diff --git a/collects/tests/mzscheme/sandbox.ss b/collects/tests/mzscheme/sandbox.ss
index 92b5a427f2..73db336351 100644
--- a/collects/tests/mzscheme/sandbox.ss
+++ b/collects/tests/mzscheme/sandbox.ss
@@ -152,7 +152,7 @@
                    (make-evaluator 'scheme/base
                                    '(define a (for/list ([i (in-range 10)])
                                                 (collect-garbage)
-                                                (make-string 1000))))))
+                                                (make-string 500000))))))
         =err> "out of memory"))
 
    ;; i/o
@@ -488,9 +488,13 @@
         --eval--
         (define a '())
         (define b 1)
-        (for ([i (in-range 20)])
-          (set! a (cons (make-bytes 500000) a))
-          (collect-garbage))
+        (length
+         (for/fold ([v null]) ([i (in-range 20)])
+           ;; Increases size of sandbox:
+           (set! a (cons (make-bytes 500000) a))
+           (collect-garbage)
+           ;; Increases size of evaluation:
+           (cons (make-bytes 500000) v)))
         =err> "out of memory"
         b => 1))
 
diff --git a/src/mzscheme/gc2/gc2.h b/src/mzscheme/gc2/gc2.h
index 04a5be7cbd..a2c2a43a7a 100644
--- a/src/mzscheme/gc2/gc2.h
+++ b/src/mzscheme/gc2/gc2.h
@@ -371,6 +371,13 @@ GC2_EXTERN void GC_fixup_variable_stack(void **var_stack,
    The `stack_mem' argument indicates the start of the allocated memory
    that contains `var_stack'. It is used for backtraces. */
 
+GC2_EXTERN int GC_merely_accounting();
+/*
+   Can be called by a mark or fixup traversal proc to determine whether
+   the traversal is merely for accounting, in which case some marking
+   can be skipped if the corresponding data should be charged to a
+   different object. */
+
 GC2_EXTERN void GC_write_barrier(void *p);
 /* 
    Explicit write barrier to ensure that a write-barrier signal is not
diff --git a/src/mzscheme/gc2/newgc.c b/src/mzscheme/gc2/newgc.c
index 479cb70c87..71c477963a 100644
--- a/src/mzscheme/gc2/newgc.c
+++ b/src/mzscheme/gc2/newgc.c
@@ -1404,6 +1404,12 @@ void GC_register_new_thread(void *t, void *c)
 #endif
 }
 
+int GC_merely_accounting()
+{
+  NewGC *gc = GC_get_GC();
+  return gc->doing_memory_accounting;
+}
+
 /*****************************************************************************/
 /* administration / initialization                                           */
 /*****************************************************************************/
diff --git a/src/mzscheme/src/mzmark.c b/src/mzscheme/src/mzmark.c
index d9d1ed3e61..36ae988ed3 100644
--- a/src/mzscheme/src/mzmark.c
+++ b/src/mzscheme/src/mzmark.c
@@ -923,8 +923,10 @@ static int cont_proc_MARK(void *p) {
   MARK_cjs(&c->cjs);
   MARK_stack_state(&c->ss);
   gcMARK(c->barrier_prompt);
-  gcMARK(c->runstack_start);
-  gcMARK(c->runstack_saved);
+  if (!GC_merely_accounting()) {
+    gcMARK(c->runstack_start);
+    gcMARK(c->runstack_saved);
+  }
 
   gcMARK(c->prompt_id);
   gcMARK(c->prompt_buf);
@@ -961,8 +963,10 @@ static int cont_proc_FIXUP(void *p) {
   FIXUP_cjs(&c->cjs);
   FIXUP_stack_state(&c->ss);
   gcFIXUP(c->barrier_prompt);
-  gcFIXUP(c->runstack_start);
-  gcFIXUP(c->runstack_saved);
+  if (!GC_merely_accounting()) {
+    gcFIXUP(c->runstack_start);
+    gcFIXUP(c->runstack_saved);
+  }
 
   gcFIXUP(c->prompt_id);
   gcFIXUP(c->prompt_buf);
@@ -1600,12 +1604,16 @@ static int thread_val_MARK(void *p) {
   gcMARK(pr->init_config);
   gcMARK(pr->init_break_cell);
 
