[go-nuts] Parallelizing for loops

On Wed, Jul 29, 2015 at 9:21 PM, Stéphane phenetas wrote:
Hello, I am dealing with go and I try to parallelize each instance of a for
loop.

My final goal is to do a ping test for 255 IP addresses. So basically my for
loop is something like

i := 0

for i < 255 {

i++

commandLinePing := fmt.Sprint("ping 10.11.204." + string(i))

err := exec.Command("bash", "-c", commandLinePing).Run()

if err != nil {
}

}



So, the loops works well. But I would like to parallelize every 255
iterations, so that all 255 execute in the time of 1 (or so).


Any good way to do so ? To considerably reduce the execution time ?

Of course, this is a ping test so I need to listen for the reply from these
255 IPs. So, at the end, I need to wait until the last thread is finished.


Thank you very much.

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On Wednesday, July 29, 2015 at 9:21:09 PM UTC+2, Stéphane phenetas wrote:

Hello, I am dealing with go and I try to parallelize each instance of a
for loop.

My final goal is to do a ping test for 255 IP addresses. So basically my
for loop is something like

i := 0

for i < 255 {

i++

commandLinePing := fmt.Sprint("ping 10.11.204." + string(i))

err := exec.Command("bash", "-c", commandLinePing).Run()

if err != nil {
}

}



So, the loops works well. But I would like to parallelize every 255
iterations, so that all 255 execute in the time of 1 (or so).


Any good way to do so ? To considerably reduce the execution time ?

Of course, this is a ping test so I need to listen for the reply from
these 255 IPs. So, at the end, I need to wait until the last thread is
finished.


Thank you very much.

goroutine 1 [semacquire]:

sync.(*WaitGroup).Wait(0x2081e4020)

/usr/local/go/src/sync/waitgroup.go:132 +0x169

On Thursday, July 30, 2015 at 1:15:38 AM UTC+2, Stéphane phenetas wrote:

Hi Roberto, I have tried your solution, using a function called in the
main, like this :

func main() {
testfunc()
}

func testfunc() {

var w sync.WaitGroup

n := 255 // do you want to ping #255 or not?

w.Add(n)

for i := 1; i < n; i++ {

go func(j int) {

defer w.Done()


commandLinePing := fmt.Sprint("ping 10.11.204." + string(j))

err := exec.Command("bash", "-c", commandLinePing).Run()


if err != nil {

}

}(i)

}

w.Wait()

}



This code just do nothing, I can run it but do nothing.

Is I use .Start() instead of .Run(), then I get an error :

fatal error: all goroutines are asleep - deadlock!

On Thursday, July 30, 2015 at 2:24:25 AM UTC+2, Stéphane phenetas wrote:

Hi Roberto, you'r right, this is so true. I changed it back to a working
code (same code but with *w.Add(n-1)*).

However I may have played too much with my new toy, because now It appears
that I have reached the limit of process running. I have blocked my
terminal and now I have a "*fork: Resource temporarily unavailable*"
message

I am not sure the process are released when they are done.

On Thursday, July 30, 2015 at 3:10:26 AM UTC+2, Stéphane phenetas wrote:

The thing is, it worked for several times. It worked well let say maybe
4/5 times.

Then the ping never stop, and even if the "ping" is done, the thread
remains there and I can see hundreds of Pings thread in the task manager.
How to really kill them when they are done in the loop ?

On Thursday, July 30, 2015 at 2:24:25 AM UTC+2, Stéphane phenetas wrote:

Hi Roberto, you'r right, this is so true. I changed it back to a working
code (same code but with *w.Add(n-1)*).

However I may have played too much with my new toy, because now It appears
that I have reached the limit of process running. I have blocked my
terminal and now I have a "*fork: Resource temporarily unavailable*"
message

I am not sure the process are released when they are done.

Please at least print out your err! And are you sure string(j) does what
you want? strconv.Itoa is what you need.

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Hi, I am back with my problems :/

I can not really use parallelism because every time after a while I get an
error message "fork/exec /bin/bash: resource temporarily unavailable"

This seams to come from the terminal, and I think that all threads are not
correctly closes or something. For exemple if I execute the same code we
were discussing about, it returns me this error after a while. Then the
terminal is blocked ( I can not execute anything else) and I have to close
the window and reopen a new terminal.)