Concurrency in Go loop polling case branch not hit - go

I am implementing a very simple concurrent program in Go. There are 2 channels todo and done that are used for signaling which task is done. There are 5 routines that are executed and each one require its own time to complete. I would like to see every 100ms the status of what is happening.
However I tried but the polling branch case <-time.After(100 * time.Millisecond): seems that is never been called. It is called sometimes (not in a consisted way) if I reduce the time to something less than 100ms.
My understanding is that go func executes the method in a separate Go scheduler thread. I do not understand therefore why the case of the polling is never hit. I tried to move the specific case branch before/after the other but nothing changed.
Any suggestions?
package main
import (
"fmt"
"math/rand"
"sync"
"time"
)
func concurrent(id int, done chan int, todo chan int) {
for {
// doing a task
t := randInt(50, 100)
time.Sleep(time.Duration(t) * time.Millisecond)
done <- id
// redo again this task
t = randInt(50, 100)
time.Sleep(time.Duration(t) * time.Millisecond)
todo <- id
}
}
func randInt(min int, max int) int {
return (min + rand.Intn(max-min))
}
func seedRandom() {
rand.Seed(time.Now().UTC().UnixNano())
}
func main() {
seedRandom()
todo := make(chan int, 5)
done := make(chan int, 5)
for i := 0; i < 5; i++ {
todo <- i
}
timeout := make(chan bool)
go func() {
time.Sleep(1 * time.Second)
timeout <- true
}()
var mu sync.Mutex
var output []int
loop:
for {
select {
case <-time.After(100 * time.Millisecond):
//this branch is never hit?
fmt.Printf("\nPolling status: %v\n", output)
case <-timeout:
fmt.Printf("\nDing ding, time is up!\n")
break loop
case id := <-done:
mu.Lock()
output = append(output, id)
fmt.Printf(".")
mu.Unlock()
case id := <-todo:
go concurrent(id, done, todo)
}
}
}
Update After following the answers I created this version in Go Playgound: https://play.golang.org/p/f08t984BdPt. That works as expected

you are creating 5 goroutines (func concurrent) and in your select case using the todo channel and this channel is being used in concurrent function so you end up creating a lot of goroutines
func concurrent(id int, done chan int, todo chan int) {
for {
// doing a task
t := randInt(50, 100)
time.Sleep(time.Duration(t) * time.Millisecond)
done <- id
// redo again this task
t = randInt(50, 100)
time.Sleep(time.Duration(t) * time.Millisecond)
by doing this call you are re-crating the go-routime
todo <- id
}
}
when I ran your code I got "runtime.NumGoroutine()"
"number of goRoutines still running 347"
as you are implementing the time.After(100 * time.Millisecond) inside the for loop it gets reset every time some other case gets hit and in your case
case id := <-todo: && id := <-done: will always get hit within 100 Milliseconds that's why you didn't get the expected output (from how your code is now i would say that the number of go-routines would increase exponentially and each of em would be waiting to send value to done and few on todo channel so your loop wont get enough time(100 ms) to wait on time.After)
loop:
for {
select {
case <-time.After(100 * time.Millisecond): ->this will always get reset ( we can use time.Ticker as it will create a single object that will signal for each and every 100ms https://golang.org/pkg/time/#NewTicker
//this branch is never hit?
fmt.Printf("\nPolling status: %v\n", output)
case <-timeout:
fmt.Printf("\nDing ding, time is up!\n")
break loop
case id := <-done: -> **this will get called**
//the mutex call is actually not very usefull as this only get called once per loop and is prefectly thread safe in this code
mu.Lock()
output = append(output, id)
fmt.Printf(".")
mu.Unlock()
case id := <-todo: -> **this will get called**
go concurrent(id, done, todo)
}
}
}
https://play.golang.org/p/SmlSIUIF5jn -> I have made some modifications to make your code work as expected..
try referring this to get a better understanding of golang channels and goroutine
https://tour.golang.org/concurrency/1

