Processor affinity

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Processor affinity is a modification of the native central queue scheduling algorithm. Each task (be it process or thread) in the queue has a tag indicating its preferred / kin processor. At allocation time, each task is allocated to its kin processor in preference to others.

Processor affinity takes advantage of the fact that some remnants of a process may remain in one processor's state (in particular, in its cache) from the last time the process ran, and so scheduling it to run on the same processor the next time could result in the process running more efficiently than if it were to run on another processor.

Actual scheduling algorithm implementations vary in how strongly they will adhere to processor affinity. Under certain circumstances some implementations will allow a task to change to another processor if this is deemed to be most efficient under the circumstances. An obvious example involves two processor-intensive tasks (A & B) having affinity to one processor while another processor lies unused. Many algorithms would shift task B to the second processor in order to maximize processor utilization. Task B would then acquire affinity with the second processor while task A would continue to have affinity with the original processor.

When Windows XP sees two or more physical or logical processors, a Set Affinity setting is added (see below).

In Windows XP, the Set Affinity dialog allows for selecting up to 32 processors to execute a process. In this images, the user has a Pentium 4 with Hyper-Threading enabled.

Processor affinity can effectively reduce cache problems but it does not curb the persistent load-balancing problem.

Processor affinity becomes more complicated in systems with non-uniform architectures. As an example, a system with two dual-core hyper-threaded CPUs presents a challenge to a scheduling algorithm. There is complete affinity between two virtual CPUs implemented on the same core via hyper-threading; partial affinity between two cores on the same physical chip (as the cores share some, but not all, cache), and no affinity between separate physical chips.

On Linux the CPU affinity of a process might be altered with the taskset(1) program.

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This article has been tagged since November 2006.