3.4. Multiprocessor Servers

A server with more than one processor is known as a multiprocessor server. One method of meeting the demand for more computing power is to include more than one processor in a server.

The term way is used to indicate the number of processors in a server. For example, a four-way server, such as the HP ProLiant DL580 G2, has four processors. An eight-way server, such as the HP ProLiant DL760, has eight processors.

Several factors can affect the performance of a multiprocessor server. For example, an operating system must be able to handle multiprocessing environments. Different operating systems handle multiprocessing differently. In some cases, the operating system sends all I/O interrupts to a single processor; in other cases, however, processors share I/O interrupts.

When designing a multiprocessor system, engineers must decide (1) how the processors will share the workload, and (2) how the processors will share memory and I/O resources.

3.4.1 Sharing the Workload

Two methods exist for a multiprocessor server to share the workload among the processors asymmetric multiprocessing (AMP) and symmetric multiprocessing (SMP). A comparison of these techniques is shown in Figure 3-10.

Figure 3-10. AMP vs. SMP.

3.4.1.1 ASYMMETRIC MULTIPROCESSING

In asymmetric multiprocessing, specific tasks are assigned to specific processors. This method can be inefficient because, depending on the type of applications, the workload among processors is unevenly distributed. Adding a new processor in an AMP system does not guarantee increased computing power. In some cases, however, keeping a single process on one processor is desirable.

3.4.1.2 SYMMETRIC MULTIPROCESSING

In symmetric multiprocessing systems, each task is executed on the next-available processor, resulting in excellent load-sharing capability. However, the operating system must be able to support SMP. Adding a new processor usually increases the computing power.

Note

Most HP industry-standard servers use SMP for multiprocessing.

3.4.2 Sharing Memory and I/O Resources

Two architectures are available for designing a multiprocessor system to share memory and I/O resources tightly coupled and loosely coupled.

3.4.2.1 TIGHTLY COUPLED

In the tightly coupled architecture, all processors share all memory and I/O resources, as shown in Figure 3-11. The operating system manages all the resources. If memory fails, all processors are affected. This requires a system bus that is common among all the processors and shared components.

Figure 3-11. Tightly coupled architecture.

Most HP industry-standard servers are based on a tightly coupled architecture.

3.4.2.2 LOOSELY COUPLED

In the loosely coupled architecture, each processor has memory and I/O resources assigned to it and, in a sense, acts as an independent computer, as illustrated in Figure 3-12. It does not share its resources with the other processors, but it does communicate and cooperate with them. If memory fails, only the processor using that memory is affected.

Figure 3-12. Loosely coupled architecture.

HP Himalaya servers use a loosely coupled architecture.

3.4.3 Other Factors

Several other factors can affect the performance of a multiprocessor server. For example, an operating system must be able to handle multiprocessing environments. Different operating systems handle multiprocessing differently. In some cases, the operating system sends all I/O interrupts to a single processor, but in other cases processors share I/O interrupts. When designing a multiprocessing solution, you must plan carefully to ensure that the solution that you design works well with the customer requirements.