06-cpu-throttling.md

title	expires_at	tags
CPU Throttling	never	garden-runc-release

CPU Throttling

Enable cpu throttling by setting the experimental_cpu_throttling flag to true in the garden config.

For details of the feature, see this blog post and this video.

Enabling and disabling cpu-throttling

Ensure you set the destroy_containers_on_start flag when changing this property, otherwise existing containers will be in the wrong part of the cgroup structure.

cgroup structure

When enabled, two extra cpu cgroup directories are inserted in the garden cgroup directory structure. App containers begin in the good cgroup subtree, and if their cpu usage allowance is exceeded, they are moved to the bad cgroup subtree. These directories are not present used when cpu-throttling is disabled. When enabling or disabling this feature there are two scenarios we want to avoid:

1. Switching from no throttling to throttling

In this case, if we have two apps - A and B (with 512 and 256 shares respectively), the cgroup structure would look like this:

garden - 1024
└ A - 512
└ B - 256

When we redeploy with throttling this will create two more cgroups under the garden one - good and bad. Any new containers will be put in the good cgroup upon creation, however, the old containers will not be moved. In this case, with a new container -D (with 256 shares), the structure would look like this:

garden - 1024
└ good - 1022
  └ D - 256
└ bad - 2
└ A - 512
└ B - 256

Obviously this is a problem, since it would mean that the sum of good + bad cgroups would not yield 100% of the cpu.

2. Switching from throttling to no throttling:

Lets say we have two apps - A and B and one of them is currently in the bad cgroup:

garden - 1024
└ good - 768
  └ A - 512
└ bad - 256
  └ B - 256

If we were to redeploy with cpu throttling disabled, this means that the app will never leave the bad cgroup and will always be throttled.