CPU Affinity

On most systems, Linux included, the interface for setting CPU affinity uses a bitmask. A bitmask is a series of n bits, where each bit individually corresponds to the status of some other object. For example, CPU affinity (on 32-bit machines) is represented by a 32-bit bitmask. Each bit represents whether the given task is bound to the corresponding processor. Count the bits from right to left, bit 0 to bit 31 and, thus, processor zero to processor 31. For example:

11111111111111111111111111111111 = 4,294,967,295

is the default CPU affinity mask for all processes. Because all bits are set, the process can run on any processor. Conversely:

00000000000000000000000000000001 = 1

Get it? What do the next two masks equal in decimal? What is the result of using them as the affinity mask of a process?

10000000000000000000000000000000
00000000000000000000000000000011

The first is equal to 2,147,483,648 and, because bit 31 is set, binds the process to processor number 31. The second is equal to 3, and it binds the process in question to processor zero and processor one.

The Linux CPU affinity interface uses a bitmask like that shown above. Unfortunately, C does not support binary constants, so you always have to use the decimal or hexadecimal equivalent. You may get a compiler warning for very large decimal constants that set bit 31, but they will work.

With the correct kernel and glibc in hand, using the system calls is easy:

#define _GNU_SOURCE
#include <sched.h>
long
sched_setaffinity(pid_t pid, unsigned int len,
                  unsigned long *user_mask_ptr);
long
sched_getaffinity(pid_t pid, unsigned int len,
                  unsigned long *user_mask_ptr);

The first system call is used to set the affinity of a process, and the second system call retrieves it.

In either system call, the PID argument is the PID of the process whose mask you wish to set or retrieve. If the PID is set to zero, the PID of the current task is used.

The second argument is the length in bytes of the CPU affinity bitmask, currently four bytes (32 bits). This number is included in case the kernel ever changes the size of the CPU affinity mask and allows the system calls to be forward-compatible with any changes; breaking syscalls is bad form, after all. The third argument is a pointer to the bitmask itself.

Let us look at retrieving the CPU affinity of a task:

unsigned long mask;
unsigned int len = sizeof(mask);
if (sched_getaffinity(0, len, &mask) < 0) {
    perror("sched_getaffinity");
    return -1;
    }
printf("my affinity mask is: %08lx\n", mask);

As a convenience, the returned mask is binary ANDed against the mask of all processors in the system. Thus, processors in your system that are not on-line have corresponding bits that are not set. For example, a uniprocessor system always returns 1 for the above call (bit 0 is set and no others).

Setting the mask is equally easy:

unsigned long mask = 7; /* processors 0, 1, and 2 */
unsigned int len = sizeof(mask);
if (sched_setaffinity(0, len, &mask) < 0) {
    perror("sched_setaffinity");
}

This example binds the current process to the first three processors in the system.

You then can call sched_getaffinity() to ensure the change took effect. What does sched_getaffinity() return for the above setup if you have only two processors? What if you have only one? The system call fails unless at least one processor in the bitmask exists. Using a mask of zero always fails. Likewise, binding to processor seven if you do not have a processor seven will fail.

It is possible to retrieve the CPU affinity mask of any process on the system. You can set the affinity of only the processes you own, however. Of course, root can set any process' affinity.

If you are not a programmer, or if you cannot modify the source for whatever reason, you still can bind processes. Listing 1 is the source code for a simple command-line utility to set the CPU affinity mask of any process, given its PID. As we discussed above, you must own the process or be root to do this.

Listing 1. bind

Usage is simple; once you learn the decimal equivalent of the CPU mask, you need:

usage: bind pid cpu_mask

As an example, assume we have a dual computer and want to bind our Quake process (with PID 1600) to processor two. We would enter the following:

bind 1600 2