CPU Affinity

In the previous example, we bound Quake to one of the two processors in our system. To ensure top-notch frame rates, we need to bind all the other processes on the system to the other processor. You can do this by hand or by writing a crafty script, but neither is efficient. Instead, make use of the fact that CPU affinity is inherited across a fork(). All of a process' children receive the same CPU affinity mask as their parent.

Then, all we need to do is have init bind itself to one processor. All other processes, by nature of init being the root of the process tree and thus the superparent of all processes, are then likewise bound to the one processor.

The cleanest way to do this type of bind is to hack this feature into init itself and pass in the desired CPU affinity mask using the kernel command line. We can accomplish our goal with a simpler solution, though, without having to modify and recompile init. Instead, we can edit the system startup script. On most systems this is /etc/rc.d/rc.sysinit or /etc/rc.sysinit, the first script run by init. Place the sample bind program in /bin, and add these lines to the start of rc.sysinit:

/bin/bind 1 1
/bin/bind $$ 1

These lines bind init (whose PID is one) and the current process to processor zero. All future processes will fork from one of these two processes and thus inherit the CPU affinity mask. You then can bind your process (whether it be a real-time nuclear control system or Quake) to processor one. All processes will run on processor zero except our special process (and any children), which will run on processor one. This ensures that the entire processor is available for our special process.

Long before Linus merged the CPU affinity system calls, the kernel supported and respected a CPU affinity mask. There was no interface by which user space could set the mask.

Each process' mask is stored in its task_struct as an unsigned long, cpus_allowed. The task_struct structure is called the process descriptor. It stores all the information about a process. The CPU affinity interface merely reads and writes cpus_allowed.

Whenever the kernel attempts to migrate a process from one processor to another, it first checks to see if the destination processor's bit is set in cpus_allowed. If the bit is not set, the kernel does not migrate the process. Further, whenever the CPU affinity mask is changed, if the process is no longer on an allowed processor it is migrated to one that is allowed. This ensures the process begins on a legal processor and can migrate only to a legal processor. Of course, if it is bound to only a single processor, it does not migrate anywhere.

The CPU affinity interface introduced in 2.5 and back-ported elsewhere provides a simple yet powerful mechanism for controlling which processes are scheduled onto which processors. Users with more than one processor may find the system calls useful in squeezing another drop of performance out of their systems or for ensuring that processor time is available for even the most demanding real-time task. Of course, users with only one processor need not feel left out. They also can use the system calls, but they aren't going to be too useful.

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