An SMP (for Symmetrical Multiprocessor) is a system in which several processors share the same memory and function under the control of just one executing operating system. An SMP generally provides—within its configurability limits—the best scalability and the best price/performance ratio. SMPs are tending to become commodities at the entry level, although there are some problems in extending SMP to support a very large number of processors—at the time of writing, somewhere around 100 processors seems the upper boundary. While this amount will increase over time, for applications that call for extreme processing capabilities (intensive numerical operations or decision support across very large databases), one turns to cluster or MPP systems.
A cluster is a collection of otherwise independent systems (“nodes”) connected by a fast interconnect. Each node runs under the control of its own copy of the operating system, although cluster software gives the user the illusion that it is a single, extremely powerful, high-capacity system. Clusters support high availability in a very natural way: any applications running on a failing node can be retried on the surviving nodes. While clusters can provide high performance, they do require that applications be written to support the programming model provided by the cluster. Both DBMSs and integrated management packages (for Enterprise Resource Planning) have been ported to cluster systems. While it can be hard for a cluster to deliver on the promise of its performance for transaction processing, it is much more straightforward for decision support.
From an architectural point of view, the only difference between a cluster and an MPP is the maximum number of processors supported—several hundreds for the MPP, instead of tens—together with a very high- performance interconnect. Key applications for MPP are intensive scientific workloads and decision support. To leverage the available raw performance, applications must be carefully written to match the platform architecture, with extreme attention being paid to extracting and exposing the maximum parallelism. Peer-to-peer networks, grid computing and clusters of PCs can offer an economic alternative to an MPP system.
Source of Information : Elsevier Server Architectures 2005
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