Multiprocessor Operating Systems

Structures

In operating systems, to improve the performance of more than one CPU can be used within one computer system called Multiprocessor operating system.

Multiple CPUs are interconnected so that a job can be divided among them for faster execution. When a job finishes, results from all CPUs are collected and compiled to give the final output. Jobs needed to share main memory and they may also share other system resources among themselves. Multiple CPUs can also be used to run multiple jobs simultaneously.

For Example: UNIX Operating system is one of the most widely used multiprocessing systems.

The basic organization of a typical multiprocessing system is shown in the given figure.

To employ a multiprocessing operating system effectively, the computer system must have the following things:

• A motherboard is capable of handling multiple processors in a multiprocessing operating system.
• Processors are also capable of being used in a multiprocessing system.

Advantages of multiprocessing operating system are:

• Increased reliability: Due to the multiprocessing system, processing tasks can be distributed among several processors. This increases reliability as if one processor fails; the task can be given to another processor for completion.
• Increased throughout: As several processors increase, more work can be done in less
• The economy of Scale: As multiprocessors systems share peripherals, secondary storage devices, and power supplies, they are relatively cheaper than single-processor systems.

Disadvantages of Multiprocessing operating System

• Operating system of multiprocessing is more complex and sophisticated as it takes care of multiple CPUs at the same time.

Types of multiprocessing systems

• Symmetrical multiprocessing operating system
• Asymmetric multiprocessing operating system

Symmetrical multiprocessing operating system:

In a Symmetrical multiprocessing system, each processor executes the same copy of the operating system, takes its own decisions, and cooperates with other processes to smooth the entire functioning of the system. The CPU scheduling policies are very simple. Any new job submitted by a user can be assigned to any processor that is least burdened. It also results in a system in which all processors are equally burdened at any time.

The symmetric multiprocessing operating system is also known as a “shared every-thing” system, because the processors share memory and the Input output bus or data path. In this system processors do not usually exceed more than 16.

Characteristics of Symmetrical multiprocessing operating system:

• In this system, any processor can run any job or process.
• In this, any processor initiates an Input and Output operation.

Advantages of Symmetrical multiprocessing operating system:

• These systems are fault-tolerant. Failure of a few processors does not bring the entire system to a halt.

Disadvantages of Symmetrical multiprocessing operating system:

• It is very difficult to balance the workload among processors rationally.
• Specialized synchronization schemes are necessary for managing multiple processors.

Asymmetric multiprocessing operating system

In an asymmetric multiprocessing system, there is a master slave relationship between the processors.

Further, one processor may act as a master processor or supervisor processor while others are treated as shown below:

In the above figure, the asymmetric processing system shows that CPU n1 acts as a supervisor whose function controls other following processors.

In this type of system, each processor is assigned a specific task, and there is a designated master processor that controls the activities of other processors.

For example, we have a math co-processor that can handle mathematical jobs better than the main CPU. Similarly, we have an MMX processor that is built to handle multimedia-related jobs. Similarly, we have a graphics processor to handle the graphics-related job better than the main processor. When a user submits a new job, the OS has to decide which processor can perform it better, and then that processor is assigned that newly arrived job. This processor acts as the master and controls the system. All other processors look for masters for instructions or have predefined tasks. It is the responsibility of the master to allocate work to other processors.

Advantages of Asymmetric multiprocessing operating system:

• In this type of system execution of Input and Output operation or an application program may be faster in some situations because many processors may be available for a single job.

Disadvantages of Asymmetric multiprocessing operating system:

• In this type of multiprocessing operating system the processors are unequally burdened. One processor may be having a long job queue, while another one may be sitting idle.

In this system, if the process handling a specific work fails, the entire system will go down.