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bitwise or and samir can set requests and limits now via cgroups!

Youcef Guichi · · #cgroups#linux#containers#bitwise#kernel

My Own Docker Samir (WIP)#

Bitwise OR and Samir can set requests and limits now via cgroups!

Bitwise OR Refresher#

Remember learning bitwise OR at university but never quite understanding its practical use? Bitwise OR works with integer values by comparing their bits position by position.

Rule: If either bit is 1, the result is 1 at that position; otherwise, the result is 0.

Example:

 001
 010
=011

So, bitwise OR between 001 and 010 equals 011.

Kernel Feature Flags with Bitwise OR#

Suppose we have the following features, each represented in 8 bits:

  • feature1 = 00000001 = 1
  • feature2 = 00000010 = 2
  • feature3 = 00000100 = 4

If you want to enable feature1 and feature3, you don't pass them as a list. Instead, the kernel uses bitwise OR:

 00000001
 00000100
=00000101 = 5

The kernel enables features based on which bits are set to 1. So, 00000101 enables the first and third features.

In Go, bitwise OR is represented by |. For example:

CreateNamespaces(unix.CLONE_NEWUTS | unix.CLONE_NEWPID | unix.CLONE_NEWNS)

Resource Requests and Limits with Cgroups#

I have implemented resource requests and limits for both CPU and memory.

To create a new cgroup, place it in the cgroup filesystem:
/sys/fs/cgroup/your-group
This will automatically populate new files.

Memory Resources#

  • Requests: Write bytes to /sys/fs/cgroup/your-group/memory.low
  • Limits: Write bytes to /sys/fs/cgroup/your-group/memory.max

CPU Resources#

  • Requests: Write to /sys/fs/cgroup/your-group/cpu.weight
  • Limits: Write to /sys/fs/cgroup/your-group/cpu.max

cpu.max Format#

cpu.max expects the format: quota period.
For example, if you specify CPU as 500m (half a core), the kernel understands it in time, not cores.

Formula:
quota = (millicores / 1000) * period

The default period is 100000 microseconds.

For 500m:

  • quota = (500 / 1000) * 100000 = 50000 microseconds

So, write to /sys/fs/cgroup/your-group/cpu.max:

50000 100000

This means every 100000 microseconds, your process can use the CPU for 50000 microseconds.

You can find the implementation code in my samir repository on GitHub.