waitgroup with concurrent limit but test fail - go

I use sync.WaitGroup with goroutine before, but I want to control the goroutine concurrency,
so I write my waitgroup with concurrency limit like:
package wglimit
import (
"sync"
)
// WaitGroupLimit ...
type WaitGroupLimit struct {
ch chan int
wg *sync.WaitGroup
}
// New ...
func New(size int) *WaitGroupLimit {
if size <= 0 {
size = 1
}
return &WaitGroupLimit{
ch: make(chan int, size), // buffer chan to limit concurrency
wg: &sync.WaitGroup{},
}
}
// Add ...
func (wgl *WaitGroupLimit) Add(delta int) {
for i := 0; i < delta; i++ {
wgl.ch <- 1
wgl.wg.Add(1)
}
}
// Done ...
func (wgl *WaitGroupLimit) Done() {
wgl.wg.Done()
<-wgl.ch
}
// Wait ...
func (wgl *WaitGroupLimit) Wait() {
close(wgl.ch)
wgl.wg.Wait()
}
And then I use it to control the goroutine concurrency, for example:
jobs := ["1", "2", "3", "4"] // some jobs
// wg := sync.WaitGroup{} // have no concurrency limit
wg := wglimit.New(2) // limit 2 goroutine
for _, job := range jobs {
wg.Add(1)
go func(job string) {
// job worker
defer wg.Done()
}(job)
}
wg.Wait()
And it looks like worked when running.
But Test Failed:
package wglimit
import (
"runtime"
"testing"
"time"
)
func TestGoLimit(t *testing.T) {
var limit int = 5
wglimit := New(limit)
for i := 0; i < 10000; i++ {
wglimit.Add(1)
go func() {
defer wglimit.Done()
time.Sleep(time.Millisecond)
if runtime.NumGoroutine() > limit+2 {
println(runtime.NumGoroutine()) // will print 9 , cocurrent limit fail ?
t.Errorf("FAIL")
}
}()
}
wglimit.Wait()
}
When testing, the goroutine numbers is bigger than my limit, it seems like the cocurrent limit fail.
Anything wrong with my WaitGroupLimit code and why?

Anything wrong with my WaitGroupLimit code [...]?
No.
The problem is runtime.NumGoroutine() doesn't do what you seem to think it does. It counts all goroutines, i.e. not only the ones you start but also the goroutines the runtime uses itself, e.g. for concurrent garbage collection. NumGoroutine is thus higher than your limit.
Your code is fine, your test isn't. Do not try to get clever in testing and test what you code really does: It blocks on Add until the limited resource is available. Test that and not a goroutine count which is just a (bad) proxy for the desired behaviour in your test.

Related

Unable to find reason for go deadlock

Unable to find a reason as to why this code deadlocks. The aim here is make the worker go routines do some work only after they are signaled.
If the signalStream channel is removed from the code, it works fine. But when this is introduced, it deadlocks. Not sure the reason for this. Also if there are any tools to explain the occurrence of deadlock, it would help.
package main
import (
"log"
"sync"
)
const jobs = 10
const workers = 5
var wg sync.WaitGroup
func main() {
// create channel
dataStream := make(chan interface{})
signalStream := make(chan interface{})
// Generate workers
for i := 1; i <= workers; i++ {
wg.Add(1)
go worker(dataStream, signalStream, i*100)
}
// Generate jobs
for i := 1; i <= jobs; i++ {
dataStream <- i
}
close(dataStream)
// start consuming data
close(signalStream)
wg.Wait()
}
func worker(c <-chan interface{}, s <-chan interface{}, id int) {
defer wg.Done()
<-s
for i := range c {
log.Printf("routine - %d - %d \n", id, i)
}
}
Generate the jobs in a separate gorouine, i.e. put the whole jobs loop into a goroutine. If you don't then dataStream <- i will block and your program will never "start consuming data"
// Generate jobs
go func() {
for i := 1; i <= jobs; i++ {
dataStream <- i
}
close(dataStream)
}()
https://go.dev/play/p/ChlbsJlgwdE

