CPU Affinity

The ability in Linux to bind one or more processes to one or more processors, called CPU affinity, is a long-requested feature. The idea is to say “always run this process on processor one” or “run these processes on all processors but processor zero”. The scheduler then obeys the order, and the process runs only on the allowed processors.

Other operating systems, such as Windows NT, have long provided a system call to set the CPU affinity for a process. Consequently, demand for such a system call in Linux has been high. Finally, the 2.5 kernel introduced a set of system calls for setting and retrieving the CPU affinity of a process.

In this article, I look at the reasons for introducing a CPU affinity interface to Linux. I then cover how to use the interface in your programs. If you are not a programmer or if you have an existing program you are unable to modify, I cover a simple utility for changing the affinity of a given process using its PID. Finally, we look at the actual implementation of the system call.

There are two types of CPU affinity. The first, soft affinity, also called natural affinity, is the tendency of a scheduler to try to keep processes on the same CPU as long as possible. It is merely an attempt; if it is ever infeasible, the processes certainly will migrate to another processor. The new O(1) scheduler in 2.5 exhibits excellent natural affinity. On the opposite end, however, is the 2.4 scheduler, which has poor CPU affinity. This behavior results in the ping-pong effect. The scheduler bounces processes between multiple processors each time they are scheduled and rescheduled. Table 1 is an example of poor natural affinity; Table 2 shows what good natural affinity looks like.

Table 1. The Ping-Pong Effect

Table 2. Good Affinitiy

Hard affinity, on the other hand, is what a CPU affinity system call provides. It is a requirement, and processes must adhere to a specified hard affinity. If a processor is bound to CPU zero, for example, then it can run only on CPU zero.

Before we cover the new system calls, let's discuss why anyone would need such a feature. The first benefit of CPU affinity is optimizing cache performance. I said the O(1) scheduler tries hard to keep tasks on the same processor, and it does. But in some performance-critical situations—perhaps a large database or a highly threaded Java server—it makes sense to enforce the affinity as a hard requirement. Multiprocessing computers go through a lot of trouble to keep the processor caches valid. Data can be kept in only one processor's cache at a time. Otherwise, the processor's cache may grow out of sync, leading to the question, who has the data that is the most up-to-date copy of the main memory? Consequently, whenever a processor adds a line of data to its local cache, all the other processors in the system also caching it must invalidate that data. This invalidation is costly and unpleasant. But the real problem comes into play when processes bounce between processors: they constantly cause cache invalidations, and the data they want is never in the cache when they need it. Thus, cache miss rates grow very large. CPU affinity protects against this and improves cache performance.

A second benefit of CPU affinity is a corollary to the first. If multiple threads are accessing the same data, it might make sense to bind them all to the same processor. Doing so guarantees that the threads do not contend over data and cause cache misses. This does diminish the performance gained from multithreading on SMP. If the threads are inherently serialized, however, the improved cache hit rate may be worth it.

The third and final benefit is found in real-time or otherwise time-sensitive applications. In this approach, all the system processes are bound to a subset of the processors on the system. The specialized application then is bound to the remaining processors. Commonly, in a dual-processor system, the specialized application is bound to one processor, and all other processes are bound to the other. This ensures that the specialized application receives the full attention of the processor.

The system calls are new, so they are not available yet in all systems. You need at least kernel 2.5.8-pre3 and glibc 2.3.1; glibc 2.3.0 supports the system calls, but it has a bug. The system calls are not yet in 2.4, but patches are available at www.kernel.org/pub/linux/kernel/people/rml/cpu-affinity.

Many distribution kernels also support the new system calls. In particular, Red Hat 9 is shipping with both kernel and glibc support for the new calls. Real-time solutions, such as MontaVista Linux, also fully support the new interface.