add runtime.GOMAXPROCS, allowing a program to, in effect, set $GOMAXPROCS

R=rsc
DELTA=29  (28 added, 1 deleted, 0 changed)
OCL=32829
CL=32837

src/pkg/runtime/extern.go

@@ -40,3 +40,6 @@ func LockOSThread()
 // If the calling goroutine has not called LockOSThread, UnlockOSThread is a no-op.
 func UnlockOSThread()
 
+// GOMAXPROCS sets the maximum number of CPUs that can be executing
+// simultaneously.   This call will go away when the scheduler improves.
+func GOMAXPROCS(n int)

src/pkg/runtime/proc.c

@@ -807,6 +807,29 @@ runtime·LockOSThread(void)
 	g->lockedm = m;
 }
 
+// delete when scheduler is stronger
+void
+runtime·GOMAXPROCS(int32 n)
+{
+	if(n < 1)
+		n = 1;
+
+	lock(&sched);
+	sched.gomaxprocs = n;
+	sched.mcpumax = n;
+	// handle fewer procs
+	while(sched.mcpu > sched.mcpumax) {
+		noteclear(&sched.stopped);
+		sched.waitstop = 1;
+		unlock(&sched);
+		notesleep(&sched.stopped);
+		lock(&sched);
+	}
+	// handle more procs
+	matchmg();
+	unlock(&sched);
+}
+
 void
 runtime·UnlockOSThread(void)
 {
@@ -821,4 +844,3 @@ runtime·mid(uint32 ret)
 	ret = m->id;
 	FLUSH(&ret);
 }
-

test/bench/spectral-norm-parallel.go

@@ -40,6 +40,7 @@ import (
 	"flag";
 	"fmt";
 	"math";
+	"runtime";
 )
 
 var n = flag.Int("n", 2000, "count")
@@ -92,6 +93,7 @@ func (v Vec) ATimesTransp(u Vec) {
 
 func main() {
 	flag.Parse();
+	runtime.GOMAXPROCS(*nCPU);
 	N := *n;
 	u := make(Vec, N);
 	for i := 0; i < N; i++ {