Rev | Author | # | Line |
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7 | AristotlePagaltzis | 1 | A system with multiple identical [CPU]s having equal access to memory and to the [I/O] subsytem(s). Contrast [NUMA] and AsymmetricMultiProcessing. |
5 | AristotlePagaltzis | 2 | |
4 | StuartYeates | 3 | 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. |
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7 | AristotlePagaltzis | 5 | 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. |
4 | StuartYeates | 6 | |
7 | 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. | ||
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7 | AristotlePagaltzis | 9 | 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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