diff --git a/src/racket/include/scheme.h b/src/racket/include/scheme.h
index d85cdcf841..b9d894c150 100644
--- a/src/racket/include/scheme.h
+++ b/src/racket/include/scheme.h
@@ -1046,6 +1046,8 @@ typedef struct Scheme_Thread {
   Scheme_Object *dead_box;      /* contains non-zero when the thread is dead */
   Scheme_Object *running_box;   /* contains pointer to thread when it's running */
 
+  struct Scheme_Thread *gc_prep_chain;
+
   struct Scheme_Thread *nester, *nestee;
 
   struct future_t *current_ft;
diff --git a/src/racket/include/schthread.h b/src/racket/include/schthread.h
index 04488499fd..aaed2fb56a 100644
--- a/src/racket/include/schthread.h
+++ b/src/racket/include/schthread.h
@@ -230,6 +230,7 @@ typedef struct Thread_Local_Variables {
   struct Scheme_Thread *scheme_current_thread_;
   struct Scheme_Thread *scheme_main_thread_;
   struct Scheme_Thread *scheme_first_thread_;
+  struct Scheme_Thread *gc_prep_thread_chain_;
   struct Scheme_Thread_Set *scheme_thread_set_top_;
   struct Scheme_Current_LWC *scheme_current_lwc_;
   int num_running_threads_;
@@ -333,6 +334,7 @@ typedef struct Thread_Local_Variables {
   int place_evts_array_size_;
   struct Evt **place_evts_;
   struct Scheme_Place_Object *place_object_;
+  struct Scheme_Place *all_child_places_;
   struct Scheme_Object **reusable_ifs_stack_;
   struct Scheme_Object *empty_self_shift_cache_;
   struct Scheme_Bucket_Table *scheme_module_code_cache_;
@@ -575,6 +577,7 @@ XFORM_GC_VARIABLE_STACK_THROUGH_THREAD_LOCAL;
 #define scheme_current_thread XOA (scheme_get_thread_local_variables()->scheme_current_thread_)
 #define scheme_main_thread XOA (scheme_get_thread_local_variables()->scheme_main_thread_)
 #define scheme_first_thread XOA (scheme_get_thread_local_variables()->scheme_first_thread_)
+#define gc_prep_thread_chain XOA (scheme_get_thread_local_variables()->gc_prep_thread_chain_)
 #define scheme_thread_set_top XOA (scheme_get_thread_local_variables()->scheme_thread_set_top_)
 #define scheme_current_lwc XOA (scheme_get_thread_local_variables()->scheme_current_lwc_)
 #define num_running_threads XOA (scheme_get_thread_local_variables()->num_running_threads_)
@@ -678,6 +681,7 @@ XFORM_GC_VARIABLE_STACK_THROUGH_THREAD_LOCAL;
 #define place_evts_array_size XOA (scheme_get_thread_local_variables()->place_evts_array_size_)
 #define place_evts XOA (scheme_get_thread_local_variables()->place_evts_)
 #define place_object XOA (scheme_get_thread_local_variables()->place_object_)
+#define all_child_places XOA (scheme_get_thread_local_variables()->all_child_places_)
 #define reusable_ifs_stack XOA (scheme_get_thread_local_variables()->reusable_ifs_stack_)
 #define empty_self_shift_cache XOA (scheme_get_thread_local_variables()->empty_self_shift_cache_)
 #define scheme_module_code_cache XOA (scheme_get_thread_local_variables()->scheme_module_code_cache_)
diff --git a/src/racket/src/mzmark_place.inc b/src/racket/src/mzmark_place.inc
index ceda0b4001..5c0f8015ab 100644
--- a/src/racket/src/mzmark_place.inc
+++ b/src/racket/src/mzmark_place.inc
@@ -57,6 +57,8 @@ static int place_val_MARK(void *p, struct NewGC *gc) {
   gcMARK2(pr->mref, gc);
   gcMARK2(pr->pumper_threads, gc);
   gcMARK2(pr->place_obj, gc);
+  gcMARK2(pr->prev, gc);
+  gcMARK2(pr->next, gc);
 
   return
   gcBYTES_TO_WORDS(sizeof(Scheme_Place));
@@ -68,6 +70,8 @@ static int place_val_FIXUP(void *p, struct NewGC *gc) {
   gcFIXUP2(pr->mref, gc);
   gcFIXUP2(pr->pumper_threads, gc);
   gcFIXUP2(pr->place_obj, gc);
+  gcFIXUP2(pr->prev, gc);
+  gcFIXUP2(pr->next, gc);
 
