Page 55 - DCAP103_Principle of operating system

Principles of Operating Systems

Notes of processes leaving the system. Thus, the long-term scheduler may need to be invoked only
when a process leaves the system. Because of the longer interval between executions, the long-
term scheduler can afford to take more time to select a process for execution.
The long-term scheduler must make a careful selection. In general, most processes can be
described as either I/O bound or CPU bound. An 110-bound process spends more of its time
doing I/O than it spends doing computations. A CPU-bound process, on the other hand,
generates I/O requests infrequently, using more of its time doing computation than an I/O-
bound process uses. The long-term scheduler should select a good process mix of I/O-bound
and CPU-bound processes. If all processes are I/O bound, the ready queue will almost always
be empty, and the short-term scheduler will have little to do. If all processes are CPU bound,
the I/O waiting queue will almost always be empty, devices will go unused, and again the
system will be unbalanced. The system with the best performance will have a combination of
CPU-bound and I/O-bound processes.
On some systems, the long-term scheduler may be absent or minimal. For example, time-
sharing systems such as UNIX often have no long-term scheduler, but simply put every
new process in memory for the short-term scheduler. The stability of these systems depends
either on a physical limitation (such as the number of available terminals) or on the self-
adjusting nature of human users. If the performance declines to unacceptable levels, some
users will simply quit. Some operating systems, such as time-sharing systems, may introduce
an additional, intermediate level of scheduling. This medium-term scheduler, diagrammed
in Figure 2.6, removes processes from memory (and from active contention for the CPU),
and thus reduces the degree of multiprogramming. At some later time, the process can be
reintroduced into memory and its execution can be continued where it left off. This scheme is
called swapping. The process is swapped out, and is later swapped in, by the medium-term
scheduler. Swapping may be necessary to improve the process mix, or because a change in
memory requirements has overcommitted available memory, requiring memory to be freed up.

Figure 2.6: Addition of Medium-term Scheduling to the Queueing Diagram

Swapin Partially executed Swap out
swapped-out processes

Ready queue CPU End

I/O waiting
I/O
queues

Give the process of PCB with all step.

2.3.3 Context Switch

Switching the CPU to another process requires saving the state of the old process and loading
the saved state for the new process. This task is known as a context switch. The context of a
process is represented in the PCB of a process; it includes the value of the CPU registers, the
process state and memory-management information. When a context switch occurs, the kernel
saves the context of the old process in its PCB and loads the saved context of the new process

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