time.After(100*time.Millisecond) creates a brand new channel, with a brand new timer, which starts at the moment that function is called.
So, in your loop :
for {
select {
// this statement resets the 100ms timer each time you execute the loop :
case <-time.After(100*time.Millisecond):
...
Your branch never gets hit because, with 5 goroutines sending signals within less than 100ms on one of the other cases, this time.After(100ms) never reaches completion.
You need to choose a way to keep the same timer between iterations.
Here is one way to adapt your time.After(...) call :
// store the timer in a variable *outside* the loop :
statusTimer := time.After(100*time.Millisecond)
for {
select {
case <-statusTimer:
fmt.Printf("\nPolling status: %v\n", output)
// reset the timer :
statusTimer = time.After(100*time.Millisecond)
case <-timeout:
...
Another way is, as #blackgreen suggests, to use a time.Ticker :
statusTicker := time.NewTicker(100*time.Millisecond)
for {
select {
case <-statusTicker.C:
fmt.Printf("\nPolling status: %v\n", output)
case <-timeout:
...
side notes
a. if the output slice is not shared with other goroutines, you don't need a mutex around its access :
for {
select {
case <-statusTicker.C:
fmt.Printf("\nPolling status: %v\n", output)
...
case i <-done:
// no race condition here : all happens within the same goroutine,
// the 'select' statement makes sure that 'case's are executed
// one at a time
output = append(output, id)
fmt.Printf(".")
b. For your timeout channel :
Another generic way to "signal" that some event occurred with a channel is to close the channel instead of sending a value on it :
// if you don't actually care about the value you send over this channel :
// you can make it unbuffered, and use the empty 'struct{}' type
timeout := make(chan struct{})
go func(){
// wait for some condition ...
<-time.After(1*time.Second)
close(timeout)
}()
select {
case <-statusTimer:
...
case <-timeout: // this branch will also be taken once timeout is closed
fmt.Printf("\nDing ding, time is up!\n")
break loop
case ...
The bug you will avoid is the following : suppose you want to use that timeout channel in two goroutines
if you send a value over the timeout channel, only one goroutine will get signaled - it will "eat up" the value from the channel, and the other goroutine will only have a blocking channel,
if you close the channel, both goroutines will correctly "receive" the signal

In absence of a default case, when multiple cases are ready, it executes one of them at random. It's not deterministic.
To make sure the case runs, you should run it in a separate goroutine. (In that case, you must synchronize accesses to the output variable).
Moreover you say "I would like to see every 100ms", but time.After sends on the channel only once.
To execute the case periodically, use <-time.NewTicker(100 * time.Millis).C instead.
var mu sync.Mutex
var output []int
go func() {
ticker := time.NewTicker(100 * time.Millisecond)
defer ticker.Stop()
for {
select {
case <-ticker.C:
// TODO: must synchronize access
fmt.Printf("\nPolling status: %v\n", output)
case <-timeout:
return
}
}
}()
loop:
for {
select {
// other cases
}
}

Related

Is it possible to cancel unfinished goroutines?