   return
   gcBYTES_TO_WORDS(sizeof(Scheme_Place));
diff --git a/src/racket/src/mzmarksrc.c b/src/racket/src/mzmarksrc.c
index f50292c75e..b2b190be6a 100644
--- a/src/racket/src/mzmarksrc.c
+++ b/src/racket/src/mzmarksrc.c
@@ -1497,6 +1497,8 @@ place_val {
   gcMARK2(pr->mref, gc);
   gcMARK2(pr->pumper_threads, gc);
   gcMARK2(pr->place_obj, gc);
+  gcMARK2(pr->prev, gc);
+  gcMARK2(pr->next, gc);
 
  size:
   gcBYTES_TO_WORDS(sizeof(Scheme_Place));
diff --git a/src/racket/src/place.c b/src/racket/src/place.c
index b7c79468d6..b2bbf82a32 100644
--- a/src/racket/src/place.c
+++ b/src/racket/src/place.c
@@ -45,6 +45,7 @@ static mzrt_mutex *id_counter_mutex;
 
 SHARED_OK mz_proc_thread *scheme_master_proc_thread;
 THREAD_LOCAL_DECL(static struct Scheme_Place_Object *place_object);
+THREAD_LOCAL_DECL(static Scheme_Place *all_child_places);
 THREAD_LOCAL_DECL(static uintptr_t force_gc_for_place_accounting);
 static Scheme_Object *scheme_place(int argc, Scheme_Object *args[]);
 static Scheme_Object *place_pumper_threads(int argc, Scheme_Object *args[]);
@@ -144,6 +145,8 @@ void scheme_init_place(Scheme_Env *env)
   PLACE_PRIM_W_ARITY("place-dead-evt",        make_place_dead, 1, 1, plenv);
 
   scheme_finish_primitive_module(plenv);
+
+  REGISTER_SO(all_child_places);
 }
 
 static Scheme_Object* scheme_place_enabled(int argc, Scheme_Object *args[]) {
@@ -457,6 +460,11 @@ Scheme_Object *scheme_place(int argc, Scheme_Object *args[]) {
 
   place_data->ready = NULL;
   place_data->place_obj = NULL;
+
+  place->next = all_child_places;
+  if (place->next)
+    place->next->prev = place;
+  all_child_places = place;
   
   {
     Scheme_Custodian_Reference *mref;
@@ -528,6 +536,14 @@ static void do_place_kill(Scheme_Place *place)
   }
 
   scheme_remove_managed(place->mref, (Scheme_Object *)place);
+
+  if (place->next)
+    place->next->prev = place->prev;
+  if (place->prev)
+    place->prev->next = place->next;
+  else
+    all_child_places = place->next;
+
   if (!refcount) {
     destroy_place_object_locks(place_obj);
   }
@@ -2206,7 +2222,7 @@ static void *place_start_proc(void *data_arg) {
   return rc;
 }
 
-void scheme_pause_one_place(Scheme_Place *p)
+static void pause_one_place(Scheme_Place *p)
 {
   Scheme_Place_Object *place_obj = p->place_obj;
 
@@ -2234,7 +2250,7 @@ static void resume_one_place_with_lock(Scheme_Place_Object *place_obj)
   }
 }
 
-void scheme_resume_one_place(Scheme_Place *p)
+static void resume_one_place(Scheme_Place *p)
 {
   Scheme_Place_Object *place_obj = p->place_obj;
 
@@ -2245,6 +2261,24 @@ void scheme_resume_one_place(Scheme_Place *p)
   }
 }
 
+static void pause_all_child_places()
+{
+  Scheme_Place *p = all_child_places;
+  while (p) {
+    pause_one_place(p);
+    p = p->next;
+  }
+}
+
+static void resume_all_child_places()
+{
+  Scheme_Place *p = all_child_places;
+  while (p) {
+    resume_one_place(p);
+    p = p->next;
+  }
+}
+
 void destroy_place_object_locks(Scheme_Place_Object *place_obj) {
   mzrt_mutex_destroy(place_obj->lock);
   if (place_obj->pause)
@@ -2279,10 +2313,10 @@ void scheme_place_check_for_interruption()
     mzrt_mutex_unlock(place_obj->lock);
     
     if (local_pause) {
-      scheme_pause_all_places();
+      pause_all_child_places();
       mzrt_sema_wait(local_pause);
       mzrt_sema_destroy(local_pause);
-      scheme_resume_all_places();
+      resume_all_child_places();
     } else
       break;
   }
@@ -2311,7 +2345,7 @@ void scheme_place_set_memory_use(intptr_t mem_use)
          custodian limits that will kill this place; pause this
          place and its children to give the original place time 
          to kill this one */
-      scheme_pause_all_places();
+      pause_all_child_places();
       mzrt_ensure_max_cas(place_obj->parent_need_gc, 1);
       scheme_signal_received_at(place_obj->parent_signal_handle);
     } else if (mem_use > (1 + place_obj->use_factor) * place_obj->prev_notify_memory_use) {
@@ -2334,7 +2368,7 @@ void scheme_place_check_memory_use()
   if (force_gc_for_place_accounting) {
     force_gc_for_place_accounting = 0;
     scheme_collect_garbage();
-    scheme_resume_all_places();
+    resume_all_child_places();
   }
 }
 
