diff --git a/src/pkg/runtime/defs_darwin.go b/src/pkg/runtime/defs_darwin.go
index 80b7a4f2779..7f22b0b8e55 100644
--- a/src/pkg/runtime/defs_darwin.go
+++ b/src/pkg/runtime/defs_darwin.go
@@ -34,6 +34,9 @@ const (
 	MAP_PRIVATE = C.MAP_PRIVATE
 	MAP_FIXED   = C.MAP_FIXED
 
+	MADV_DONTNEED = C.MADV_DONTNEED
+	MADV_FREE     = C.MADV_FREE
+
 	MACH_MSG_TYPE_MOVE_RECEIVE   = C.MACH_MSG_TYPE_MOVE_RECEIVE
 	MACH_MSG_TYPE_MOVE_SEND      = C.MACH_MSG_TYPE_MOVE_SEND
 	MACH_MSG_TYPE_MOVE_SEND_ONCE = C.MACH_MSG_TYPE_MOVE_SEND_ONCE
diff --git a/src/pkg/runtime/defs_darwin_386.h b/src/pkg/runtime/defs_darwin_386.h
index 3606bad9413..92732f46024 100644
--- a/src/pkg/runtime/defs_darwin_386.h
+++ b/src/pkg/runtime/defs_darwin_386.h
@@ -12,6 +12,9 @@ enum {
 	MAP_PRIVATE	= 0x2,
 	MAP_FIXED	= 0x10,
 
+	MADV_DONTNEED	= 0x4,
+	MADV_FREE	= 0x5,
+
 	MACH_MSG_TYPE_MOVE_RECEIVE	= 0x10,
 	MACH_MSG_TYPE_MOVE_SEND		= 0x11,
 	MACH_MSG_TYPE_MOVE_SEND_ONCE	= 0x12,
diff --git a/src/pkg/runtime/defs_darwin_amd64.h b/src/pkg/runtime/defs_darwin_amd64.h
index c28225f4cfe..d4fbfef4995 100644
--- a/src/pkg/runtime/defs_darwin_amd64.h
+++ b/src/pkg/runtime/defs_darwin_amd64.h
@@ -12,6 +12,9 @@ enum {
 	MAP_PRIVATE	= 0x2,
 	MAP_FIXED	= 0x10,
 
+	MADV_DONTNEED	= 0x4,
+	MADV_FREE	= 0x5,
+
 	MACH_MSG_TYPE_MOVE_RECEIVE	= 0x10,
 	MACH_MSG_TYPE_MOVE_SEND		= 0x11,
 	MACH_MSG_TYPE_MOVE_SEND_ONCE	= 0x12,
diff --git a/src/pkg/runtime/mem_darwin.c b/src/pkg/runtime/mem_darwin.c
index 3af7af99ae9..cde5601cf78 100644
--- a/src/pkg/runtime/mem_darwin.c
+++ b/src/pkg/runtime/mem_darwin.c
@@ -23,9 +23,8 @@ runtime·SysAlloc(uintptr n)
 void
 runtime·SysUnused(void *v, uintptr n)
 {
-	USED(v);
-	USED(n);
-	// TODO(rsc): call madvise MADV_DONTNEED
+	// Linux's MADV_DONTNEED is like BSD's MADV_FREE.
+	runtime·madvise(v, n, MADV_FREE);
 }
 
 void
diff --git a/src/pkg/runtime/sys_darwin_386.s b/src/pkg/runtime/sys_darwin_386.s
index 6ce1ba5533c..b358509b191 100644
--- a/src/pkg/runtime/sys_darwin_386.s
+++ b/src/pkg/runtime/sys_darwin_386.s
@@ -48,6 +48,13 @@ TEXT runtime·mmap(SB),7,$0
 	INT	$0x80
 	RET
 
+TEXT runtime·madvise(SB),7,$0
+	MOVL	$75, AX
+	INT	$0x80
+	JAE	2(PC)
+	CALL	runtime·notok(SB)
+	RET
+
 TEXT runtime·munmap(SB),7,$0
 	MOVL	$73, AX
 	INT	$0x80
diff --git a/src/pkg/runtime/sys_darwin_amd64.s b/src/pkg/runtime/sys_darwin_amd64.s
index e0dbdb7c169..095ba2fa532 100644
--- a/src/pkg/runtime/sys_darwin_amd64.s
+++ b/src/pkg/runtime/sys_darwin_amd64.s
@@ -55,6 +55,16 @@ TEXT runtime·setitimer(SB), 7, $0
 	SYSCALL
 	RET
 
+TEXT runtime·madvise(SB), 7, $0
+	MOVQ	8(SP), DI		// arg 1 addr
+	MOVQ	16(SP), SI		// arg 2 len
+	MOVL	24(SP), DX		// arg 3 advice
+	MOVL	$(0x2000000+75), AX	// syscall entry madvise
+	SYSCALL
+	JCC	2(PC)
+	CALL	runtime·notok(SB)
+	RET
+
 // func now() (sec int64, nsec int32)
 TEXT time·now(SB), 7, $32
 	MOVQ	SP, DI	// must be non-nil, unused