I am trying to understand, how golang channel works. I read a book about the go language and found the following example.
package main
import (
"fmt"
)
// Send the sequence 2, 3, 4, ... to returned channel
func generate() chan int {
ch := make(chan int)
go func() {
for i := 2; i <= 100 ; i++ {
ch <- i
}
}()
return ch
}
// Filter out input values divisible by 'prime', send rest to returned channel
func filter(in chan int, prime int) chan int {
out := make(chan int)
go func() {
for {
if i := <-in; i%prime != 0 {
out <- i
}
}
}()
return out
}
func sieve() chan int {
out := make(chan int)
go func() {
ch := generate()
for {
prime := <-ch
ch = filter(ch, prime)
out <- prime
}
}()
return out
}
func main() {
primes := sieve()
for {
fmt.Println(<-primes)
}
}
When I run this programm, I've got a deadlock, but when I change the generate function to
// Send the sequence 2, 3, 4, ... to returned channel
func generate() chan int {
ch := make(chan int)
go func() {
for i := 2; ; i++ {
ch <- i
}
}()
return ch
}
Then the programm will run the infinite loop, but not deadlock. Why do I get deadlock, when I remove the condition in for loop?
What do you mean with blocking principle?
You can see it illustrated in the blog post "The Nature Of Channels In Go "
for an unbuffered channel:
(Illustration from blog post "The Nature Of Channels In Go ", written by William Kennedy, Feb. 2014)
Unbuffered channels have no capacity and therefore require both goroutines to be ready to make any exchange.
When a goroutine attempts to write a resource to an unbuffered channel and there is no goroutine waiting to receive the resource, the channel will lock the goroutine and make it wait.
When a goroutine attempts to read from an unbuffered channel, and there is no goroutine waiting to send a resource, the channel will lock the goroutine and make it wait.
That is what happens in your case with your reader:
func main() {
primes := sieve()
for {
fmt.Println(<-primes)
}
}
since primes is never closed, main remains blocked.
It (main) is in step 3:
in step 3, the goroutine on the right places his hand into the channel or performs a read.
That goroutine is also locked in the channel until the exchange is complete.
The sender never calls close(primes).
Let's consider a simpler example:
func generate() chan int {
ch := make(chan int)
go func() {
for i := 2; /*i < 100*/; i++ {
ch <- i
}
}()
return ch
}
func main() {
for i := range generate() {
fmt.Println(i)
}
}
With the condition i < 100 uncommented, the goroutine spawned by generate stops after sending 98 numbers. However, it does not close the channel, so main has no way of knowing that no more numbers are going to be sent, and it just keeps blocking on the channel. Since main is now the only goroutine still in existence (the other one has returned), and it's blocking, you have a deadlock.
Related
Im studying Golang now on my freetime and I am trying sample exams online to test what i learned,
I came about this coding exam task but I cant seem to make it work/run without a crash,
im getting fatal error: all goroutines are asleep - deadlock! error, can anybody help what I am doing wrong here?
func executeParallel(ch chan<- int, done chan<- bool, functions ...func() int) {
ch <- functions[1]()
done <- true
}
func exampleFunction(counter int) int {
sum := 0
for i := 0; i < counter; i++ {
sum += 1
}
return sum
}
func main() {
expensiveFunction := func() int {
return exampleFunction(200000000)
}
cheapFunction := func() int {return exampleFunction(10000000)}
ch := make(chan int)
done := make(chan bool)
go executeParallel(ch, done, expensiveFunction, cheapFunction)
var isDone = <-done
for result := range ch {
fmt.Printf("Result: %d\n", result)
if isDone {
break;
}
}
}
Your executeParallel function will panic if less than 2 functions are provided - and will only run the 2nd function:
ch <- functions[1]() // runtime panic if less then 2 functions
I think it should look more like this: running all input functions in parallel and grabbing the first result.
for _, fn := range functions {
fn := fn // so each iteration/goroutine gets the proper value
go func() {
select {
case ch <- fn():
// first (fastest worker) wins
default:
// other workers results are discarded (if reader has not read results yet)
// this ensure we don't leak goroutines - since reader only reads one result from channel
}
}()
}
As such there's no need for a done channel - as we just need to read the one and only (quickest) result:
ch := make(chan int, 1) // big enough to capture one result - even if reader is not reading yet
executeParallel(ch, expensiveFunction, cheapFunction)
fmt.Printf("Result: %d\n", <-ch)
https://play.golang.org/p/skXc3gZZmRn
package main
import "fmt"
func executeParallel(ch chan<- int, done chan<- struct{}, functions ...func() int) {
// Only execute the second function [1], if available.
if len(functions) > 1 {
ch <- functions[1]()
}
// Close the done channel to signal the for-select to break and the main returns.
close(done)
}
// example returns the number of iterations for [0..counter-1]
func example(counter int) int {
sum := 0
for i := 0; i < counter; i++ {
sum += 1
}
return sum
// NOTE(SS): This function could just return "counter-1"
// to avoid the unnecessary calculation done above.
