• Simple schedulers.

  • Slightly more complex schedulers.

$ cat announce.txt

  • Two weeks to complete ASST2.

  • Screen casts are up, and test161 targets and a new test161 release should be out Monday.

  • Please contact the course staff if you are working alone. We have some important information for you.

  • Any questions about submitting using test161?

Review: Scheduling: What?

What is scheduling?
  • Scheduling is the process of choosing the next thread (or threads) to run on the CPU (or CPUs).

Review: Scheduling: Why?

Why schedule threads?
  • CPU multiplexing: we have more threads that cores to run them on.

  • Kernel privilege: we are in charge of allocating the CPU and must try to make good decisions. Applications rely on it.

Review: Scheduling: When?

When does scheduling happen?
  1. When a thread voluntarily gives up the CPU by calling yield().

  2. When a thread makes a blocking system call and must sleep until the call completes.

  3. When a thread exits.

  4. When the kernel decides that a thread has run for long enough—​and uses a timer interrupt to take control.

Review: Human-Computer Interaction (and Expectations)

  1. What do you expect from your machine?

    • Respond (Click)

    • Continue (Watch, or active waiting)

    • Finish (Expect, or passive waiting)

Review: Scheduling Goals

  • How well does it meet deadlines—unpredictable or predictable?

  • How completely does it allocate system resources?

    • No point having idle CPU, memory, or disk bandwidth when something useful could be happening.

  • On human-facing systems, deadlines (or interactivity) usually wins. Why?

    • Your time is more valuable than your computer’s.

  • Performance: making great scheduling decisions is futile if the decision process itself takes forever.

Review: Conflicting Goals

  • Scheduling is a balance between meeting deadlines and optimizing resource allocation.

  • Responsiveness and continuity require meeting deadlines—unpredictable or predictable.

Why Schedule: Questions?

Scheduling Information

Today we will talk about schedulers that use three kinds of additional information in order to choose what thread to run next:

  • What will happen next? Oracular schedulers can cannot be implemented but can be a good point of comparison.

  • What just happened? Typical schedulers—and many other operating system algorithms—use the past to predict the future.

  • What does the user want? Schedulers usually have ways to incorporate user input.

The Know Nothings

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Random Scheduling

  • Choose a scheduling quantum. This is the maximum amount of time any thread will be able to run at one time.

  • Then:

    1. Choose a thread a random from the ready pile.

    2. Run the thread until it it blocks or the scheduling quantum expires.

What happens when a thread leaves the waiting state?
  • Just return it to the ready pile!

Round-Robin Scheduling

  • Choose a scheduling quantum. This is the maximum amount of time any thread will be able to run at one time.

  • Establish an ordered ready queue. For example, when a thread is created add it to the tail of the ready queue.

  • Then:

    1. Choose the thread at the head of the ready queue.

    2. Run the thread until it it blocks or the scheduling quantum expires.

    3. If its scheduling quantum expires, place it at the tail of the ready queue.

  • What happens when a thread leaves the waiting state?

    • Could put it at the head of the ready queue, or at the tail.

The Know Nothings

  • The random and round robin scheduling algorithms:

    • require no information about a threads past, present, or future, and

    • accept no user input.

  • These are rarely useful algorithms except as straw men to compare other approaches to.

  • Both penalize—or at least do not reward—threads that give up the CPU before their quantums expire.

  • As one exception, round robin scheduling is sometimes used once other scheduling decisions have been made and a set of threads are considered equivalent.

    • As an example, you might rotate round-robin though a set of threads with the same priority.

The Know-It-Alls

Let’s say we can predict the future. What might we like to know about the thread we are about to execute?

  • How long is it going to use the CPU!

  • Will it block or yield?

  • How long will it wait?

The Know-It-Alls

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Shortest-Job First

Why would we use this algorithm?
  • Minimizes waiting time!

More generally, why would we prefer threads that give up the CPU before their time quantum ends?
  • They are probably waiting for something else, which can be done in parallel with CPU use.

The Know-It-Alls

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Monday: The Story of the Linux Completely Fair Scheduler

  • How one Australian anaesthetist changed the Linux kernel forever…​ and then gave up.

  • The scheduler may be fair, but is Linux development?

  • A fascinating view into the Linux community, and a good story highlighting many aspects of large-scale community-driven operating systems development.

  • Including discussion of the BFS (Brain F*ck Scheduler).