Uniprocessor systems, like the one shown in Figure 11-1, are the simplest systems in terms of architecture. Each of these systems (typically a standard personal computer) contains a single CPU and a single I/O channel and is made entirely with industry-standard components. They are most often used as single-user stand-alone machines; for example, for database development or as client machines in a network. Some uniprocessor machines are also used as small servers for databases. Versions of this architecture made with more exotic RISC-based CPUs are typically used as engineering, scientific, or graphics workstations today.
Until the 1990s, uniprocessor systems were frequently used as servers because of their low price and the limited ability of relational databases to fully utilize other types of systems. However, Oracle evolved to take advantage of systems containing multiple CPUs through improved parallelism and more sophisticated optimization. At the same time, the price points of Symmetric Multiprocessing (SMP) systems (described in the next section) have plummeted dramatically, making SMP systems the database hardware servers of choice.
Although there is only a single processor in a uniprocessor system, the server operating systems used by these systems now support multiple threads. Each thread can be used to support a concurrent process, which can execute in parallel. By default, the PARALLEL_THREADS_PER_CPU parameter in the initialization file is set at 2 for most platforms on which Oracle runs. Oracle can further determine the degree of parallelism based on parameters set in the initialization file or using the adaptive degree of parallelism feature. This adaptive multiuser feature makes use of algorithms that take into account the number of threads. Additional tuning parameters can also affect parallelism. For more information about the parallel thread capability of Oracle, see Chapter 6.