Restricting a CPU From Scheduled Work

Restricting a CPU From Scheduled Work

For best performance of a real-time process or for minimum interrupt response time, you need to use one or more CPUs without competition from other scheduled processes. You can exert three levels of increasing control: restricted, isolated, and nonpreemptive.

In general, the IRIX scheduling algorithms will run a process that is ready to run on any CPU. This is modified by considerations of

affinity--CPUs are made to execute the processes that have developed affinity to them
processor group assignments--the pset command can force a specified group of CPUs to service only a given scheduling queue

You can restrict one or more CPUs from running any scheduled processes at all. The only processes that can use a restricted CPU are processes that you assign to those CPUs.

Note: Restricting a CPU overrides any group assignment made with pset. A restricted CPU remains part of a group, but does not perform any work you assign to the group using pset. You can find out the number of CPUs that exist, and the number that are still unrestricted, using the sysmp() function as in Example 6-9.

Example 6-9 : Number of Processors Available and Total

#include <sys/sysmp.h>
int CPUsInSystem = sysmp(MP_NPROCS);
int CPUsNotRestricted = sysmp(MP_NAPROCS);

To restrict one or more CPUs, you can use mpadmin. For example, to restrict CPUs 4 and 5, you can use

mpadmin -r 4 mpadmin -r 5

The equivalent operation from within a program uses sysmp() as in Example 6-10 (see also the sysmp(2) reference page).

Example 6-10 : Restricting a CPU

#include <sys/sysmp.h>
int restrictCpuN(int cpu)
{
   int ret = sysmp(MP_RESTRICT,cpu);
   if (-1 == ret) perror("sysmp(MP_RESTRICT)");
   return ret;
}

You remove the restriction, allowing the CPU to execute any scheduled process, with mpadmin -u or with sysmp(MP_EMPOWER).

Note: The following points are important to remember:

The CPU assigned to handle the scheduling clock ("Assigning the Clock Processor") must not be restricted.
The REACT/Pro Frame Scheduler automatically restricts and isolates any CPU it uses. See Chapter 7.

Assigning Work to a Restricted CPU

After restricting a CPU, you can assign processes to it using the command runon (see the runon(1) reference page). For example, to run a program on CPU 3, you could use

runon 3 ~rt/bin/rtapp

The equivalent operation from within a program uses sysmp() as in Example 6-11 (see also the sysmp(2) reference page).

Example 6-11 : Assigning the Calling Process to a CPU

#include <sys/sysmp.h>
int runMeOn(int cpu)
{
   int ret = sysmp(MP_MUSTRUN,cpu);
   if (-1 == ret) perror("sysmp(MP_MUSTRUN)");
   return ret;
}

You remove the assignment, allowing the process to execute on any available CPU, with sysmp(MP_RUNANYWHERE). There is no command equivalent.

The assignment to a specified CPU is inherited by processes created by the assigned process. Thus if you assign a real-time program with runon, all the processes it creates run on that same CPU. More often you will want to run multiple processes concurrently on multiple CPUs. There are three approaches you can take:

Use the REACT/Pro Frame Scheduler, letting it restrict CPUs for you.
Let the parent process be scheduled normally using a nondegrading real-time priority. After creating child processes with sproc(), use schedctl(SCHEDMODE,SGS_GANG) to cause the share group to be gang-scheduled. Assign a processor group to service the gang-scheduled process queue.
The CPUs that service the gang queue cannot be restricted. However, if yours is the only gang-scheduled program, those CPUs will effectively be dedicated to your program.
Let the parent process be scheduled normally. Let it restrict as many CPUs as it will have child processes. Have each child process invoke sysmp(MP_MUSTRUN,cpu) when it starts, each specifying a different restricted CPU.