Consider a group of check works, each of which has independent logic, so they seem to be good to run concurrently, like:
type Work struct {
// ...
}
// This Check could be quite time-consuming
func (w *Work) Check() bool {
// return succeed or not
//...
}
func CheckAll(works []*Work) {
num := len(works)
results := make(chan bool, num)
for _, w := range works {
go func(w *Work) {
results <- w.Check()
}(w)
}
for i := 0; i < num; i++ {
if r := <-results; !r {
ReportFailed()
break;
}
}
}
func ReportFailed() {
// ...
}
When concerned about the results, if the logic is no matter which one work fails, we assert all works totally fail, the remaining values in the channel are useless. Let the remaining unfinished goroutines continue to run and send results to the channel is meaningless and waste, especially when w.Check() is quite time-consuming. The ideal effect is similar to:
for _, w := range works {
if !w.Check() {
ReportFailed()
break;
}
}
This only runs necessary check works then break, but is in sequential non-concurrent scenario.
So, is it possible to cancel these unfinished goroutines, or sending to channel?
Cancelling a (blocking) send
Your original question asked how to cancel a send operation. A send on a channel is basically "instant". A send on a channel blocks if the channel's buffer is full and there is no ready receiver.
You can "cancel" this send by using a select statement and a cancel channel which you close, e.g.:
cancel := make(chan struct{})
select {
case ch <- value:
case <- cancel:
}
Closing the cancel channel with close(cancel) on another goroutine will make the above select abandon the send on ch (if it's blocking).
But as said, the send is "instant" on a "ready" channel, and the send first evaluates the value to be sent:
results <- w.Check()
This first has to run w.Check(), and once it's done, its return value will be sent on results.
Cancelling a function call
So what you really need is to cancel the w.Check() method call. For that, the idiomatic way is to pass a context.Context value which you can cancel, and w.Check() itself must monitor and "obey" this cancellation request.
See Terminating function execution if a context is cancelled
Note that your function must support this explicitly. There is no implicit termination of function calls or goroutines, see cancel a blocking operation in Go.
So your Check() should look something like this:
// This Check could be quite time-consuming
func (w *Work) Check(ctx context.Context, workDuration time.Duration) bool {
// Do your thing and monitor the context!
select {
case <-ctx.Done():
return false
case <-time.After(workDuration): // Simulate work
return true
case <-time.After(2500 * time.Millisecond): // Simulate failure after 2.5 sec
return false
}
}
And CheckAll() may look like this:
func CheckAll(works []*Work) {
ctx, cancel := context.WithCancel(context.Background())
defer cancel()
num := len(works)
results := make(chan bool, num)
wg := &sync.WaitGroup{}
for i, w := range works {
workDuration := time.Second * time.Duration(i)
wg.Add(1)
go func(w *Work) {
defer wg.Done()
result := w.Check(ctx, workDuration)
// You may check and return if context is cancelled
// so result is surely not sent, I omitted it here.
select {
case results <- result:
case <-ctx.Done():
return
}
}(w)
}
go func() {
wg.Wait()
close(results) // This allows the for range over results to terminate
}()
for result := range results {
fmt.Println("Result:", result)
if !result {
cancel()
break
}
}
}
Testing it:
CheckAll(make([]*Work, 10))
Output (try it on the Go Playground):
Result: true
Result: true
Result: true
Result: false
We get true printed 3 times (works that complete under 2.5 seconds), then the failure simulation kicks in, returns false, and terminates all other jobs.
Note that the sync.WaitGroup in the above example is not strictly needed as results has a buffer capable of holding all results, but in general it's still good practice (should you use a smaller buffer in the future).
See related: Close multiple goroutine if an error occurs in one in go
The short answer is: No.
You can not cancel or close any goroutine unless the goroutine itself reaches the return or end of its stack.
If you want to cancel something, the best approach is to pass a context.Context to them and listen to this context.Done() inside of the routine. Whenever context is canceled, you should return and the goroutine will automatically die after executing defers(if any).
package main
import "fmt"
type Work struct {
// ...
Name string
IsSuccess chan bool
}
// This Check could be quite time-consuming
func (w *Work) Check() {
// return succeed or not
//...
if len(w.Name) > 0 {
w.IsSuccess <- true
}else{
w.IsSuccess <- false
}
}
//堆排序
func main() {
works := make([]*Work,3)
works[0] = &Work{
Name: "",
IsSuccess: make(chan bool),
}
works[1] = &Work{
Name: "111",
IsSuccess: make(chan bool),
}
works[2] =&Work{
Name: "",
IsSuccess: make(chan bool),
}
for _,w := range works {
go w.Check()
}
for i,w := range works{
select {
case checkResult := <-w.IsSuccess :
fmt.Printf("index %d checkresult %t \n",i,checkResult)
}
}
}
enter image description here

Why write channel blocked in spite of a goroutine is selecting on this channel?

I am confused about the following code, I write down some comment in the code which point out my confusion. And there is a result of execution at the end of the code, I also write down what result I expect.
package main
import (
"fmt"
"time"
)
func sendRPC() bool {
time.Sleep(5 * time.Second)
return true
}
func main() {
done := make(chan struct{})
ch := make(chan bool)
go func() { // goroutine A
select {
case ch <- sendRPC():
fmt.Println("RPC return")
case <-done:
fmt.Println("exit")
}
}()
select {
case <-ch:
case <-time.After(1000 * time.Millisecond):
fmt.Println("timeout")
if len(done) == 0 {
fmt.Println("1")
// here write done channel will block until sendRPC() return, why?
// I expect that the write is nonblock because goroutine A is select on done channel.
done <- struct{}{}
fmt.Println("2")
}
}
// result:
// timeout (after about 1 second)
// 1
// exit (after about 5 seconds, I expect that it is printed after about 1 second too.)
// 2
}
The specification says:
For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the "select" statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed.
The set of channels for the select in goroutine A waits on the evaluation of sendRPC(). It may help to look at this equivalent goroutine:
go func() { // goroutine A
v := sendRPC() // waits for 5 seconds
select {
case ch <- v:
fmt.Println("RPC return")
case <-done:
fmt.Println("exit")
}
}()
The receive on done is delayed for 5 seconds.

