diff --git a/src/pkg/runtime/malloc.h b/src/pkg/runtime/malloc.h
index 20cf6fb96b0..36166543ee3 100644
--- a/src/pkg/runtime/malloc.h
+++ b/src/pkg/runtime/malloc.h
@@ -157,8 +157,9 @@ struct MLink
 //
 // SysUnused notifies the operating system that the contents
 // of the memory region are no longer needed and can be reused
-// for other purposes.  The program reserves the right to start
-// accessing those pages in the future.
+// for other purposes.
+// SysUsed notifies the operating system that the contents
+// of the memory region are needed again.
 //
 // SysFree returns it unconditionally; this is only used if
 // an out-of-memory error has been detected midway through
@@ -174,6 +175,7 @@ struct MLink
 void*	runtime·SysAlloc(uintptr nbytes);
 void	runtime·SysFree(void *v, uintptr nbytes);
 void	runtime·SysUnused(void *v, uintptr nbytes);
+void	runtime·SysUsed(void *v, uintptr nbytes);
 void	runtime·SysMap(void *v, uintptr nbytes);
 void*	runtime·SysReserve(void *v, uintptr nbytes);
 
diff --git a/src/pkg/runtime/mem_darwin.c b/src/pkg/runtime/mem_darwin.c
index 7aa607f8ee7..ef5674e3408 100644
--- a/src/pkg/runtime/mem_darwin.c
+++ b/src/pkg/runtime/mem_darwin.c
@@ -27,6 +27,13 @@ runtime·SysUnused(void *v, uintptr n)
 	runtime·madvise(v, n, MADV_FREE);
 }
 
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	USED(v);
+	USED(n);
+}
+
 void
 runtime·SysFree(void *v, uintptr n)
 {
diff --git a/src/pkg/runtime/mem_freebsd.c b/src/pkg/runtime/mem_freebsd.c
index 805e74cffba..e47ea7a436d 100644
--- a/src/pkg/runtime/mem_freebsd.c
+++ b/src/pkg/runtime/mem_freebsd.c
@@ -31,6 +31,13 @@ runtime·SysUnused(void *v, uintptr n)
 	runtime·madvise(v, n, MADV_FREE);
 }
 
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	USED(v);
+	USED(n);
+}
+
 void
 runtime·SysFree(void *v, uintptr n)
 {
diff --git a/src/pkg/runtime/mem_linux.c b/src/pkg/runtime/mem_linux.c
index bacd568d9e0..d96eb69e899 100644
--- a/src/pkg/runtime/mem_linux.c
+++ b/src/pkg/runtime/mem_linux.c
@@ -77,6 +77,13 @@ runtime·SysUnused(void *v, uintptr n)
 	runtime·madvise(v, n, MADV_DONTNEED);
 }
 
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	USED(v);
+	USED(n);
+}
+
 void
 runtime·SysFree(void *v, uintptr n)
 {
diff --git a/src/pkg/runtime/mem_netbsd.c b/src/pkg/runtime/mem_netbsd.c
index e5bdac0ef65..8a7ef17e84f 100644
--- a/src/pkg/runtime/mem_netbsd.c
+++ b/src/pkg/runtime/mem_netbsd.c
@@ -31,6 +31,13 @@ runtime·SysUnused(void *v, uintptr n)
 	runtime·madvise(v, n, MADV_FREE);
 }
 
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	USED(v);
+	USED(n);
+}
+
 void
 runtime·SysFree(void *v, uintptr n)
 {
diff --git a/src/pkg/runtime/mem_openbsd.c b/src/pkg/runtime/mem_openbsd.c
index e5bdac0ef65..8a7ef17e84f 100644
--- a/src/pkg/runtime/mem_openbsd.c
+++ b/src/pkg/runtime/mem_openbsd.c
@@ -31,6 +31,13 @@ runtime·SysUnused(void *v, uintptr n)
 	runtime·madvise(v, n, MADV_FREE);
 }
 
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	USED(v);
+	USED(n);
+}
+
 void
 runtime·SysFree(void *v, uintptr n)
 {
diff --git a/src/pkg/runtime/mem_plan9.c b/src/pkg/runtime/mem_plan9.c
index 26ca367f11e..3aa16eb3a63 100644
--- a/src/pkg/runtime/mem_plan9.c
+++ b/src/pkg/runtime/mem_plan9.c
@@ -55,6 +55,12 @@ runtime·SysUnused(void *v, uintptr nbytes)
 	USED(v, nbytes);
 }
 
