Challenge 1 - Scheduler

Write a monitoring tool that inspects goroutine behavior.

Challenge: bounded parallelism

You will write a program that processes 1000 jobs, but at most GOMAXPROCS jobs may run at once. Spawning unbounded goroutines strains the scheduler and memory; a fixed number of workers uses the CPU fully.

isler := make(chan int)
sonuc := make(chan int)
for w := 0; w < runtime.GOMAXPROCS(0); w++ {
    go isci(isler, sonuc)
}

Workers take jobs with for i := range isler and finish when the channel closes.

Plan

1) Start n workers with wg.Add(n). 2) A goroutine sends 1..1000 to isler and closes it. 3) Another goroutine does close(sonuc) after wg.Wait(). 4) main sums with for r := range sonuc. Each worker sends the square of its number; the expected total is 333,833,500.

Exercise Worker pool

Write the pool following the plan and print the sum of squares.

Show example solution

package main

import (
    "fmt"
    "runtime"
    "sync"
)

func main() {
    isler := make(chan int)
    sonuc := make(chan int)
    n := runtime.GOMAXPROCS(0)
    var wg sync.WaitGroup
    wg.Add(n)
    for w := 0; w < n; w++ {
        go func() {
            defer wg.Done()
            for i := range isler {
                sonuc <- i * i
            }
        }()
    }
    go func() {
        for i := 1; i <= 1000; i++ {
            isler <- i
        }
        close(isler)
    }()
    go func() {
        wg.Wait()
        close(sonuc)
    }()
    toplam := 0
    for r := range sonuc {
        toplam += r
    }
    fmt.Println(toplam)
}

Test yourself

Who should close the result channel, and when?
- main, as soon as it sends the jobs
- It should never be closed
- A separate goroutine, after all workers finish (after the WaitGroup wait)
- Each worker when its job ends
Show answer

A separate goroutine, after all workers finish (after the WaitGroup wait) — With several senders, if one worker closes the channel the others write to a closed channel and panic; it is closed after all senders are done.

Close the results

go func() {
    wg.____()
    close(sonuc)
}()

Show answer

go func() {
    wg.Wait()
    close(sonuc)
}()

Piece and role

The isler channel

Job distribution
GOMAXPROCS(0) workers

Parallelism bound
wg.Wait + close

Safe close
range sonuc

Collecting

Next lesson: Challenge 2 - Performance

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