How to run for x seconds in a http handler

I want to run my function InsertRecords for 30 seconds and test how many records I can insert in a given time.
How can I stop processing InsertRecords after x seconds and then return a result from my handler?
func benchmarkHandler(w http.ResponseWriter, r *http.Request) {
counter := InsertRecords()
w.WriteHeader(200)
io.WriteString(w, fmt.Sprintf("counter is %d", counter))
}
func InsertRecords() int {
counter := 0
// db code goes here
return counter
}
Cancellations and timeouts are often done with a context.Context.
While this simple example could be done with a channel alone, using the context here makes it more flexible, and can take into account the client disconnecting as well.
func benchmarkHandler(w http.ResponseWriter, r *http.Request) {
ctx, cancel := context.WithTimeout(r.Context(), 30*time.Second)
defer cancel()
counter := InsertRecords(ctx)
w.WriteHeader(200)
io.WriteString(w, fmt.Sprintf("counter is %d", counter))
}
func InsertRecords(ctx context.Context) int {
counter := 0
done := ctx.Done()
for {
select {
case <-done:
return counter
default:
}
// db code goes here
counter++
}
return counter
}
This will run for at least 30 seconds, returning the number of complete database iterations. If you want to be sure that the handler always returns immediately after 30s, even if the DB call is blocked, then you need to push the DB code into another goroutine and let it return later. The shortest example would be to use a similar pattern as above, but synchronize access to the counter variable, since it could be written by the DB loop while returning.
func InsertRecords(ctx context.Context) int {
counter := int64(0)
done := ctx.Done()
go func() {
for {
select {
case <-done:
return
default:
}
// db code goes here
atomic.AddInt64(&counter, 1)
}
}()
<-done
return int(atomic.LoadInt64(&counter))
}
See #JoshuaKolden's answer for an example with a producer and a timeout, which could also be combined with the existing request context.
As JimB pointed out cancelation for limiting the time taken by an http requests can be handled with context.WithTimeout, however since you asked for the purposes of benchmarking you may want to use a more direct method.
The purpose of context.Context is to allow for numerous cancelation events to occur and have the same net effect of gracefully stopping all downstream tasks. In JimB's example it's possible that some other process will cancel the context before the 30 seconds have elapsed, and this is desirable from the resource utilization point of view. For example, if the connection is terminated prematurely there is no point in doing any more work on building a response.
If benchmarking is your goal you'd want to minimized the effect of superfluous events on the code being benchmarked. Here is an example of how to do that:
func InsertRecords() int {
stop := make(chan struct{})
defer close(stop)
countChan := make(chan int)
go func() {
defer close(countChan)
for {
// db code goes here
select {
case countChan <- 1:
case <-stop:
return
}
}
}()
var counter int
timeoutCh := time.After(30 * time.Second)
for {
select {
case n := <-countChan:
counter += n
case <-timeoutCh:
return counter
}
}
}
Essentially what we are doing is creating an infinite loop over discrete db operations, and counting iterations through the loop, we stop when time.After is triggered.
A problem in JimB's example is that despite checking ctx.Done() in the loop the loop can still block if the "db code" blocks. This is because ctx.Done() is only evaluated inline with the "db code" block.
To avoid this problem we separate the timing function and the benchmarking loop so that nothing can prevent us from receiving the timeout event when it occurs. Once the time out even occurs we immediately return the result of the counter. The "db code" may still be in mid execution but InsertRecords will exit and return its results anyway.
If the "db code" is in mid-execution when InsertRecords exits, the goroutine will be left running, so to clean this up we defer close(stop) so that on function exit we'll be sure to signal the goroutine to exit on the next iteration. When the goroutine exits, it cleans up the channel it was using to send the count.
As a general pattern the above is an example of how you can get precise timing in Go without regard to the actual execution time of the code being timed.
sidenote: A somewhat more advanced observation is that my example does not attempt to synchronize the start times between the timer and the goroutine. It seemed a bit pedantic to address that issue here. However, you can easily synchronize the two threads by creating a channel that blocks the main thread until the goroutine closes it just before starting the loop.

