How to use the Noop IO Scheduler

Solution Verified - Updated -

Environment

  • Red Hat Enterprise Linux (RHEL) 4
  • Red Hat Enterprise Linux (RHEL) 5
  • Red Hat Enterprise Linux (RHEL) 6
  • Red Hat Enterprise Linux (RHEL) 7

Issue

  • How to turn on the noop scheduler for a device?
  • What are the tunables for noop scheduler and what do they do?
  • How does the logic within the scheduler work in choosing which IO to dispatch next?

Resolution

 
Enable noop io scheduler

  • Per device:

    $ echo 'noop' > /sys/block/sda/queue/scheduler
    $ cat           /sys/block/sda/queue/scheduler
    [noop] anticipatory deadline cfq
    
  • All devices (at boot):
    • RHEL 4, 5, 6: you can set noop for all devices via adding elevator=noop to the end of the kernel line in /etc/grub.conf file:

      title Red Hat Enterprise Linux Server (2.6.9-67.EL)
      root (hd0,0)
      kernel /vmlinuz-2.6.9-67.EL ro root=/dev/vg0/lv0 elevator=noop
      initrd /initrd-2.6.9-67.EL.img

    • RHEL 7: you can set noop for all devices via editting /etc/default/grub as show below and then rebuilding the grub2 configuration file:

      [root@host ~]# cat /etc/default/grub
      GRUB_TIMEOUT=5
      GRUB_DISTRIBUTOR="$(sed 's, release .*$,,g' /etc/system-release)"
      GRUB_DEFAULT=saved
      GRUB_DISABLE_SUBMENU=true
      GRUB_TERMINAL_OUTPUT="console"
      GRUB_CMDLINE_LINUX="crashkernel=auto rd.lvm.lv=rhel00/root rd.lvm.lv=rhel00/swap elevator=noop"
      GRUB_DISABLE_RECOVERY="true"
      • Rebuild the /boot/grub2/grub.cfg file by running the grub2-mkconfig command as follows:
        • On BIOS-based machines:

          [root@host ~]# grub2-mkconfig -o /boot/grub2/grub.cfg 
          Generating grub configuration file ...
          Found linux image: /boot/vmlinuz-3.10.0-327.el7.x86_64
          Found initrd image: /boot/initramfs-3.10.0-327.el7.x86_64.img
          Found linux image: /boot/vmlinuz-0-rescue-537dacc8159a4d4caaa419342da0b820
          Found initrd image: /boot/initramfs-0-rescue-537dacc8159a4d4caaa419342da0b820.img
          done

        • On UEFI-based machines:
          [root@host ~]# grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg 
          Generating grub configuration file ...
          Found linux image: /boot/vmlinuz-3.10.0-327.el7.x86_64
          Found initrd image: /boot/initramfs-3.10.0-327.el7.x86_64.img
          Found linux image: /boot/vmlinuz-0-rescue-537dacc8159a4d4caaa419342da0b820
          Found initrd image: /boot/initramfs-0-rescue-537dacc8159a4d4caaa419342da0b820.img
          done

 
noop tunables

$ ls            /sys/block/sda/queue/iosched
$

That's right, the directory of tunables is empty - there are no tunables for noop.

See Understanding the Noop IO Scheduler for further information.

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3 Comments

On UEFI- based machines, one must execute grub2-mkconfig -o /boot/efi/EFI/redhat/grub.cfg instead of grub2-mkconfig -o /boot/grub2/grub.cfg.

For more info, see: Customizing the GRUB 2 Configuration File.

Is this persistent across kernel updates? Or will we, say after upgrade to newer kernel release, perform this change again? If so, wouldn't a udev rule based setting be more efficient ?

grubby --update-kernel=ALL --remove-args="rhgb quiet" --args="elevator=noop" works carries over to new kernels as well. It modifies /boot/grub2/grub.cfg

My question is why does it not change /etc/default/grub or am I doing something wrong?