}
func main() {
var (
cheap = func() int { return example(10000000) }
expensive = func() int { return example(200000000) }
ch = make(chan int)
done = make(chan struct{})
)
// executeParallel takes ch, done channel followed by variable
// number of functions where on the second i.e., indexed 1
// function is executed on a separated goroutine which is then
// sent to ch channel which is then received by the for-select
// reciever below i.e., <-ch is the receiver.
go executeParallel(ch, done, expensive, cheap)
for {
select {
// Wait for something to be sent to done or the done channel
// to be closed.
case <-done:
return
// Keep receiving from ch (if something is sent to it)
case result := <-ch:
fmt.Println("Result:", result)
}
}
}
I have commented on the code so that it's understandable. As you didn't the actual question the logic could be still wrong.
How to get only the result of the fastest goroutine. For example let's we have this function.
func Add(num int)int{
return num+5
}
And we have this function.
func compute(){
for i := 0; i < 5; i++ {
go Add(i)
}
}
I want to get the result of the goroutine that finishes first.
Use a buffered channel to get first result of many grouting execution
func Test(t *testing.T) {
ch := make(chan int, 1)
go func() {
for i := 0; i < 5; i++ {
go func(c chan<- int, i int) {
res := Add(i)
c <- res
}(ch, i)
}
}()
res := <-ch // blocking here, before get first result
//close(ch) - writing to close channel produce a panic
fmt.Println(res)
}
PLAYGROUND
It hasn't memory leak when leave a channel open and never close it. When the channel is no longer used, it will be garbage collected.
can someone give me some insight about this code, Why this get deadlock error in for x:=range c
func main() {
c:=make(chan int,10)
for i:=0;i<5;i++{
go func(chanel chan int,i int){
println("i",i)
chanel <- 1
}(c,i)
}
for x:=range c {
println(x)
}
println("Done!")
}
Because this:
for x:=range c {
println(x)
}
will loop until the channel c closes, which is never done here.
Here is one way you can fix it, using a WaitGroup:
package main
import "sync"
func main() {
var wg sync.WaitGroup
c := make(chan int, 10)
for i := 0; i < 5; i++ {
wg.Add(1)
go func(chanel chan int, i int) {
defer wg.Done()
println("i", i)
chanel <- 1
}(c, i)
}
go func() {
wg.Wait()
close(c)
}()
for x := range c {
println(x)
}
println("Done!")
}
Try it on Go Playground
You create five goroutines, each sending an integer value to the channel. Once all those five values are written, there's no other goroutine left that writes to the channel.
The main goroutine reads those five values from the channel. But there are no goroutines that can possibly write the sixth value or close the channel. So, you're deadlocked waiting data from the channel.
Close the channel once all the writes are completed. It should be an interesting exercise to figure out how you can do that with this code.
Channel needs to be closed to indicate task is complete.
Coordinate with a sync.WaitGroup:
c := make(chan int, 10)
var wg sync.WaitGroup // here
for i := 0; i < 5; i++ {
wg.Add(1) // here
go func(chanel chan int, i int) {
defer wg.Done()
println("i", i)
chanel <- 1
}(c, i)
}
go func() {
wg.Wait() // and here
close(c)
}()
for x := range c {
println(x)
}
println("Done!")
https://play.golang.org/p/VWcBC2YGLvM
Let's say I have an int channel in Go:
theint := make(chan int)
I want to wrap this channel in a new channel called incremented
incremented := make(chan int)
Such that:
go func() { theint <- 1 }
<- incremented // 2
appended can be assumed to be the only one that reads from the int.
It will work if a run a goroutine in the background
go func() {
for num := range theint {
incremented <- num + 1
}
}
However, I prefer to do it without a goroutine since I can't control it in my context.
Is there a simpler way to do it?
One thing that came to mind is python's yield:
for num in theint:
yield num + 1
Is something like this possible in go?