Idiomatic way for reading from the channel for a certain time

I need to read data from the Go channel for a certain period of time (say 5 seconds). The select statement with timeout doesn't work for me, as I need to read as many values as available and stop exactly after 5 seconds. So far, I've come up with a solution using an extra time channel https://play.golang.org/p/yev9CcvzRIL
package main
import "time"
import "fmt"
func main() {
// I have no control over dataChan
dataChan := make(chan string)
// this is a stub to demonstrate some data coming from dataChan
go func() {
for {
dataChan <- "some data"
time.Sleep(time.Second)
}
}()
// the following is the code I'm asking about
timeChan := time.NewTimer(time.Second * 5).C
for {
select {
case d := <-dataChan:
fmt.Println("Got:", d)
case <-timeChan:
fmt.Println("Time's up!")
return
}
}
}
I'm wondering is there any better or more idiomatic way for solving this problem?
That's pretty much it. But if you don't need to stop or reset the timer, simply use time.After() instead of time.NewTimer(). time.After() is "equivalent to NewTimer(d).C".
afterCh := time.After(5 * time.Second)
for {
select {
case d := <-dataChan:
fmt.Println("Got:", d)
case <-afterCh:
fmt.Println("Time's up!")
return
}
}
Alternatively (to your liking), you may declare the after channel in the for statement like this:
for afterCh := time.After(5 * time.Second); ; {
select {
case d := <-dataChan:
fmt.Println("Got:", d)
case <-afterCh:
fmt.Println("Time's up!")
return
}
}
Also I know this is just an example, but always think how a goroutine you start will properly end, as the goroutine producing data in your case will never terminate.
Also don't forget that if multiple cases may be executed without blocking, one is chosen randomly. So if dataChan is ready to receive from "non-stop", there is no guarantee that the loop will terminate immediately after the timeout. In practice this is usually not a problem (starting with that even the timeout is not a guarantee, see details at Golang Timers with 0 length), but you should not forget about it in "mission-critial" applications. For details, see related questions:
force priority of go select statement
golang: How the select worked when multiple channel involved?
It looks like context with deadline would be good fit, something like
func main() {
dataChan := make(chan string)
ctx, cancel := context.WithDeadline(context.Background(), time.Now().Add(5*time.Second))
defer cancel()
go func(ctx context.Context) {
for {
select {
case dataChan <- "some data":
time.Sleep(time.Second)
case <-ctx.Done():
fmt.Println(ctx.Err())
close(dataChan)
return
}
}
}(ctx)
for d := range dataChan {
fmt.Println("Got:", d)
}
}

golang channel behaviour with for loops

I'm curious about the behaviour of channels and how they work in relation to loops. Suppose I have the following code:
Consumer
tick := time.Tick(time.Duration(2) * time.Second)
for {
select {
case <-tick:
p.channel <-true
}
}
And I have a goroutine that has the following:
Processor
for {
select {
case canProcess := <-p.channel:
// synchronous process that takes longer than 2 seconds
case <-p.stop:
return
}
}
What happens when the Consumer pushes to the channel faster than the Processor can complete its synchronous process?
Do they pile up waiting for the Processor to complete or do they skip a "beat" as such?
If they pile up, is there potential for memory leaking?
I know I can put the synchronous process in a goroutine instead, but this is really to understand how channels behave. (i.e. my example has a 2-second tick, but it doesn't have to).
select will only invoke next time.Tick if previous case corresponding code was completed,
you can still have some lever by specifying size of channel i.e channel := make(chan bool,10)
See below:
func main() {
channel := make(chan bool, 10)
go func() {
tick := time.Tick(time.Duration(1) * time.Second)
for {
select {
case <-tick:
fmt.Printf("Producer: TICK %v\n", time.Now())
channel <- true
}
}
}()
for {
select {
case canProcess := <-channel:
time.Sleep(3* time.Second)
fmt.Printf("Consumer: Completed : %v\n")
fmt.Printf("%v\n", canProcess)
}
}
}

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