golang sync.WaitGroup never finishes

the below code that get worker from channel and execute function "call", all routines finish and print that they are done but wait never finishes,
i traced the counter of WaitGroup by making varible counter incresing when add to wg and decresing when done and it was zero at the end of for loop
any help please
package mapreduce
import (
"fmt"
"sync"
)
func schedule(jobName string, mapFiles []string, nReduce int, phase jobPhase, registerChan chan string) {
var ntasks int
var n_other int // number of inputs (for reduce) or outputs (for map)
switch phase {
case mapPhase:
ntasks = len(mapFiles)
n_other = nReduce
case reducePhase:
ntasks = nReduce
n_other = len(mapFiles)
}
fmt.Printf("Schedule: %v %v tasks (%d I/Os)\n", ntasks, phase, n_other)
var wg sync.WaitGroup
for i := 0; i < ntasks; i++ {
worker := <-registerChan
doTaskArg := DoTaskArgs{jobName, mapFiles[i], phase, i, n_other}
wg.Add(1)
go func() {
defer wg.Done()
done := call(worker, "Worker.DoTask", doTaskArg, nil)
if done {
registerChan <- worker
} else {
i = i - 1
}
}()
}
wg.Wait()
fmt.Printf("Schedule: %v phase done\n", phase)
}
The channel is blocking your goroutine. If you put some data into a unbuffered channel the goroutine waits until the receiver gets the the data from the channel. In your case your routine blocks at register <- worker and defer wg.Done() is never called, because the function is waiting.

Undetected "deadlock" while reading from channel

How do I deal with a situation where undetected deadlock occurs when reading results of execution of uncertain number tasks from a channel in a complex program, e.g. web server?
package main
import (
"fmt"
"math/rand"
"time"
)
func main() {
rand.Seed(time.Now().UTC().UnixNano())
results := make(chan int, 100)
// we can't know how many tasks there will be
for i := 0; i < rand.Intn(1<<8)+1<<8; i++ {
go func(i int) {
time.Sleep(time.Second)
results <- i
}(i)
}
// can't close channel here
// because it is still written in
//close(results)
// something else is going on other threads (think web server)
// therefore a deadlock won't be detected
go func() {
for {
time.Sleep(time.Second)
}
}()
for j := range results {
fmt.Println(j)
// we just stuck in here
}
}
In case of simpler programs go detects a deadlock and properly fails. Most examples either fetch a known number of results, or write to the channel sequentially.
The trick is to use sync.WaitGroup and wait for the tasks to finish in a non-blocking way.
var wg sync.WaitGroup
// we can't know how many tasks there will be
for i := 0; i < rand.Intn(1<<8)+1<<8; i++ {
wg.Add(1)
go func(i int) {
time.Sleep(time.Second)
results <- i
wg.Done()
}(i)
}
// wait for all tasks to finish in other thread
go func() {
wg.Wait()
close(results)
}()
// execution continues here so you can print results
See also: Go Concurrency Patterns: Pipelines and cancellation - The Go Blog

