View Source: SymmetricMultiProcessing - Waikato Linux Users Group

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A system with multiple identical [CPU]s having equal access to memory and to the [I/O] subsytem(s). Contrast [NUMA] and AsymmetricMultiProcessing.

It is an accepted rule of thumb that due to the InterProcessCommunication "friction" required to synchronize tasks across [CPU]s, each additional [CPU] contributes about 1/6 less performance than the previous one. As a result, fitting more than four [CPU]s in a machine tends to cost heaps of money with surprisingly little to show for it.

On the OperatingSystem level, there is a wide span for the degree of symmetry expressed. On a fully symmetric system, any [CPU] may run any userland process, any kernel process, and any interrupt handler. This is rare in practice due to limitations in various platforms -- most of the time, the interrupt handler can only be run by the first [CPU]. In simpler OperatingSystem~s, this is also the only [CPU] that may kernel tasks. In yet simpler system designs, even user processes are assigned to a fixed [CPU] and cannot move among [CPU]s.

In practice, a less symmetric system OperatingSystem design may not be a drawback. It is desirable to tie processes to a certain [CPU] as long the system load is evenly distributed among [CPU]s, because this maximizes the efficiency of [CPU] caches. Even in a completely symmetric system, an intelligently written scheduler will take this into account and try not to shuffle tasks around needlessly.

Applications have to be explicitly designed to exploit the additional processing power available in an [SMP] system. This is not often the case. It only commonly applies to multithreaded applications, which are common in the [Java] world where the [GUI], GarbageCollection, and application logic will often run on sepreate [CPU]s. (What else can you do when all your [I/O] is blocking? At least there are nonblocking [I/O] classes in more recent [Java] versions, but of course they add yet another layer to the already byzantine library.)

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Last edited on Tuesday, November 2, 2004 4:53:14 am by AristotlePagaltzis

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