Lesson 06 · Two tasks interleave - Build a Green-Thread Scheduler

Two tasks interleave

This is the moment cooperative scheduling comes alive. Two tasks that each yield once will hand the CPU back and forth, and the scheduler weaves them into a single exact interleaving. Pinning that A,B,A,B trace is the heart of the whole project.

The goal

Run two tasks that each yield once, and assert the exact interleaved trace.

Start here - the target

TO DO

Scenario: Two yielding tasks alternate

Giventasks 1 and 2 spawned in that order, each returning Ready on its first step and Done on its second

WhenRun is called

Thenthe run trace is exactly [1, 2, 1, 2]

Andboth tasks finish and the run queue ends empty

Background

Yielding is a task voluntarily giving up the CPU: it returns Ready instead of running to the end, trusting the scheduler to come back to it. Because a yielded task goes to the back of the run queue, two tasks that both yield naturally take turns. Task 1 runs and yields, task 2 runs and yields, task 1 resumes and finishes, task 2 resumes and finishes - the interleaving [1, 2, 1, 2] falls straight out of the FIFO rule with no extra machinery.

Trace it by hand once: the queue starts [1, 2]; step 1 yields so it goes to the back, leaving [2, 1]; step 2 yields, leaving [1, 2]; step 1 finishes, leaving [2]; step 2 finishes. That exact alternation is what people mean by “green threads running concurrently” on a single driver - not two things at once, but two things taking precise, fair turns. Everything else in this project is a variation on this one trace.

Make it work

// a task that yields once then finishes
func twoStep() func() Status {
  done := false
  return func() Status {
    if !done { done = true; return Ready } // step 1: yield
    return Done                             // step 2: finish
  }
}
// spawn two of these; the scheduler already does the rest

CheckpointDONE

Two yielding tasks interleave in an exact A,B,A,B trace. Commit and stop here.