Generator pattern
What you are trying to implement is generator pattern. To use channels and goroutines for implementation of this pattern is totally common practice.
However, I prefer to do it without a goroutine since I can't control it in my context.
I believe the problem is deadlock
fatal error: all goroutines are asleep - deadlock!
To avoid deadlocks and orphaned (not closed) channels use sync.WaitGroup. This is an idiomatic way to control goroutines.
Playground
package main
import (
"fmt"
"sync"
)
func incGenerator(n []int) chan int {
ch := make(chan int)
var wg sync.WaitGroup
wg.Add(len(n))
for _, i := range n {
incremented := i + 1
go func() {
wg.Done()
ch <- incremented
}()
}
go func() {
wg.Wait()
close(ch)
}()
return ch
}
func main() {
n := []int{1, 2, 3, 4, 5}
for x := range incGenerator(n) {
fmt.Println(x)
}
}
One thing you can also consider is having a select on the int channel and an exit channel - in an infinite for loop. You can choose a variable increment value too. Please see code below:
package main
import (
"fmt"
"sync"
"time"
)
func main() {
var accum int //accumulator of incremented values
var wg sync.WaitGroup
c1 := make(chan int)
exChan := make(chan bool)
wg.Add(1)
go func() {
time.Sleep(time.Second * 1)
c1 <- 1
wg.Done()
}()
wg.Add(1)
go func() {
time.Sleep(time.Second * 2)
c1 <- 2
wg.Done()
}()
wg.Add(1)
go func() {
time.Sleep(time.Second * 2)
c1 <- 5
wg.Done()
}()
go func() {
wg.Wait()
close(exChan)
}()
for {
var done bool
select {
case incBy := <-c1: //Increment by value in channel
accum += incBy
fmt.Println("Received value to increment:", incBy, "; Accumulated value is", accum)
case d := <-exChan:
done = !(d)
}
if done == true {
break
}
}
fmt.Println("Final accumulated value is", accum)
}
Playground: https://play.golang.org/p/HmdRmMCN7U
Exit channel not needed, if we are having non-zero increments always. I like #I159 's approach too!
Anyways, hope this helps.
I'm reading/working through Go Concurrency Patterns: Pipelines and cancellation, but i'm having trouble understanding the Stopping short section. We have the following functions:
func sq(in <-chan int) <-chan int {
out := make(chan int)
go func() {
for n := range in {
out <- n * n
}
close(out)
}()
return out
}
func gen(nums ...int) <-chan int {
out := make(chan int)
go func() {
for _, n := range nums {
out <- n
}
close(out)
}()
return out
}
func merge(cs ...<-chan int) <-chan int {
var wg sync.WaitGroup
out := make(chan int, 1) // enough space for the unread inputs
// Start an output goroutine for each input channel in cs. output
// copies values from c to out until c is closed, then calls wg.Done.
output := func(c <-chan int) {
for n := range c {
out <- n
}
wg.Done()
}
wg.Add(len(cs))
for _, c := range cs {
go output(c)
}
// Start a goroutine to close out once all the output goroutines are
// done. This must start after the wg.Add call.
go func() {
wg.Wait()
close(out)
}()
return out
}
func main() {
in := gen(2, 3)
// Distribute the sq work across two goroutines that both read from in.
c1 := sq(in)
c2 := sq(in)
// Consume the first value from output.
out := merge(c1, c2)
fmt.Println(<-out) // 4 or 9
return
// Apparently if we had not set the merge out buffer size to 1
// then we would have a hanging go routine.
}
Now, if you notice line 2 in merge, it says we make the out chan with buffer size 1, because this is enough space for the unread inputs. However, I'm almost positive that we should allocate a chan with buffer size 2. In accordance with this code sample:
c := make(chan int, 2) // buffer size 2
c <- 1 // succeeds immediately
c <- 2 // succeeds immediately
c <- 3 // blocks until another goroutine does <-c and receives 1
Because this section implies that a chan of buffer size 3 would not block. Can anyone please clarify/assist my understanding?
The program sends two values to the channel out and reads one value from the channel out. One of the values is not received.
If the channel is unbuffered (capacity 0), then one of the sending goroutines will block until the program exits. This is a leak.
If the channel is created with a capacity of 1, then both goroutines can send to the channel and exit. The first value sent to the channel is received by main. The second value remains in the channel.
If the main function does not receive a value from the channel out, then a channel of capacity 2 is required to prevent the goroutines from blocking indefinitely.