Always have x number of goroutines running at any time

I see lots of tutorials and examples on how to make Go wait for x number of goroutines to finish, but what I'm trying to do is have ensure there are always x number running, so a new goroutine is launched as soon as one ends.
Specifically I have a few hundred thousand 'things to do' which is processing some stuff that is coming out of MySQL. So it works like this:
db, err := sql.Open("mysql", connection_string)
checkErr(err)
defer db.Close()
rows,err := db.Query(`SELECT id FROM table`)
checkErr(err)
defer rows.Close()
var id uint
for rows.Next() {
err := rows.Scan(&id)
checkErr(err)
go processTheThing(id)
}
checkErr(err)
rows.Close()
Currently that will launch several hundred thousand threads of processTheThing(). What I need is that a maximum of x number (we'll call it 20) goroutines are launched. So it starts by launching 20 for the first 20 rows, and from then on it will launch a new goroutine for the next id the moment that one of the current goroutines has finished. So at any point in time there are always 20 running.
I'm sure this is quite simple/standard, but I can't seem to find a good explanation on any of the tutorials or examples or how this is done.
You may find Go Concurrency Patterns article interesting, especially Bounded parallelism section, it explains the exact pattern you need.
You can use channel of empty structs as a limiting guard to control number of concurrent worker goroutines:
package main
import "fmt"
func main() {
maxGoroutines := 10
guard := make(chan struct{}, maxGoroutines)
for i := 0; i < 30; i++ {
guard <- struct{}{} // would block if guard channel is already filled
go func(n int) {
worker(n)
<-guard
}(i)
}
}
func worker(i int) { fmt.Println("doing work on", i) }
Here I think something simple like this will work :
package main
import "fmt"
const MAX = 20
func main() {
sem := make(chan int, MAX)
for {
sem <- 1 // will block if there is MAX ints in sem
go func() {
fmt.Println("hello again, world")
<-sem // removes an int from sem, allowing another to proceed
}()
}
}
Thanks to everyone for helping me out with this. However, I don't feel that anyone really provided something that both worked and was simple/understandable, although you did all help me understand the technique.
What I have done in the end is I think much more understandable and practical as an answer to my specific question, so I will post it here in case anyone else has the same question.
Somehow this ended up looking a lot like what OneOfOne posted, which is great because now I understand that. But OneOfOne's code I found very difficult to understand at first because of the passing functions to functions made it quite confusing to understand what bit was for what. I think this way makes a lot more sense:
package main
import (
"fmt"
"sync"
)
const xthreads = 5 // Total number of threads to use, excluding the main() thread
func doSomething(a int) {
fmt.Println("My job is",a)
return
}
func main() {
var ch = make(chan int, 50) // This number 50 can be anything as long as it's larger than xthreads
var wg sync.WaitGroup
// This starts xthreads number of goroutines that wait for something to do
wg.Add(xthreads)
for i:=0; i<xthreads; i++ {
go func() {
for {
a, ok := <-ch
if !ok { // if there is nothing to do and the channel has been closed then end the goroutine
wg.Done()
return
}
doSomething(a) // do the thing
}
}()
}
// Now the jobs can be added to the channel, which is used as a queue
