Multiprocessor Systems:
Multiprocessing is a type of processing in which two or more processors work together to process more than one program simultaneously. It allows the system to do more work in a shorter period of time. UNIX is one of the most widely used multiprocessing systems.
Multiprocessor system is also known as parallel system or tightly-coupled system. It means that multiple processors are tied together in some manner. Generally, the processors are in close communication with each other. They share common data structures and a common system clock.
Advantages of Multiprocessor Systems:
Some advantages of multiprocessor system are as follows:
Reduced Cost: multiple processors share the same resources. Separate power ;supply or mother board for each chip is not required. This reduces the cost.
Increased Reliability: The reliability of system is also increased. The failure of one processor does not affect the other processors though it will slow down the machine. Several mechanisms are required to achieve increased reliability. If a processor fails, a job running on that processor also fails. The system must be able to reschedule the failed job or to alert the user that the job was not successfully completed.
Increased Throughput: An increase in the number of processes completes the work in less time. It is important to note that doubling the number of processors does not halve the time to complete a job. It is due to the overhead in communication between processors and contention for shared resources etc.
Types of Multiprocessor Systems:
There are two main types of parallel processing:
1. Symmetric Multiprocessing:
A method of processing in which multiple processors work together on the same task is called symmetric multiprocessing. Each processor runs an identical copy of the same operating system. Symmetric multiprocessing treats all processors equally. I/O can be processed on any processor. The processors communicate with each other as needed. It enables many processes to be run at once without degrading performance. Each CPU is separate, it is possible for one CPU to be idle while another CPU is overloaded.
Symmetric multiprocessing is easier to implement in operating systems. It is the method mostly used by the operating systems that support multiple processors. The most common multiprocessor systems today use SMP architecture.
2. Asymmetric Multiprocessing:
A multiprocessing technique in which individual processors are dedicated to particular tasks such as running the operating system or performing user requests is called asymmetric multiprocessing. It has one master processor and remainder processors are called slave. The master distributes tasks among the slaves. I/O is usually done by the master only.
An example is disk management where the responsibility of managing disk activities is not handled by main processor. It is performed by a microprocessor that has its own memory. It only performs disk management tasks such as how to move the head of disk to read data from disk into memory etc.
