scheduler

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A process migrates between the various scheduling queues throughout its life-time purposes.
The OS must select for scheduling processes from these queues in some fashion.
This selection process is carried out by the appropriate scheduler.
In a batch system, more processes are submittedand then
executed immediately.
So these processes are spooled to a mass storage device like disk, where they
are kept for later execution.

Types of schedulers:

There are 3 types of schedulers mainly used:

1. Long term scheduler:
Long term scheduler selects process from the disk & loads them into
memory for execution.
It controls the degreeof multi-programming i.e. no. of processes in
memory.
It executes less frequently than other schedulers.
If the degree of multiprogramming is stable than the average rate of process creation is equal to the average departure rate of processes leaving the system.
So, the long term scheduler is needed to be invoked only when a process leaves the system. Due to longer intervals between executions
it can afford to take more time to decide which process should be selected for execution.

2. Short - term scheduler:
The short term scheduler selects among the process that are ready
to execute & allocates the CPU to one of them.
The primary distinction between these two schedulers is the frequency of their execution.
The short-term scheduler must select a new
process for the CPU quite frequently.
It must execute at least one in 100ms.
Due to the short duration of time between executions, it must be very fast.

3. Medium - term scheduler:
some operating systems introduce an additional intermediate
level of scheduling known as medium - term scheduler.
The main idea behind this scheduler is that sometimes it is advantageous to remove processes from memory & thus reduce the
degree of multiprogramming.
At some later time, the process can be reintroduced into memory & its execution can be continued from where it had left off.
This is called as swapping.
The process is swapped out & swapped in later by medium term scheduler.
Swapping is necessary to improve theprocess miss or due to some change in memory

Process control block:

Each process is represented in the OS by a process control block. It is also by a process control block.
It is also known as task control block.
A process control block contains many pieces of information associated with a specific process.

It includes the following informations.

• Process state: The state may be new, ready, running, waiting or terminated state.

• Program counter:it indicates the address of the next instruction to be executed for this purpose.

• CPU registers: The registers vary in number & type depending on the computer architecture.
It includes accumulators, index registers, stack pointer & general purpose
registers, plus any condition- code information must be saved when an interrupt occurs to allow the process to be continued correctly after- ward.

• CPU scheduling information:This information includes process priority pointers to scheduling queues & any other scheduling parameters.

• Memory management information: This information may include such information as the value of the bar & limit registers, the page tables or the segment tables, depending upon the memory system used by the operating system.

• Accounting information: This information includes the amount of CPU and real time used, time limits, account number, job or process numbers and so on.

• I/O Status Information: This information includes the list of I/O devices allocated to this process, a list of open files and so on. The PCB simply serves as the repository for any information that may vary from process to process.