Fair Share Scheduler (FSS)

Container Manager uses the fair share scheduler (FSS) to ensure the minimum CPU shares you set. The fair share scheduler is the default scheduler. The fair share scheduler calculates the proportion of CPU allocated to a project by dividing the shares for the project by the total number of active projects' shares. An active project is a project with at least one process that uses the CPU. Shares for idle projects, that is, projects with no active processes, are not used in the calculations.

For example, three projects, sales, marketing, and database, have two, one, and four shares allocated respectively. All projects are active. The CPU resources for the resource pool is distributed this way: the sales project receives to 2/7ths, the marketing project receives 1/7th, and the database project receives 4/7ths of the CPU resources. If the sales project is idle, then the marketing project receives 1/5th and the database project receives 4/5ths of the CPU resources.

Note that the fair share scheduler only limits CPU usage if there is competition for the CPU. A project that is the only active project on the system can use 100 percent of the CPU, regardless of the number of shares it holds. CPU cycles are not wasted. If a project does not use all of the CPU that it is entitled to use because it has no work to perform, the remaining CPU resources are distributed among other active processes. If a project does not have any CPU shares defined, it is assigned one share. Processes in projects with zero (0) shares are run at the lowest system priority. These processes only run when projects with nonzero shares are not using CPU resources.