Review question: process  (Page 2/4)

The system operator can review this quantity to determine the degree of "stress" on the system. By reducing the number of activejobs allowed on the system, this average can be kept high. A typical guidelineis that this average should be kept above 2 minutes [IBM86]. This may seem likea lot, but it isn't.

What main advantage does a system with virtual memory have over a system without it? how does virtual memory help with memory protection?

With virtual memory, a process image does not need to be entirely in physical memory for it to run. In addition, it is not restrictedto the size of physical memory. The process can address as much memory as it wants, and the operating system brings blocks from secondary storage intophysical memory as they are needed. Virtual memory also helps with memory protection because one process cannot address memory that belongs to anotherprocess. All addresses are mapped by the operating system into its own address space.

What is the difference between a symmetric multiprocessing system and a distributed operating system?

In a symmetric multiprocessing system, a group of processors with the same capabilities share memory and are located within thesame computer system. The existence of multiple processors is transparent to the user. In a distributed operating system, the group of processors each addressesits own memory, are located in different computers connected by a network, and can have differing capabilities. The operating system allows the user to treatthe collection of systems as one computer.

What shortcoming does the two-state process model have? how does the blocked state fix this problem?

In the two-state process model, if a process is interrupted for I/O, it is placed in a queue with other processes that are readyto run. When the operating system wants to find a new process to run on the CPU, it must search through the queue for a process that is ready. Using a model witha Blocked state means that there are two separate queues: one for ready processes and one for those that are waiting for I/O. When the operating systemwants to find a process that is ready, it can simply take the first process on the ready queue.

What shortcoming does the five-state process model have? how does swapping solve this problem?

Assuming virtual memory is not being used, then only a limited number of processes can be active at any time. In the five-stateprocess model, it is possible that all active processes are waiting on an I/O event and none of them are in the ready queue. In this case, the CPU will beidle.

Swapping can solve this problem by taking a process out of memory and writing it out to disk, then bringing in a new process thatcan run on the CPU. The five-state model is augmented with a new state called Suspended to indicate the process is swapped out.

What is contained in a process image? some older operating systems require the entire process image to be stored in main memory while the processIs executing. what problem does this create and how is it solved?

The process image contains user data (for program data and the stack), the user program, the system stack (for procedure calls),and the process control block (process identifiers, processor state, various types of control information).