diff --git a/src/pkg/runtime/defs_freebsd.go b/src/pkg/runtime/defs_freebsd.go
index 306e32197e..0302527936 100644
--- a/src/pkg/runtime/defs_freebsd.go
+++ b/src/pkg/runtime/defs_freebsd.go
@@ -7,8 +7,9 @@
 /*
 Input to cgo.
 
-GOARCH=amd64 cgo -cdefs defs.go >amd64/defs.h
-GOARCH=386 cgo -cdefs defs.go >386/defs.h
+GOARCH=amd64 go tool cgo -cdefs defs_freebsd.go >defs_freebsd_amd64.h
+GOARCH=386 go tool cgo -cdefs defs_freebsd.go >defs_freebsd_386.h
+GOARCH=arm go tool cgo -cdefs defs_freebsd.go >defs_freebsd_arm.h
 */
 
 package runtime
@@ -38,6 +39,8 @@ const (
 	MAP_PRIVATE = C.MAP_PRIVATE
 	MAP_FIXED   = C.MAP_FIXED
 
+	MADV_FREE = C.MADV_FREE
+
 	SA_SIGINFO = C.SA_SIGINFO
 	SA_RESTART = C.SA_RESTART
 	SA_ONSTACK = C.SA_ONSTACK
diff --git a/src/pkg/runtime/defs_freebsd_386.h b/src/pkg/runtime/defs_freebsd_386.h
index 29fcb8b576..eb8c7f629f 100644
--- a/src/pkg/runtime/defs_freebsd_386.h
+++ b/src/pkg/runtime/defs_freebsd_386.h
@@ -11,6 +11,7 @@ enum {
 	MAP_ANON = 0x1000,
 	MAP_PRIVATE = 0x2,
 	MAP_FIXED = 0x10,
+	MADV_FREE = 0x5,
 	SA_SIGINFO = 0x40,
 	SA_RESTART = 0x2,
 	SA_ONSTACK = 0x1,
diff --git a/src/pkg/runtime/defs_freebsd_amd64.h b/src/pkg/runtime/defs_freebsd_amd64.h
index 8a222dca4f..a933a9c024 100644
--- a/src/pkg/runtime/defs_freebsd_amd64.h
+++ b/src/pkg/runtime/defs_freebsd_amd64.h
@@ -11,6 +11,7 @@ enum {
 	MAP_ANON = 0x1000,
 	MAP_PRIVATE = 0x2,
 	MAP_FIXED = 0x10,
+	MADV_FREE = 0x5,
 	SA_SIGINFO = 0x40,
 	SA_RESTART = 0x2,
 	SA_ONSTACK = 0x1,
diff --git a/src/pkg/runtime/defs_freebsd_arm.h b/src/pkg/runtime/defs_freebsd_arm.h
index 351259eee9..995129f291 100644
--- a/src/pkg/runtime/defs_freebsd_arm.h
+++ b/src/pkg/runtime/defs_freebsd_arm.h
@@ -11,6 +11,7 @@ enum {
 	MAP_ANON = 0x1000,
 	MAP_PRIVATE = 0x2,
 	MAP_FIXED = 0x10,
+	MADV_FREE = 0x5,
 	SA_SIGINFO = 0x40,
 	SA_RESTART = 0x2,
 	SA_ONSTACK = 0x1,
diff --git a/src/pkg/runtime/mem_freebsd.c b/src/pkg/runtime/mem_freebsd.c
index d1c22583dc..cc31711f8e 100644
--- a/src/pkg/runtime/mem_freebsd.c
+++ b/src/pkg/runtime/mem_freebsd.c
@@ -23,9 +23,7 @@ runtime·SysAlloc(uintptr n)
 void
 runtime·SysUnused(void *v, uintptr n)
 {
-	USED(v);
-	USED(n);
-	// TODO(rsc): call madvise MADV_DONTNEED
+	runtime·madvise(v, n, MADV_FREE);
 }
 
 void
diff --git a/src/pkg/runtime/sys_freebsd_386.s b/src/pkg/runtime/sys_freebsd_386.s
index c386268ff5..9049f1e75b 100644
--- a/src/pkg/runtime/sys_freebsd_386.s
+++ b/src/pkg/runtime/sys_freebsd_386.s
@@ -102,6 +102,13 @@ TEXT runtime·munmap(SB),7,$-4
 	MOVL	$0xf1, 0xf1  // crash
 	RET
 
+TEXT runtime·madvise(SB),7,$-4
+	MOVL	$75, AX	// madvise
+	INT	$0x80
+	JAE	2(PC)
+	MOVL	$0xf1, 0xf1  // crash
+	RET
+
 TEXT runtime·setitimer(SB), 7, $-4
 	MOVL	$83, AX
 	INT	$0x80
diff --git a/src/pkg/runtime/sys_freebsd_amd64.s b/src/pkg/runtime/sys_freebsd_amd64.s
index 3d25db2ce7..d66aed107c 100644
--- a/src/pkg/runtime/sys_freebsd_amd64.s
+++ b/src/pkg/runtime/sys_freebsd_amd64.s
@@ -184,6 +184,17 @@ TEXT runtime·munmap(SB),7,$0
 	MOVL	$0xf1, 0xf1  // crash
 	RET
 
+TEXT runtime·madvise(SB),7,$0
+	MOVQ	8(SP), DI
+	MOVQ	16(SP), SI
+	MOVQ	24(SP), DX
+	MOVQ	$75, AX	// madvise
+	SYSCALL
+	CMPQ	AX, $0xfffffffffffff001
+	JLS	2(PC)
+	MOVL	$0xf1, 0xf1  // crash
+	RET
+	
 TEXT runtime·sigaltstack(SB),7,$-8
 	MOVQ	new+8(SP), DI
 	MOVQ	old+16(SP), SI
diff --git a/src/pkg/runtime/sys_freebsd_arm.s b/src/pkg/runtime/sys_freebsd_arm.s
index 097fcd0457..1edf8071bb 100644
--- a/src/pkg/runtime/sys_freebsd_arm.s
+++ b/src/pkg/runtime/sys_freebsd_arm.s
@@ -187,6 +187,15 @@ TEXT runtime·munmap(SB),7,$0
 	MOVW.CS R9, (R9)
 	RET
 
+TEXT runtime·madvise(SB),7,$0
+	MOVW 0(FP), R0		// arg 1 addr
+	MOVW 4(FP), R1		// arg 2 len
+	MOVW 8(FP), R2		// arg 3 flags
+	SWI $75
+	MOVW.CS $0, R9 // crash on syscall failure
+	MOVW.CS R9, (R9)
+	RET
+	
 TEXT runtime·sigaltstack(SB),7,$-8
 	MOVW new+0(FP), R0
 	MOVW old+4(FP), R1