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tags: 作業系統
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# 作業系統 ch5 scheduling

select next precess -> dispatcher module -> context switch (kernal mode)
dispatcher latency: time between stop one process and start another running (including context switch)
time quantum：a small unit of CPU time

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- FCFS: convey effect
- SJF (Shortest-Job-First): nonpreemptive, type of priority(time)
- SRTF (Shortest-remaining-time-first): SJF && preemptive
- RR (Round Robin):preemptive, better response (time quantum > context switch time)
- Priority

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### determine next CPU burst time:
next_predict_t = x(actual_t) + (1-x)(prior_predict_t)

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### Process type
- batch（批次）: 大量 && 不急 -> background (RR)
- interactive：互動 && 即時 -> foreground (FCFS)

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## Single CPU

### Multi-Level
- Multi-Level queue: process cann't move between queues
- Multi-level feedback queue: process can move between queues(method to determine process enter which queue)

### schedule between queue
- Fixed-priority (foreground -> background)
- Time slice (80% foreground)

### Thread scheduling
- support thread schedule, not process
- user-mode
- run on LWP(light-weight-process)
- process contention scope (PCS): threads 在同一個 process 爭取 LWP
- kernal-mode
- system-contention-scope(SCS): 爭取 CPU

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## Multiple-Processor ( Multi-core )
- processor affinity: process perfer the same CPU to avoid transfer data time
- load balancing (push - if overload、pull - idle)
### hardware type
- homogeneous: each CPU is the same
- heterogeneous: (<->homogeneous)

### software type
- asymmetric: only one CPU can execute kernal process
- symmetric(SMP): each CPU can run user and kernal process

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## Real-Time CPU Scheduling

[ note ]
- Interrupt latency: time between interrupt arrivial and run ISR(interpute service routine)

- conflict: (inside dispatch latency) 準備 ISR 所需要的資源

- preemptive && priority-based

* interrupt latency -> dispatcher latency

type
- software real-time system: priority high, not sure complete before deadline
- hardware real-time system: make sure complete before deadline

process change way
(p: periodic, d: deadline, t: processing time)
- rate montonic scheduling (period time)
- earlist deadline first scheduling (deadline)

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## Pthread

- PTHREAD_SCOPE_PROCESS: PCS scheduling(user mode)
- PTHREAD_SCOPE_SYSTEM: SCS scheduling(kernal mode)
- 受 OS 所限制