125 lines
4.8 KiB
Markdown
125 lines
4.8 KiB
Markdown
05/10/20
|
|
---
|
|
The OS is responsible for *managing* and *scheduling processes*
|
|
>Decide when to admit processes to the system (new -> ready)
|
|
>
|
|
>Decide which process to run next (ready -> run)
|
|
>
|
|
>Decide when and which processes to interrupt (running -> ready)
|
|
|
|
It relies on the *scheduler* (dispatcher) to decide which process to run next, which uses a scheduling algorithm to do so.
|
|
|
|
The type of algorithm used by the scheduler is influenced by the type of operating system e.g. real time vs batch.
|
|
|
|
**Long Term**
|
|
- Applies to new processes and controls the degree of multi-programming by deciding which processes to admit to the system when:
|
|
- A good mix of CPU and I/O bound processes is favourable to keep all resources as bust as possible
|
|
- Usually absent in popular modern OS
|
|
|
|
**Medium Term**
|
|
|
|
>Controls swapping and the degree of multi-programming
|
|
|
|
**Short Term**
|
|
- Decide which process to run next
|
|
- Manages the *ready queue*
|
|
- Invoked very frequency, hence must be fast
|
|
- Usually called in response to *clock interrupts*, *I/O interrupts*, or *blocking system calls*
|
|
|
|
|
|
|
|
**Non-preemptive** processes are only interrupted voluntarily (e.g. I/O operation or "nice" system call `yield()`)
|
|
>Windows 3.1 and DOS were non-preemptive
|
|
>
|
|
>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.
|
|
|
|
**Preemptive** processes can be interrupted forcefully or voluntarily
|
|
|
|
>This required context switches which generate *overhead*, too many of them show me avoided.
|
|
>
|
|
>Prevents processes from monopolising the CPU
|
|
>
|
|
>Most popular modern OS use this kind.
|
|
|
|
Overhead - wasted CPU cycles
|
|
How can we objectively critic the OS?
|
|
|
|
**User Oriented criteria**
|
|
*Response time* minimise the time between creating the job and its first execution (time between clicking the button and it starting)
|
|
*Turnaround time* minimise the time between creating the job and finishing it
|
|
*Predictability* minimise the variance in processing times
|
|
|
|
**System oriented criteria**
|
|
*Throughput*: maximise the number of jobs processed per hour
|
|
*Fairness*:
|
|
|
|
> Are processing power/waiting time equally distributed?
|
|
> Are some processes kept waiting excessively long - **starvation**
|
|
|
|
### Different types of Scheduling Algorithms
|
|
[NOTE: FCFS = FIFO]
|
|
|
|
**First come first serve**
|
|
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.
|
|
|
|
| Pros | Cons |
|
|
| ----------- | ----------- |
|
|
| Positional fairness | Favours long processes over short ones (think supermarket checkout) || |
|
|
| Easy to implement | Could compromise resource utilisation |
|
|
|
|
|
|
|
|
**Shortest job first**
|
|
A non-preemptive algorithm that starts processes in order of ascending processing time using a provided estimate of the processing
|
|
|
|
| Pros | Cons |
|
|
| ----------- | ----------- |
|
|
| Always results an optimal turnaround time | Starvation might occur |
|
|
| - | Fairness and predictability are compromised |
|
|
| - | Processing times need to be known in advanced |
|
|
|
|
|
|
|
|
**Round Robin**
|
|
A preemptive version of FCFS that focuses context switches at periodic intervals or time slices
|
|
|
|
>Processes run in order that they were added to the queue.
|
|
>Processes are forcefully interrupted by the timer.
|
|
|
|
| Pros | Cons |
|
|
| ----------- | ----------- |
|
|
| Improved response time | Increased context switching and overhead |
|
|
| Effective for general purpose interactive/time sharing systems | Favours CPU processes over I/O |
|
|
| - | Can reduce to FCFS |
|
|
|
|
Exam 2013: Round Robin is said to favour CPU bound processes over I/O bound processes. Explain why this may be the case.
|
|
>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.
|
|
|
|
If the time slice is only used partially the next process starts immediately
|
|
The length of the time slice must be carefully considered.
|
|
>A small time slice (~ 1ms) gives a good response time.
|
|
>A large time slice (~ 1000ms) gives a high throughput.
|
|
|
|
|
|
|
|
**Priority Queue**
|
|
A preemptive algorithm that schedules processes by priority
|
|
|
|
>A round robin is used for processes with the same priority level
|
|
>The process priority is saved in the process control block
|
|
|
|
| Pros | Cons |
|
|
| ----------- | ----------- |
|
|
|Can prioritise I/O bound jobs | Low priority processes might suffer from starvation |
|
|
|
|
Low priority starvation only happens when a static priority level is used.
|
|
You could give higher priority processes a larger time slice to improve efficiency
|
|
|
|
|
|
|
|
Exam Q 2013: Which algorithms above lead to starvation?
|
|
>Shortest job first and highest priority first.
|
|
|
|
|
|
|