@@ -2462,7 +2496,7 @@ static void *place_start_proc_after_stack(void *data_arg, void *stack_base) {
   REGISTER_SO(place_object);
   place_object = place_obj;
   place_obj->refcount++;
-  
+
   {
     void *signal_handle;
     signal_handle = scheme_get_signal_handle();
diff --git a/src/racket/src/schpriv.h b/src/racket/src/schpriv.h
index 4f3832b568..53f1ff153c 100644
--- a/src/racket/src/schpriv.h
+++ b/src/racket/src/schpriv.h
@@ -3815,6 +3815,8 @@ typedef struct Scheme_Place {
   struct GC_Thread_Info *gc_info; /* managed by the GC */
 #endif
   Scheme_Object *pumper_threads; /* Scheme_Vector of scheme threads */
+
+  struct Scheme_Place *prev, *next; /* keeping a list of child places */
 } Scheme_Place;
 
 typedef struct Scheme_Place_Object {
@@ -3882,9 +3884,4 @@ void scheme_place_set_memory_use(intptr_t amt);
 void scheme_place_check_memory_use();
 void scheme_clear_place_ifs_stack();
 
-void scheme_pause_all_places();
-void scheme_pause_one_place(Scheme_Place *p);
-void scheme_resume_all_places();
-void scheme_resume_one_place(Scheme_Place *p);
-
 #endif /* __mzscheme_private__ */
diff --git a/src/racket/src/thread.c b/src/racket/src/thread.c
index b43e90829d..dbcbcb9c96 100644
--- a/src/racket/src/thread.c
+++ b/src/racket/src/thread.c
@@ -156,6 +156,7 @@ THREAD_LOCAL_DECL(static int buffer_init_size);
 THREAD_LOCAL_DECL(Scheme_Thread *scheme_current_thread = NULL);
 THREAD_LOCAL_DECL(Scheme_Thread *scheme_main_thread = NULL);
 THREAD_LOCAL_DECL(Scheme_Thread *scheme_first_thread = NULL);
+THREAD_LOCAL_DECL(static Scheme_Thread *gc_prep_thread_chain = NULL);
 
 XFORM_NONGCING Scheme_Thread *scheme_get_current_thread() { return scheme_current_thread; }
 XFORM_NONGCING intptr_t scheme_get_multiple_count() { return scheme_current_thread->ku.multiple.count; }
@@ -292,10 +293,16 @@ typedef struct {
 # define MALLOC_MREF() (Scheme_Custodian_Reference *)scheme_make_late_weak_box(NULL)
 # define CUSTODIAN_FAM(x) ((Scheme_Custodian_Weak_Box *)x)->val
 # define xCUSTODIAN_FAM(x) SCHEME_BOX_VAL(x)
+# define SET_MREF_POSITION(mref, i) (((Scheme_Custodian_Weak_Box *)mref)->hash_key = (i & 0xFFFF))
+# define EXTRACT_MREF_START_POSITION(mref, c) (((Scheme_Custodian_Weak_Box *)mref)->hash_key | ((c) & ~0xFFFF))
+# define EXTRACT_MREF_POSITION_DELTA(mref, c) 0x10000
 #else
 # define MALLOC_MREF() MALLOC_ONE_WEAK(Scheme_Custodian_Reference)
 # define CUSTODIAN_FAM(x) (*(x))
 # define xCUSTODIAN_FAM(x) (*(x))
+# define SET_MREF_POSITION(mref, i) /* empty */
+# define EXTRACT_MREF_START_POSITION(mref, c) ((c)-1)
+# define EXTRACT_MREF_POSITION_DELTA(mref, c) 1
 #endif
 
 typedef struct Proc_Global_Rec {
@@ -891,7 +898,7 @@ static void add_managed_box(Scheme_Custodian *m,
 			    Scheme_Object **box, Scheme_Custodian_Reference *mref,
 			    Scheme_Close_Custodian_Client *f, void *data)
 {
-  int i;
+  int i, saw = 0;
 
   for (i = m->count; i--; ) {
     if (!m->boxes[i]) {
@@ -899,11 +906,16 @@ static void add_managed_box(Scheme_Custodian *m,
       m->closers[i] = f;
       m->data[i] = data;
       m->mrefs[i] = mref;
+      SET_MREF_POSITION(mref, i);
 
       m->elems++;
       adjust_limit_table(m);
 
       return;
+    } else {
+      saw++;
+      if (i + saw == m->elems)
+        break; /* no empty spaces left */
     }
   }
 
