active scheduler has square brackets around. If there are other schedulers compiled in the kernel they are also shown.

    Example:

    # cat /sys/block/sda/queue/scheduler
[mq-deadline] none

# uname -r
5.4.0-26-generic

    The mq-deadline scheduler does not support the mechanism used by ionice at the moment, see: https://unix.stackexchange.com/a/160081/27458

    Solution: Switch to BFQ scheduler

    The bfq scheduler does not need to be compiled in the kernel, it can be loaded afterwards using a kernel module.

    Switch to the BFQ scheduler:

    # modprobe  "bfq"
# echo "bfq" > /sys/block/sda/queue/scheduler
# echo "bfq" > /etc/modules-load.d/bfq.conf
# echo 'ACTION=="add|change", KERNEL=="sd*[!0-9]|sr*", ATTR{queue/scheduler}="bfq"' > /etc/udev/rules.d/60-scheduler.rules

    Check:

    # cat /sys/block/sda/queue/scheduler
mq-deadline [bfq] none

    It is probably a good idea to also do a reboot and check again.

    BFQ not available on "virtual" kernel

    If you are using a "virtual" kernel, you will probably not have the bfq kernel module available because it does not include the linux-modules-extra-5.xxx package.

    You can solve this by switching to the "generic-HWE" kernel:

    # sudo apt-get install linux-generic-hwe-20.04  linux-tools-generic-hwe-20.04
# reboot

    After reboot you should be on kernel 5.8.0-xxx-generic. You can check this:

    # uname -r
5.8.0-59-generic

    Now you can apply the above solution.

    Alternative solution: Systemd scope

    If you don't want to switch the IO scheduler, you can use a Systemd scope with a lower IO weight.

    Create a file /usr/local/bin/mh_ionice with contents:

    #!/bin/bash
if (( EUID == 0 )); then USERMODE=''; else USERMODE='--user'; fi
systemd-run \
  --collect \
  --quiet \
  --scope \
  $USERMODE \
  --nice=19 \
  --property="IOAccounting=yes" \
  --property="IOWeight=1" \
  "$@"

    Make it executable:

    chmod 755 /usr/local/bin/mh_ionice

    Now you can run:

    mh_ionice  [heavy_command] [arg] [arg] [arg]
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