effective_go: provide reference to runtime.NumCPU()

R=golang-dev, robert.hencke, r
CC=golang-dev
https://golang.org/cl/5538050

doc/effective_go.html

@@ -2623,8 +2623,10 @@ is if you want CPU parallelism you must tell the run-time
 how many goroutines you want executing code simultaneously.  There
 are two related ways to do this.  Either run your job with environment
 variable <code>GOMAXPROCS</code> set to the number of cores to use
-(default 1); or import the <code>runtime</code> package and call
+or import the <code>runtime</code> package and call
 <code>runtime.GOMAXPROCS(NCPU)</code>.
+A helpful value might be <code>runtime.NumCPU()</code>, which reports the number
+of logical CPUs on the local machine.
 Again, this requirement is expected to be retired as the scheduling and run-time improve.
 </p>
 

doc/effective_go.tmpl

@@ -2560,8 +2560,10 @@ is if you want CPU parallelism you must tell the run-time
 how many goroutines you want executing code simultaneously.  There
 are two related ways to do this.  Either run your job with environment
 variable <code>GOMAXPROCS</code> set to the number of cores to use
-(default 1); or import the <code>runtime</code> package and call
+or import the <code>runtime</code> package and call
 <code>runtime.GOMAXPROCS(NCPU)</code>.
+A helpful value might be <code>runtime.NumCPU()</code>, which reports the number
+of logical CPUs on the local machine.
 Again, this requirement is expected to be retired as the scheduling and run-time improve.
 </p>
 

src/pkg/runtime/debug.go

@@ -19,6 +19,7 @@ func UnlockOSThread()
 // GOMAXPROCS sets the maximum number of CPUs that can be executing
 // simultaneously and returns the previous setting.  If n < 1, it does not
 // change the current setting.
+// The number of logical CPUs on the local machine can be queried with NumCPU.
 // This call will go away when the scheduler improves.
 func GOMAXPROCS(n int) int
 

src/pkg/runtime/extern.go

@@ -68,7 +68,7 @@ func funcline_go(*Func, uintptr) (string, int)
 // mid returns the current os thread (m) id.
 func mid() uint32
 
-// NumCPU returns the number of CPUs on the local machine.
+// NumCPU returns the number of logical CPUs on the local machine.
 func NumCPU() int
 
 // Semacquire waits until *s > 0 and then atomically decrements it.