--- /sys/src/9k/k10/arch.c
+++ /sys/src/9k/k10/arch.c
@@ -95,6 +95,8 @@ kprocchild(Proc* p, void (*func)(void*), void* arg)
 	p->kparg = arg;
 }
 
+static int idle_spin;
+
 /*
  *  put the processor in the halt state if we've no processes to run.
  *  an interrupt will get us going again.
@@ -102,6 +104,16 @@ kprocchild(Proc* p, void (*func)(void*), void* arg)
 void
 idlehands(void)
 {
-	if(sys->nonline == 1)
+	/*
+	 * we used to halt only on single-core setups. halting in an smp system
+	 * can result in a startup latency for processes that become ready.
+	 * if idle_spin is zero, we care more about saving energy
+	 * than reducing this latency.
+	 *
+	 * the performance loss with idle_spin == 0 seems to be slight
+	 * and it reduces lock contention (thus system time and real time)
+	 * on many-core systems with large values of NPROC.
+	 */
+	if(sys->nonline == 1 || idle_spin == 0)
 		halt();
 }