3.2.3.2.2. Is Symmetric Multiprocessing Right for You?

Symmetric multiprocessing (also known as SMP) makes it possible for a computer system to have more than one CPU sharing all system resources. This means that, unlike a uniprocessor system, an SMP system may actually have more than one process running at the same time.

At first glance, this seems like any system administrator's dream. First and foremost, SMP makes it possible to increase a system's CPU power even if CPUs with faster clock speeds are not available -- just by adding another CPU. However, this flexibility comes with some caveats.

The first caveat is that not all systems are capable of SMP operation. Your system must have a motherboard designed to support multiple processors. If it does not, a motherboard upgrade (at the least) would be required.

The second caveat is that SMP increases system overhead. This makes sense if you stop to think about it; with more CPUs to schedule work for, the operating system requires more CPU cycles for overhead. Another aspect to this is that with multiple CPUs, there can be more contention for system resources. Because of these factors, upgrading a dual-processor system to a quad-processor unit does not result in a 100% increase in available CPU power. In fact, depending on the actual hardware, the workload, and the processor architecture, it is possible to reach a point where the addition of another processor could actually reduce system performance.

Another point to keep in mind is that SMP does not help workloads consisting of one monolithic application with a single stream of execution. In other words, if a large compute-bound simulation program runs as one process and without threads, it will not run any faster on an SMP system than on a single-processor machine. In fact, it may even run somewhat slower, due to the increased overhead SMP brings. For these reasons, many system administrators feel that when it comes to CPU power, single stream processing power is the way to go. It provides the most CPU power with the fewest restrictions on its use.

While this discussion seems to indicate that SMP is never a good idea, there are circumstances in which it makes sense. For example, environments running multiple highly compute-bound applications are good candidates for SMP. The reason for this is that applications that do nothing but compute for long periods of time keep contention between active processes (and therefore, the operating system overhead) to a minimum, while the processes themselves keep every CPU busy.

One other thing to keep in mind about SMP is that the performance of an SMP system tends to degrade more gracefully as the system load increases. This does make SMP systems popular in server and multi-user environments, as the ever-changing process mix can impact the system-wide load less on a multi-processor machine.