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ECS150-L10

2 min read

📗 -> 01/30/26: ECS150-L10

[Lecture Slide Link]

🎤 Vocab

❗ Unit and Larger Context

Small summary

✒️ -> Scratch Notes

Implementing a process scheduler:

  • Handling context, sharing computer resources and storing/loading environments for the threads
    • static ucontext_t - “A context/environment box”
      • Snapshot of registers, like the setup/cleanup when calling functions in assembly

??

What EXACTLY needs to be preserved between threads?

  • Obviously registers, but do we have to worry about maintaining the stack? i guess the stack pointer kinda takes care of that
  • Variable names and pointers are resolved at compile time so not those either
  • is there anything else?

Scheduing concepts

Process States
  • Ready
  • Running
  • Blocked
  • Zombie

Scheduling Algos

Vocab
  • Submission time: time at which a process is created
  • Turnaround time: total time between process submission and completion
  • Response time: time between process submission and first execution or first response (e.g., screen output, or input from user)
  • Waiting time: total time spent in the ready queue

FCFS / FIFO

A queue

Pathology:

  • Long job hogging because of unlucky scheduling

SJF

Shortest Job First

  • Optimal scheduling, but needs to know task lengths in advance

Pathology:

  • Long job entered first, and hogs

Preemptive SJF / SRTF

  • Also known as SRTF (shortest remaining time first)
  • New shorted jobs can interrupt longer jobs

Pathology:

  • Long jobs continually interrupted by shorter jobs and unable to complete

Metrics

Turnaround time - Best for batch systems where reactivity is not needed (scientific computing, clusters)

  • Best when length of tasks known and tasks mostly CPU-bound
    Response time - Best for interactive systems (user initiated, browsers)
  • Best for tasks of unknown length

Round-robin (RR)

Tasks run only for a (short) time slice at a time
Relies of preemption (via timer interrupts)

Characteristics:

  • NO starvation (everyone gets a turn)
  • Time slice duration matters (response time vs context switching overhead)
  • Poor turnaround time

Multi-level Queue Scheduling

Combine approaches to take advantage of each’s advantages

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🔗 -> Links

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