-  {
+  if (!pr->runstack_owner
+      || !GC_merely_accounting()
+      || (*pr->runstack_owner == pr)) {
     Scheme_Object **rs = pr->runstack_start;
     gcMARK( pr->runstack_start);
-    pr->runstack = pr->runstack_start + (pr->runstack - rs);
+    if (pr->runstack != pr->runstack_start + (pr->runstack - rs))
+      pr->runstack = pr->runstack_start + (pr->runstack - rs);
+
+    gcMARK(pr->runstack_saved);
   }
-  gcMARK(pr->runstack_saved);
   gcMARK(pr->runstack_owner);
   gcMARK(pr->runstack_swapped);
   pr->spare_runstack = NULL; /* just in case */
@@ -1706,12 +1714,16 @@ static int thread_val_FIXUP(void *p) {
   gcFIXUP(pr->init_config);
   gcFIXUP(pr->init_break_cell);
 
-  {
+  if (!pr->runstack_owner
+      || !GC_merely_accounting()
+      || (*pr->runstack_owner == pr)) {
     Scheme_Object **rs = pr->runstack_start;
     gcFIXUP_TYPED_NOW(Scheme_Object **, pr->runstack_start);
-    pr->runstack = pr->runstack_start + (pr->runstack - rs);
+    if (pr->runstack != pr->runstack_start + (pr->runstack - rs))
+      pr->runstack = pr->runstack_start + (pr->runstack - rs);
+
+    gcFIXUP(pr->runstack_saved);
   }
-  gcFIXUP(pr->runstack_saved);
   gcFIXUP(pr->runstack_owner);
   gcFIXUP(pr->runstack_swapped);
   pr->spare_runstack = NULL; /* just in case */
@@ -1858,7 +1870,8 @@ static int prompt_val_SIZE(void *p) {
 static int prompt_val_MARK(void *p) {
   Scheme_Prompt *pr = (Scheme_Prompt *)p;
   gcMARK(pr->boundary_overflow_id);
-  gcMARK(pr->runstack_boundary_start);
+  if (!GC_merely_accounting())
+    gcMARK(pr->runstack_boundary_start);
   gcMARK(pr->tag);
   gcMARK(pr->id);
   return
@@ -1868,7 +1881,8 @@ static int prompt_val_MARK(void *p) {
 static int prompt_val_FIXUP(void *p) {
   Scheme_Prompt *pr = (Scheme_Prompt *)p;
   gcFIXUP(pr->boundary_overflow_id);
-  gcFIXUP(pr->runstack_boundary_start);
+  if (!GC_merely_accounting())
+    gcFIXUP(pr->runstack_boundary_start);
   gcFIXUP(pr->tag);
   gcFIXUP(pr->id);
   return
diff --git a/src/mzscheme/src/mzmarksrc.c b/src/mzscheme/src/mzmarksrc.c
index b398e8113f..4ac017da7b 100644
--- a/src/mzscheme/src/mzmarksrc.c
+++ b/src/mzscheme/src/mzmarksrc.c
@@ -355,8 +355,10 @@ cont_proc {
   MARK_cjs(&c->cjs);
   MARK_stack_state(&c->ss);
   gcMARK(c->barrier_prompt);
-  gcMARK(c->runstack_start);
-  gcMARK(c->runstack_saved);
+  if (!GC_merely_accounting()) {
+    gcMARK(c->runstack_start);
+    gcMARK(c->runstack_saved);
+  }
 
   gcMARK(c->prompt_id);
   gcMARK(c->prompt_buf);
@@ -615,12 +617,16 @@ thread_val {
   gcMARK(pr->init_config);
   gcMARK(pr->init_break_cell);
 
-  {
+  if (!pr->runstack_owner
+      || !GC_merely_accounting()
+      || (*pr->runstack_owner == pr)) {
     Scheme_Object **rs = pr->runstack_start;
     gcFIXUP_TYPED_NOW(Scheme_Object **, pr->runstack_start);
-    pr->runstack = pr->runstack_start + (pr->runstack - rs);
+    if (pr->runstack != pr->runstack_start + (pr->runstack - rs))
+      pr->runstack = pr->runstack_start + (pr->runstack - rs);
+
+    gcMARK(pr->runstack_saved);
   }
-  gcMARK(pr->runstack_saved);
   gcMARK(pr->runstack_owner);
   gcMARK(pr->runstack_swapped);
   pr->spare_runstack = NULL; /* just in case */
@@ -738,7 +744,8 @@ prompt_val {
  mark: 
   Scheme_Prompt *pr = (Scheme_Prompt *)p;
   gcMARK(pr->boundary_overflow_id);
-  gcMARK(pr->runstack_boundary_start);
+  if (!GC_merely_accounting())
+    gcMARK(pr->runstack_boundary_start);
   gcMARK(pr->tag);
   gcMARK(pr->id);
  size: