Mapping Processes and CPUs

Mapping Processes and CPUs

Normally IRIX tries to keep all CPUs busy, dispatching the next ready process to the next available CPU. (This simple picture is complicated by the needs of affinity scheduling, deadline scheduling, and gang scheduling). Since the number of ready processes changes all the time, dispatching is a random process. A process cannot predict how often or when it will next be able to run. For normal programs this does not matter, as long as each process continues to run at a satisfactory average rate.

Real-time processes cannot tolerate this unpredictability. To reduce it, you can dedicate one or more CPUs to real-time work. There are two steps:

Restrict one or more CPUs from normal scheduling, so that they can run only the processes that are specifically assigned to them.
Assign one or more processes to run on the restricted CPUs.

A process on a dedicated CPU runs when it needs to run, delayed only by interrupt service and by kernel scheduling cycles (if scheduling is enabled on that CPU). For details, see "Assigning Work to a Restricted CPU". The REACT/Pro Frame Scheduler takes care of both steps automatically; see "REACT/Pro Frame Scheduler".