Early computers ran one process at a time. While the process waited for servicing by another device, the CPU was idle. In an I/O intensive process, the CPU could be idle as much as 80% of the time. Advancements in operating systems led to computers that load several independent processes into memory and switch the CPU from one job to another when the first becomes blocked while waiting for servicing by another device. This idea of multiprogramming reduces the idle time of the CPU. Multiprogramming accelerates the throughput of the system by efficiently using the CPU time.
Programs in a multiprogrammed environment appear to run at the same time. Processes running in a multiprogrammed environment are called concurrent processes. In actuality, the CPU processes one instruction at a time, but can execute instructions from any active process.
As the illustration shows, CPU utilization of a system can be improved by using multiprogramming. Let P be the fraction of time that a process spends away from the CPU. If there is one process in memory, the CPU utilization is (1-P). If there are N processes in memory, the probability of N processes waiting for an I/O is P*P...*P (N times). The CPU utilization is ( 1 - P^N ) where N is called the multiprogramming level (MPL) or the degree of multiprogramming. As N increases, the CPU utilization increases. While this equation indicates that a CPU continues to work more efficiently as more and more processes are added, logically, this cannot be true. Once the system passes the point of optimal CPU utilization, it thrashes.
(For more on performance issues of OS, refer to Operating System Performance Module )
In order to use the multiprogramming concept, processes must be loaded into independent sections or partitions of memory. So, main memory is divided into fixed-sized or variable-sized partitions. Since a partition may not be large enough for the entire process, virtual memory is implemented to keep the processes executing. The answers to several questions are important to implementing an efficient virtual memory system in a multiprogrammed environment.
Follow the combined yellow and blue lines to find the answers.
