How to pass time.Duration type to a go function?

Question

I am learning GOLANG, in particular for its concurrency capabilities.

Have tried to further develop one of the worker_pool examples so that each worker receives a job id and a job load, represented in a random duration of the job.

The duration is used by a time.sleep command to wait the alloted number of nano seconds, which are calculated randomly.

Code looks like this...

//worker_pool improved example

package main

import "fmt"
import "time"
import "math/rand"

// Here's the worker, of which we'll run several
// concurrent instances. These workers will receive
// work on the `jobs` channel and send the corresponding
// results on `results`. We'll sleep a random number of seconds between
// 1 and 5 to simulate an expensive task.
func worker(id int, jobs <-chan int, loads <-chan time.Duration, results chan<- int) {
   for j := range jobs {
        fmt.Println("worker", id, "started  job", j, time.Now())
        time.Sleep(loads*time.Second)  
        fmt.Println("worker", id, "finished job", j, time.Now())
        results <- j * 2
    }
}

func main() {

    // In order to use our pool of workers we need to send
    // them work and collect their results. We make 2
    // channels for this.
    jobs := make(chan int)
    loads := make(chan time.Duration)
    results := make(chan int)

    // This starts up 3 workers, initially blocked
    // because there are no jobs yet.
    for w := 1; w <= 3; w++ {
        go worker(w, jobs, results)
    }

    // Here we send 24 `jobs` and then `close` that
    // channel to indicate that's all the work we have.
    for j := 1; j <= 24; j++ {
        jobs <- j
        r := rand.New(rand.NewSource(99))
        load := r.Int63n(5000000)
        loads <- load
    }
    close(jobs)
    close(loads)


    // Finally we collect all the results of the work.
    for a := 1; a <= 24; a++ {
        <-results
    }
}

I keep getting this error messages...

prog.go:18:33: cannot convert loads (type <-chan int) to type time.Duration

prog.go:36:18: not enough arguments in call to worker have (int, chan int, chan int) want (int, <-chan int, <-chan int, chan<- int)

prog.go:45:15: cannot use load (type int64) as type int in send

What am I doing wrong?

Answer 1

Even if you fix your compile errors, you still have problems.

//worker_pool improved example

package main

import "fmt"
import "time"
import "math/rand"

// Here's the worker, of which we'll run several
// concurrent instances. These workers will receive
// work on the `jobs` channel and send the corresponding
// results on `results`. We'll sleep a random number of seconds between
// 1 and 5 to simulate an expensive task.
func worker(id int, jobs <-chan int, loads <-chan time.Duration, results chan<- int) {
    for j := range jobs {
        fmt.Println("worker", id, "started  job", j, time.Now())
        time.Sleep(<-loads * time.Second)
        fmt.Println("worker", id, "finished job", j, time.Now())
        results <- j * 2
    }
}

func main() {

    // In order to use our pool of workers we need to send
    // them work and collect their results. We make 2
    // channels for this.
    jobs := make(chan int)
    loads := make(chan time.Duration)
    results := make(chan int)

    // This starts up 3 workers, initially blocked
    // because there are no jobs yet.
    for w := 1; w <= 3; w++ {
        go worker(w, jobs, loads, results)
    }

    // Here we send 24 `jobs` and then `close` that
    // channel to indicate that's all the work we have.
    for j := 1; j <= 24; j++ {
        jobs <- j
        r := rand.New(rand.NewSource(99))
        load := time.Duration(r.Int63n(5000000))
        loads <- load
    }
    close(jobs)
    close(loads)

    // Finally we collect all the results of the work.
    for a := 1; a <= 24; a++ {
        <-results
    }
}

Playground: https://play.golang.org/p/tVdlKFHunKN

Output:

worker 3 started  job 1 2009-11-10 23:00:00 +0000 UTC m=+0.000000001
worker 1 started  job 2 2009-11-10 23:00:00 +0000 UTC m=+0.000000001
worker 2 started  job 3 2009-11-10 23:00:00 +0000 UTC m=+0.000000001
worker 1 finished job 2 2009-12-27 17:05:41 +0000 UTC m=+4039541.000000001
worker 3 finished job 1 2009-12-27 17:05:41 +0000 UTC m=+4039541.000000001
worker 2 finished job 3 2009-12-27 17:05:41 +0000 UTC m=+4039541.000000001
fatal error: all goroutines are asleep - deadlock!

Answer 2

Indeed there was a problem with the code.

The channels I created were not buffered. I included a buffer of 100 (per original example) for the jobs and results channels, and the program worked OK.

//worker_pool improved example

package main

import "fmt"
import "time"
import "math/rand"

func worker(id int, jobs <-chan int, loads <-chan time.Duration, results chan<- int) {
    for j := range jobs {
        before := time.Now()
        fmt.Println("worker", id, "started  job", j)
        time.Sleep(<-loads)
        after := time.Now()
        fmt.Println("worker", id, "finished job", j, "in", after.Sub(before))
        results <- j
    }
}

func main() {

    jobs := make(chan int, 100) // Buffered channel
    loads := make(chan time.Duration)
    results := make(chan int, 100) // Buffered channel

    for w := 1; w <= 3; w++ {
        go worker(w, jobs, loads, results)
    }

    for j := 1; j <= 24; j++ {
        jobs <- j
        r := rand.New(rand.NewSource(int64(j*10)))
        loads <- time.Duration(r.Int63n(500000000)) // In nano seconds
    }
    close(jobs)
    close(loads)

    for a := 1; a <= 24; a++ {
        <-results
    }
}

Playground: https://play.golang.org/p/bz-JIkD1OoG

I am still getting used to the random generator logic. It returns the exact same numbers every time, given a certain seed, which in my mind defeats the concept of randomness.

Partial results...

worker 3 started  job 1
worker 1 started  job 2
worker 2 started  job 3
worker 1 finished job 2 in 168.00641ms
worker 1 started  job 4
worker 3 finished job 1 in 205.826435ms
worker 3 started  job 5
worker 3 finished job 5 in 160.909863ms
worker 3 started  job 6
worker 2 finished job 3 in 381.707665ms