Throttling CPU usage with Linux cgroups

There are a number of reasons you may want to throttle rather than limit a process's CPU usage on your system. One very good reason is to keep the CPU temperature down or to simply reduce the amount of energy a certain process uses.

Limiting versus throttling

The term "limit" is nearly always used where throttling is actually required. A good example of why the two are not interchangeable would be the current ISP industry:

Example 1: Sally signs up for super fast broadband (100 Mbps) but hasn't read the small print: she can only download 10 GB of data before her connection is terminated and she has to wait for the next month before she can continue to use her service. Sally's service is not throttled but it is limited.

Example 2: Tony signs up for a basic package (1 Mbps) as he doesn't need to use the Internet a great deal. However he had the good sense to use an unlimited package so that he doesn't hit any usage caps. His router syncs at 100 Mbps but he only receives a 1 Mbps service. The ISPs equipment is throttling his service, but not limiting it.

Example 3: Benedict has signed up for some deal without reading any of the details. He receives a 20 Mbps service and is happy with the speeds. Unfortunately for him after downloading 5 GB of data his download rate drops to 1 Mbps. He has limits on his service which has led to it being throttled.

There are many instances where you may wish to both limit and throttle CPU usage. The former is very easy and well documented, the latter not so much.