Multiprocessingis the use of two or morecentral processing units(CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them.There are many variations on this basic theme, and the definition of multiprocessing can vary with context, mostly as a function of how CPUs are defined (multiple coreson onedie, multiple dies in onepackage, multiple packages in onesystem unit, etc.).
Multiprocessingsometimes refers to the execution of multiple concurrent software processes in a system as opposed to a single process at any one instant. However, the termsmultitaskingormultiprogrammingare more appropriate to describe this concept, which is implemented mostly in software, whereas multiprocessing is more appropriate to describe the use of multiple hardware CPUs. A system can be both multiprocessing and multi programming, only one of the two, or neither of the two of them.
In multiprocessor configurations that contain four CPUs, the processor can be run in one of two modes: as a single four-CPU processor, or as two independent two-CPU processors. When operating as two independent processors, resources such as main storage and channels are split between the two processors. Depending on operational needs, the installation may switch from one mode to another.
Quite frequently, the workload of an installation is held up because many jobs are waiting for execution by a single processing unit. Even with efficient operating systems such as MVS, a single processor can execute only one instruction at a time, which tends to cause bottlenecks—usually in the most crucial moments. To overcome this problem, computer installations use two or more processing units. This is referred to as multiprocessing.
To understand what a multiprocessor does, let us focus on the term single processor. A single processor, or uniprocessor, contains its own main storage and is controlled by a single operating system in a vacuum. This means that single processor has no direct communication with other processors in the system. A multiprocessor, on the other hand, can share system resources as well as work. For example, while one of the processors initiate and I/O operation, the other can handle the interrupt that occurs at the end of the operation. This is all possible because MVS breaks up its load into a number of tasks, allowing each task to be handled separately.
Multiprocessing allows more work to be done in the same amount of time and enhances availability; when one processor is unavailable, others can pick up the work and carry on.
There are two types of multiprocessing mechanisms. The first one is called loosely coupled multiprocessing, where processors operate under separate operating systems, yet share access to data. A shared DASD or channel-to-channel adapters connect the processors. In this particular situation, each processor has its own operating system and jobs can, if necessary, be routed through a particular processor.
The second kind of multiprocessing mechanism is referred to as tightly coupled multi processing. A tightly coupled processor configuration consists of a number of hardware components such as processors, channel paths, storage and I/O devices! In a multiprocessing environment, it is possible to reconfigure the existing system to either and/or remove some of the previously mentioned components. The ability to reconfigure the system on the fly is considered an efficient way to maintain operations despite possible malfunctions in the system. You might also need to perform maintenance on some of the components, or a change in the workload might require reconfiguring a single system into two or more systems.
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