Why doesn't the Go statement execute in parallel?

Question

I'm testing this Go code on my VirtualBoxed Ubuntu 11.4

package main

import ("fmt";"time";"big")
var c chan *big.Int

func sum( start,stop,step int64) {
    bigStop := big.NewInt(stop)
    bigStep := big.NewInt(step)
    bigSum  := big.NewInt(0)
    for i := big.NewInt(start);i.Cmp(bigStop)<0 ;i.Add(i,bigStep){
        bigSum.Add(bigSum,i)
    }
    c<-bigSum           
}

func main() {
    s := big.NewInt( 0 )
    n := time.Nanoseconds()

    step := int64(4)
    c = make( chan *big.Int , int(step))
    stop := int64(100000000)
    for j:=int64(0);j<step;j++{
        go sum(j,stop,step)     
    }
    for j:=int64(0);j<step;j++{
        s.Add(s,<-c)
    }
    n = time.Nanoseconds() - n
    fmt.Println(s,float64(n)/1000000000.)
}

Ubuntu has access to all my 4 cores. I checked this with simultaneous run of several executables and System Monitor. But when I'm trying to run this code, it's using only one core and is not gaining any profit of parallel processing.

What I'm doing wrong?

Kyle Lemons · Accepted Answer · 2013-09-02 18:20:18Z

24

You probably need to review the Concurrency section of the Go FAQ, specifically these two questions, and work out which (if not both) apply to your case:

Why doesn't my multi-goroutine program use multiple CPUs?

You must set the GOMAXPROCS shell environment variable or use the similarly-named function of the runtime package to allow the run-time support to utilize more than one OS thread.

Programs that perform parallel computation should benefit from an increase in GOMAXPROCS. However, be aware that concurrency is not parallelism.

Why does using GOMAXPROCS > 1 sometimes make my program slower?

It depends on the nature of your program. Programs that contain several goroutines that spend a lot of time communicating on channels will experience performance degradation when using multiple OS threads. This is because of the significant context-switching penalty involved in sending data between threads.

Go's goroutine scheduler is not as good as it needs to be. In future, it should recognize such cases and optimize its use of OS threads. For now, GOMAXPROCS should be set on a per-application basis.

For more detail on this topic see the talk entitled Concurrency is not Parallelism.

edited Sep 2, 2013 at 18:20

Kyle Lemons

4,7661 gold badge22 silver badges24 bronze badges

answered Jun 4, 2011 at 7:09

Nicholas Knight

16.1k5 gold badges47 silver badges58 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

mnagel Over a year ago

also note that the default seems to be "1"

Xeoncross Over a year ago

The default with go1.6.3 on OSX 10.11 is the number of CPU cores. I believe it was changed in Go v1.5.

kristianp · Accepted Answer · 2025-04-13 05:03:12Z

0

Note that as of a long time ago (Go 1.5, on 19 Aug 2015), GOMAXPROCS now defaults to the number of threads on your machine. So if you have an intel processor with 4 cores and each has hyperthreading enabled, runtime.GOMAXPROCS(0) will return 8.

edited Apr 13 at 5:03

answered Apr 13 at 2:23

kristianp

6,01542 silver badges63 bronze badges

Collectives™ on Stack Overflow

Why doesn't the Go statement execute in parallel?

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related