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?

Answer 1

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.