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John Gatward
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docs/lectures/osc/01_scheduling_algorithms.md
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docs/lectures/osc/01_scheduling_algorithms.md
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05/10/20
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The OS is responsible for *managing* and *scheduling processes*
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>Decide when to admit processes to the system (new -> ready)
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>
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>Decide which process to run next (ready -> run)
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>
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>Decide when and which processes to interrupt (running -> ready)
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It relies on the *scheduler* (dispatcher) to decide which process to run next, which uses a scheduling algorithm to do so.
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The type of algorithm used by the scheduler is influenced by the type of operating system e.g. real time vs batch.
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**Long Term**
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- Applies to new processes and controls the degree of multi-programming by deciding which processes to admit to the system when:
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- A good mix of CPU and I/O bound processes is favourable to keep all resources as bust as possible
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- Usually absent in popular modern OS
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**Medium Term**
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>Controls swapping and the degree of multi-programming
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**Short Term**
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- Decide which process to run next
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- Manages the *ready queue*
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- Invoked very frequency, hence must be fast
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- Usually called in response to *clock interrupts*, *I/O interrupts*, or *blocking system calls*
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**Non-preemptive** processes are only interrupted voluntarily (e.g. I/O operation or "nice" system call `yield()`)
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>Windows 3.1 and DOS were non-preemptive
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>
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>The issue with this is if the process in control goes wrong or gets stuck in a infinite loop then the CPU will never regain control.
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**Preemptive** processes can be interrupted forcefully or voluntarily
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>This required context switches which generate *overhead*, too many of them show me avoided.
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>
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>Prevents processes from monopolising the CPU
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>
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>Most popular modern OS use this kind.
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Overhead - wasted CPU cycles
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How can we objectively critic the OS?
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**User Oriented criteria**
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*Response time* minimise the time between creating the job and its first execution (time between clicking the button and it starting)
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*Turnaround time* minimise the time between creating the job and finishing it
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*Predictability* minimise the variance in processing times
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**System oriented criteria**
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*Throughput*: maximise the number of jobs processed per hour
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*Fairness*:
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> Are processing power/waiting time equally distributed?
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> Are some processes kept waiting excessively long - **starvation**
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### Different types of Scheduling Algorithms
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[NOTE: FCFS = FIFO]
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**First come first serve**
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Concept: a non-preemptive algorithm that operates as a strict queuing mechanism and schedules the processes in the same order that they were added to the queue.
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| Pros | Cons |
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| ----------- | ----------- |
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| Positional fairness | Favours long processes over short ones (think supermarket checkout) || |
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| Easy to implement | Could compromise resource utilisation |
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**Shortest job first**
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A non-preemptive algorithm that starts processes in order of ascending processing time using a provided estimate of the processing
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| Pros | Cons |
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| ----------- | ----------- |
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| Always results an optimal turnaround time | Starvation might occur |
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| - | Fairness and predictability are compromised |
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| - | Processing times need to be known in advanced |
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**Round Robin**
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A preemptive version of FCFS that focuses context switches at periodic intervals or time slices
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>Processes run in order that they were added to the queue.
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>Processes are forcefully interrupted by the timer.
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| Pros | Cons |
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| ----------- | ----------- |
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| Improved response time | Increased context switching and overhead |
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| Effective for general purpose interactive/time sharing systems | Favours CPU processes over I/O |
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| - | Can reduce to FCFS |
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Exam 2013: Round Robin is said to favour CPU bound processes over I/O bound processes. Explain why this may be the case.
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>I/O processes will spend a lot of their allocated time waiting for data to come back from memory, therefore less processing can occur before the time slice runs out.
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If the time slice is only used partially the next process starts immediately
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The length of the time slice must be carefully considered.
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>A small time slice (~ 1ms) gives a good response time.
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>A large time slice (~ 1000ms) gives a high throughput.
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**Priority Queue**
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A preemptive algorithm that schedules processes by priority
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>A round robin is used for processes with the same priority level
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>The process priority is saved in the process control block
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| Pros | Cons |
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| ----------- | ----------- |
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|Can prioritise I/O bound jobs | Low priority processes might suffer from starvation |
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Low priority starvation only happens when a static priority level is used.
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You could give higher priority processes a larger time slice to improve efficiency
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Exam Q 2013: Which algorithms above lead to starvation?
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>Shortest job first and highest priority first.
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