for i:=0; i<50; i++ {
ch <- i // add i to the queue
}
close(ch) // This tells the goroutines there's nothing else to do
wg.Wait() // Wait for the threads to finish
}
Create channel for passing data to goroutines.
Start 20 goroutines that processes the data from channel in a loop.
Send the data to the channel instead of starting a new goroutine.
Grzegorz Żur's answer is the most efficient way to do it, but for a newcomer it could be hard to implement without reading code, so here's a very simple implementation:
type idProcessor func(id uint)
func SpawnStuff(limit uint, proc idProcessor) chan<- uint {
ch := make(chan uint)
for i := uint(0); i < limit; i++ {
go func() {
for {
id, ok := <-ch
if !ok {
return
}
proc(id)
}
}()
}
return ch
}
func main() {
runtime.GOMAXPROCS(4)
var wg sync.WaitGroup //this is just for the demo, otherwise main will return
fn := func(id uint) {
fmt.Println(id)
wg.Done()
}
wg.Add(1000)
ch := SpawnStuff(10, fn)
for i := uint(0); i < 1000; i++ {
ch <- i
}
close(ch) //should do this to make all the goroutines exit gracefully
wg.Wait()
}
playground
This is a simple producer-consumer problem, which in Go can be easily solved using channels to buffer the paquets.
To put it simple: create a channel that accept your IDs. Run a number of routines which will read from the channel in a loop then process the ID. Then run your loop that will feed IDs to the channel.
Example:
func producer() {
var buffer = make(chan uint)
for i := 0; i < 20; i++ {
go consumer(buffer)
}
for _, id := range IDs {
buffer <- id
}
}
func consumer(buffer chan uint) {
for {
id := <- buffer
// Do your things here
}
}
Things to know:
Unbuffered channels are blocking: if the item wrote into the channel isn't accepted, the routine feeding the item will block until it is
My example lack a closing mechanism: you must find a way to make the producer to wait for all consumers to end their loop before returning. The simplest way to do this is with another channel. I let you think about it.
I've wrote a simple package to handle concurrency for Golang. This package will help you limit the number of goroutines that are allowed to run concurrently:
https://github.com/zenthangplus/goccm
Example:
package main
import (
"fmt"
"goccm"
"time"
)
func main() {
// Limit 3 goroutines to run concurrently.
c := goccm.New(3)
for i := 1; i <= 10; i++ {
// This function have to call before any goroutine
c.Wait()
go func(i int) {
fmt.Printf("Job %d is running\n", i)
time.Sleep(2 * time.Second)
// This function have to when a goroutine has finished
// Or you can use `defer c.Done()` at the top of goroutine.
c.Done()
}(i)
}
// This function have to call to ensure all goroutines have finished
// after close the main program.
c.WaitAllDone()
}
Also can take a look here: https://github.com/LiangfengChen/goutil/blob/main/concurrent.go
The example can refer the test case.
func TestParallelCall(t *testing.T) {
format := "test:%d"
data := make(map[int]bool)
mutex := sync.Mutex{}
val, err := ParallelCall(1000, 10, func(pos int) (interface{}, error) {
mutex.Lock()
defer mutex.Unlock()
data[pos] = true
return pos, errors.New(fmt.Sprintf(format, pos))
})
for i := 0; i < 1000; i++ {
if _, ok := data[i]; !ok {
t.Errorf("TestParallelCall pos not found: %d", i)
}
if val[i] != i {
t.Errorf("TestParallelCall return value is not right (%d,%v)", i, val[i])
}
if err[i].Error() != fmt.Sprintf(format, i) {
t.Errorf("TestParallelCall error msg is not correct (%d,%v)", i, err[i])
}
}
}