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	USED(v, nbytes);
+}
+
 void
 runtime·SysMap(void *v, uintptr nbytes)
 {
diff --git a/src/pkg/runtime/mem_windows.c b/src/pkg/runtime/mem_windows.c
index 1a778b77527..0a1ea38d567 100644
--- a/src/pkg/runtime/mem_windows.c
+++ b/src/pkg/runtime/mem_windows.c
@@ -11,6 +11,7 @@
 enum {
 	MEM_COMMIT = 0x1000,
 	MEM_RESERVE = 0x2000,
+	MEM_DECOMMIT = 0x4000,
 	MEM_RELEASE = 0x8000,
 	
 	PAGE_READWRITE = 0x0004,
@@ -31,8 +32,21 @@ runtime·SysAlloc(uintptr n)
 void
 runtime·SysUnused(void *v, uintptr n)
 {
-	USED(v);
-	USED(n);
+	uintptr r;
+
+	r = runtime·stdcall(runtime·VirtualFree, 3, v, n, (uintptr)MEM_DECOMMIT);
+	if(r == 0)
+		runtime·throw("runtime: failed to decommit pages");
+}
+
+void
+runtime·SysUsed(void *v, uintptr n)
+{
+	uintptr r;
+
+	r = runtime·stdcall(runtime·VirtualAlloc, 4, v, n, (uintptr)MEM_COMMIT, (uintptr)PAGE_READWRITE);
+	if(r != v)
+		runtime·throw("runtime: failed to commit pages");
 }
 
 void
diff --git a/src/pkg/runtime/mheap.c b/src/pkg/runtime/mheap.c
index c03b13bdfd8..d7713965cfc 100644
--- a/src/pkg/runtime/mheap.c
+++ b/src/pkg/runtime/mheap.c
@@ -156,6 +156,7 @@ HaveSpan:
 		// is just a unique constant not seen elsewhere in the
 		// runtime, as a clue in case it turns up unexpectedly in
 		// memory or in a stack trace.
+		runtime·SysUsed((void*)(s->start<<PageShift), s->npages<<PageShift);
 		*(uintptr*)(s->start<<PageShift) = (uintptr)0xbeadbeadbeadbeadULL;
 	}
 	s->npreleased = 0;
@@ -350,8 +351,10 @@ MHeap_FreeLocked(MHeap *h, MSpan *s)
 	if(sizeof(void*) == 8)
 		p -= (uintptr)h->arena_start >> PageShift;
 	if(p > 0 && (t = h->spans[p-1]) != nil && t->state != MSpanInUse) {
-		tp = (uintptr*)(t->start<<PageShift);
-		*tp |= *sp;	// propagate "needs zeroing" mark
+		if(t->npreleased == 0) {  // cant't touch this otherwise
+			tp = (uintptr*)(t->start<<PageShift);
+			*tp |= *sp;	// propagate "needs zeroing" mark
+		}
 		s->start = t->start;
 		s->npages += t->npages;
 		s->npreleased = t->npreleased; // absorb released pages
@@ -364,8 +367,10 @@ MHeap_FreeLocked(MHeap *h, MSpan *s)
 		mstats.mspan_sys = h->spanalloc.sys;
 	}
 	if((p+s->npages)*sizeof(h->spans[0]) < h->spans_mapped && (t = h->spans[p+s->npages]) != nil && t->state != MSpanInUse) {
-		tp = (uintptr*)(t->start<<PageShift);
-		*sp |= *tp;	// propagate "needs zeroing" mark
+		if(t->npreleased == 0) {  // cant't touch this otherwise
+			tp = (uintptr*)(t->start<<PageShift);
+			*sp |= *tp;	// propagate "needs zeroing" mark
+		}
 		s->npages += t->npages;
 		s->npreleased += t->npreleased;
 		h->spans[p + s->npages - 1] = s;
@@ -401,7 +406,7 @@ scavengelist(MSpan *list, uint64 now, uint64 limit)
 
 	sumreleased = 0;
 	for(s=list->next; s != list; s=s->next) {
-		if((now - s->unusedsince) > limit) {
+		if((now - s->unusedsince) > limit && s->npreleased != s->npages) {
 			released = (s->npages - s->npreleased) << PageShift;
 			mstats.heap_released += released;
 			sumreleased += released;