What is Scheduler in Operating System ?

If we consider batch systems, there will often be more processes submitted than the number
of processes that can be executed immediately. So incoming processes are spooled (to a
disk). The long-term scheduler selects processes from this process pool and loads selected
processes into memory for execution.
The short-term scheduler selects the process to get the processor from among the
processes which are already in memory.

The short-time scheduler will be executing frequently (mostly at least once every 10
milliseconds). So it has to be very fast in order to achieve a better processor utilization. The
short-time scheduler, like all other OS programs, has to execute on the processor. If it takes
1 millisecond to choose a process that means ( 1 / ( 10 + 1 )) = 9% of the processor time is
being used for short time scheduling and only 91% may be used by processes for execution.
The long-term scheduler on the other hand executes much less frequently. It controls the
degree of multiprogramming (no. of processes in memory at a time). If the degree of
multiprogramming is to be kept stable (say 10 processes at a time), then the long-term
scheduler may only need to be invoked when a process finishes execution.

The long-term scheduler must select a good process mix of I/O-bound and processor bound
processes. If most of the processes selected are I/O-bound, then the ready queue will
almost be empty while the device queue(s) will be very crowded. If most of the processes
are processor-bound, then the device queue(s) will almost be empty while the ready queue
is very crowded and that will cause the short-term scheduler to be invoked very frequently.
Time-sharing systems (mostly) have no long-term scheduler.

The stability of these systems  either depends upon a physical limitation (no. of available terminals) or the self-adjusting nature of users (if you can’t get response, you quit).

It can sometimes be good to reduce the degree of multiprogramming by removing processes
from memory and storing them on disk. These processes can then be reintroduced into
memory by the medium-term scheduler. This operation is also known as swapping.
Swapping may be necessary to improve the process mix or to free memory.

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