How to wait for all goroutines to finish without using time.Sleep?

This code selects all xml files in the same folder, as the invoked executable and asynchronously applies processing to each result in the callback method (in the example below, just the name of the file is printed out).
How do I avoid using the sleep method to keep the main method from exiting? I have problems wrapping my head around channels (I assume that's what it takes, to synchronize the results) so any help is appreciated!
package main
import (
"fmt"
"io/ioutil"
"path"
"path/filepath"
"os"
"runtime"
"time"
)
func eachFile(extension string, callback func(file string)) {
exeDir := filepath.Dir(os.Args[0])
files, _ := ioutil.ReadDir(exeDir)
for _, f := range files {
fileName := f.Name()
if extension == path.Ext(fileName) {
go callback(fileName)
}
}
}
func main() {
maxProcs := runtime.NumCPU()
runtime.GOMAXPROCS(maxProcs)
eachFile(".xml", func(fileName string) {
// Custom logic goes in here
fmt.Println(fileName)
})
// This is what i want to get rid of
time.Sleep(100 * time.Millisecond)
}
You can use sync.WaitGroup. Quoting the linked example:
package main
import (
"net/http"
"sync"
)
func main() {
var wg sync.WaitGroup
var urls = []string{
"http://www.golang.org/",
"http://www.google.com/",
"http://www.somestupidname.com/",
}
for _, url := range urls {
// Increment the WaitGroup counter.
wg.Add(1)
// Launch a goroutine to fetch the URL.
go func(url string) {
// Decrement the counter when the goroutine completes.
defer wg.Done()
// Fetch the URL.
http.Get(url)
}(url)
}
// Wait for all HTTP fetches to complete.
wg.Wait()
}
WaitGroups are definitely the canonical way to do this. Just for the sake of completeness, though, here's the solution that was commonly used before WaitGroups were introduced. The basic idea is to use a channel to say "I'm done," and have the main goroutine wait until each spawned routine has reported its completion.
func main() {
c := make(chan struct{}) // We don't need any data to be passed, so use an empty struct
for i := 0; i < 100; i++ {
go func() {
doSomething()
c <- struct{}{} // signal that the routine has completed
}()
}
// Since we spawned 100 routines, receive 100 messages.
for i := 0; i < 100; i++ {
<- c
}
}
sync.WaitGroup can help you here.
package main
import (
"fmt"
"sync"
"time"
)
func wait(seconds int, wg * sync.WaitGroup) {
defer wg.Done()
time.Sleep(time.Duration(seconds) * time.Second)
fmt.Println("Slept ", seconds, " seconds ..")
}
func main() {
var wg sync.WaitGroup
for i := 0; i <= 5; i++ {
wg.Add(1)
go wait(i, &wg)
}
wg.Wait()
}
Although sync.waitGroup (wg) is the canonical way forward, it does require you do at least some of your wg.Add calls before you wg.Wait for all to complete. This may not be feasible for simple things like a web crawler, where you don't know the number of recursive calls beforehand and it takes a while to retrieve the data that drives the wg.Add calls. After all, you need to load and parse the first page before you know the size of the first batch of child pages.
I wrote a solution using channels, avoiding waitGroup in my solution the the Tour of Go - web crawler exercise. Each time one or more go-routines are started, you send the number to the children channel. Each time a go routine is about to complete, you send a 1 to the done channel. When the sum of children equals the sum of done, we are done.
My only remaining concern is the hard-coded size of the the results channel, but that is a (current) Go limitation.
// recursionController is a data structure with three channels to control our Crawl recursion.
// Tried to use sync.waitGroup in a previous version, but I was unhappy with the mandatory sleep.
// The idea is to have three channels, counting the outstanding calls (children), completed calls
// (done) and results (results). Once outstanding calls == completed calls we are done (if you are
// sufficiently careful to signal any new children before closing your current one, as you may be the last one).
//
type recursionController struct {
results chan string
children chan int
done chan int
}
// instead of instantiating one instance, as we did above, use a more idiomatic Go solution
func NewRecursionController() recursionController {
// we buffer results to 1000, so we cannot crawl more pages than that.
return recursionController{make(chan string, 1000), make(chan int), make(chan int)}
}
// recursionController.Add: convenience function to add children to controller (similar to waitGroup)
func (rc recursionController) Add(children int) {
rc.children <- children
}
// recursionController.Done: convenience function to remove a child from controller (similar to waitGroup)
func (rc recursionController) Done() {
rc.done <- 1
}
// recursionController.Wait will wait until all children are done
func (rc recursionController) Wait() {
fmt.Println("Controller waiting...")
var children, done int
for {
select {
case childrenDelta := <-rc.children:
children += childrenDelta
// fmt.Printf("children found %v total %v\n", childrenDelta, children)
case <-rc.done:
done += 1
// fmt.Println("done found", done)
default:
if done > 0 && children == done {
fmt.Printf("Controller exiting, done = %v, children = %v\n", done, children)
close(rc.results)
return
}
}
}
}
Full source code for the solution
Here is a solution that employs WaitGroup.
First, define 2 utility methods:
package util
import (
"sync"
)
var allNodesWaitGroup sync.WaitGroup
func GoNode(f func()) {
allNodesWaitGroup.Add(1)
go func() {
defer allNodesWaitGroup.Done()
f()
}()
}
func WaitForAllNodes() {
allNodesWaitGroup.Wait()
}
Then, replace the invocation of callback:
go callback(fileName)
With a call to your utility function:
util.GoNode(func() { callback(fileName) })
Last step, add this line at the end of your main, instead of your sleep. This will make sure the main thread is waiting for all routines to finish before the program can stop.
func main() {
// ...
util.WaitForAllNodes()
}

Resources