Page 94 - DCAP403_Operating System

Unit 5: Scheduling

2. Turnaround time: The interval between the submission of a process and the completion Notes
of its execution, including the actual running time, plus the time spent sleeping before
being dispatched or while waiting to access various resources. This is the appropriate
responsiveness measure for batch production, as well as for time-shared systems that
maintain multiple batch queues, sharing CPU time among them.

3. Meeting deadlines: The ability of the OS to meet pre-defined deadlines for job completion.
It makes sense only when the minimal execution time of an application can be accurately
predicted.
4. Predictability: The ability of the system to ensure that a given task is executed within a
certain time interval, and/or to ensure that a certain constant response time is granted
within a strict tolerance, no matter what the machine load is.

When the overall system performance is considered, additional scheduling criteria must be taken
into account:
1. Throughput: The rate of completion of processes (processes completed per unit time). This
is a ‘‘raw’’ measure of how much work is performed, since it depends on the execution
length of processes, but it’s obviously affected by the scheduling policy.
2. User processor utilisation: Time (percentage of unit time) during which the CPU is running
user processes. This is a measure of how well the system can serve the payload and keep at
minimum time spent in housekeeping chores.

3. Overall processor utilisation: Time percentage during which the CPU is busy. It’s a
significant criterion for expensive hardware, that must be kept busy as much as possible in

order to be justify its cost (e.g. supercomputers for numerical calculus applications).
4. Resource utilisation balance: It extends the idea of processor utilisation to take into account
all system resources. A good scheduler should try to keep all the hardware resources in use
at any time.

New State Ready Queue

CPU

Suspended
Ready Queue

Suspended
Block Queue

Block Queue

Long Term Scheduling
Medium Term Scheduling
Short Term Scheduling

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