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CPU Isolation#

The Replicated PV Mayastor will fully utilize each CPU core that it was configured to run on. It will spawn a thread on each and the thread will run in an endless loop serving tasks dispatched to it without sleeping or blocking. There are also other IO engine threads that are not bound to the CPU and those are allowed to block and sleep. However, the bound threads (also called reactors) rely on being interrupted by the kernel and other userspace processes as little as possible. Otherwise, the latency of I/O may suffer.

Ideally, the only thing that interrupts IO engine's reactor would be only kernel time-based interrupts responsible for CPU accounting. However, that is far from trivial. isolcpus option that we will be using does not prevent:

  • kernel threads and
  • other k8s pods to run on the isolated CPU

However, it prevents system services including kubelet from interfering with IO engine.

Set Linux kernel Boot Parameter#


The best way to accomplish this step may differ, based on the Linux distro that you are using.

Add the isolcpus kernel boot parameter to GRUB_CMDLINE_LINUX_DEFAULT in the grub configuration file, with a value which identifies the CPUs to be isolated (indexing starts from zero here). The location of the configuration file to change is typically /etc/default/grub but may vary. For example when running Ubuntu 20.04 in AWS EC2 Cloud boot parameters are in /etc/default/grub.d/50-cloudimg-settings.cfg.

In the following example we assume a system with 4 CPU cores in total, and that the third and the fourth CPU cores are to be dedicated to the IO engine.

GRUB_CMDLINE_LINUX_DEFAULT="quiet splash isolcpus=2,3"

Update Grub#


sudo update-grub

Example Output

Sourcing file `/etc/default/grub'
Sourcing file `/etc/default/grub.d/40-force-partuuid.cfg'
Sourcing file `/etc/default/grub.d/50-cloudimg-settings.cfg'
Sourcing file `/etc/default/grub.d/init-select.cfg'
Generating grub configuration file ...
Found linux image: /boot/vmlinuz-5.8.0-29-generic
Found initrd image: /boot/microcode.cpio /boot/initrd.img-5.8.0-29-generic
Found linux image: /boot/vmlinuz-5.4.0-1037-aws
Found initrd image: /boot/microcode.cpio /boot/initrd.img-5.4.0-1037-aws
Found Ubuntu 20.04.2 LTS (20.04) on /dev/xvda1

Reboot the System#


sudo reboot

Verify Isolcpus#

Basic verification is by outputting the boot parameters of the currently running kernel:


cat /proc/cmdline

Example Output

BOOT_IMAGE=/boot/vmlinuz-5.8.0-29-generic root=PARTUUID=7213a253-01 ro console=tty1 console=ttyS0 nvme_core.io_timeout=4294967295 isolcpus=2,3 panic=-1

You can also print a list of isolated CPUs:


cat /sys/devices/system/cpu/isolated

Example Output


Update Replicated PV Mayastor Helm Chart for CPU Core Specification#


Update the Helm value in the Replicated PV Mayastor Helm chart to specify the CPU core.

To allot specific CPU cores for IO engine's reactors, follow these steps:

  1. Ensure that you have the Replicated PV Mayastor kubectl plugin installed, matching the version of your Replicated PV Mayastor Helm chart deployment (releases). You can find installation instructions in the kubectl plugin documentation.

  2. Execute the following command to update Replicated PV Mayastor's configuration. Replace <namespace> with the appropriate Kubernetes namespace where Replicated PV Mayastor is deployed.

kubectl mayastor upgrade -n <namespace> --set-args 'io_engine.coreList={3,4}'

In the above command, io_engine.coreList={3,4} specifies that IO engine's reactors should operate on the third and fourth CPU cores.

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