@@ -913,6 +925,7 @@ static void add_managed_box(Scheme_Custodian *m,
   m->closers[m->count] = f;
   m->data[m->count] = data;
   m->mrefs[m->count] = mref;
+  SET_MREF_POSITION(mref, m->count);
 
   m->elems++;
   adjust_limit_table(m);
@@ -924,7 +937,7 @@ static void remove_managed(Scheme_Custodian_Reference *mr, Scheme_Object *o,
 			   Scheme_Close_Custodian_Client **old_f, void **old_data)
 {
   Scheme_Custodian *m;
-  int i;
+  int i, delta;
 
   if (!mr)
     return;
@@ -932,21 +945,27 @@ static void remove_managed(Scheme_Custodian_Reference *mr, Scheme_Object *o,
   if (!m)
     return;
 
-  for (i = m->count; i--; ) {
-    if (m->boxes[i] && SAME_OBJ((xCUSTODIAN_FAM(m->boxes[i])),  o)) {
-      xCUSTODIAN_FAM(m->boxes[i]) = 0;
-      m->boxes[i] = NULL;
-      CUSTODIAN_FAM(m->mrefs[i]) = 0;
-      m->mrefs[i] = NULL;
-      if (old_f)
-	*old_f = m->closers[i];
-      if (old_data)
-	*old_data = m->data[i];
-      m->data[i] = NULL;
-      --m->elems;
-      adjust_limit_table(m);
-      break;
+  i = EXTRACT_MREF_START_POSITION(mr, m->count);
+  delta = EXTRACT_MREF_POSITION_DELTA(mr, m->count);
+
+  while (i >= 0) {
+    if (i < m->count) {
+      if (m->boxes[i] && SAME_OBJ((xCUSTODIAN_FAM(m->boxes[i])),  o)) {
+        xCUSTODIAN_FAM(m->boxes[i]) = 0;
+        m->boxes[i] = NULL;
+        CUSTODIAN_FAM(m->mrefs[i]) = 0;
+        m->mrefs[i] = NULL;
+        if (old_f)
+          *old_f = m->closers[i];
+        if (old_data)
+          *old_data = m->data[i];
+        m->data[i] = NULL;
+        --m->elems;
+        adjust_limit_table(m);
+        break;
+      }
     }
+    i -= delta;
   }
 
   while (m->count && !m->boxes[m->count - 1]) {
@@ -1265,11 +1284,12 @@ Scheme_Thread *scheme_do_close_managed(Scheme_Custodian *m, Scheme_Exit_Closer_F
 	CUSTODIAN_FAM(m->mrefs[i]) = NULL;
 	
 	/* Set m->count to i in case a GC happens while
-	   the closer is running. If there's a GC, then
-	   for_each_managed will be called. */
+	   the closer is running. */
 	m->count = i;
 
-	if (is_thread && !the_thread) {
+        if (!o) {
+          /* weak link disappeared */
+        } else if (is_thread && !the_thread) {
 	  /* Thread is already collected, so skip */
 	} else if (cf) {
 	  cf(o, f, data);
@@ -1357,53 +1377,6 @@ Scheme_Thread *scheme_do_close_managed(Scheme_Custodian *m, Scheme_Exit_Closer_F
   return kill_self;
 }
 
-typedef void (*Scheme_For_Each_Func)(Scheme_Object *);
-
-static void for_each_managed(Scheme_Type type, Scheme_For_Each_Func cf)
-  XFORM_SKIP_PROC
-/* This function must not allocate. */
-{
-  Scheme_Custodian *m;
-  int i;
-
-  if (SAME_TYPE(type, scheme_thread_type))
-    type = scheme_thread_hop_type;
-
-  /* back to front so children are first: */
-  m = last_custodian;
-
-  while (m) {
-    for (i = m->count; i--; ) {
-      if (m->boxes[i]) {
-	Scheme_Object *o;
-
-	o = xCUSTODIAN_FAM(m->boxes[i]);
-      
-	if (SAME_TYPE(SCHEME_TYPE(o), type)) {
-	  if (SAME_TYPE(type, scheme_thread_hop_type)) {
-	    /* We've added an indirection and made it weak. See mr_hop note above. */
-	    Scheme_Thread *t;
-	    t = (Scheme_Thread *)WEAKIFIED(((Scheme_Thread_Custodian_Hop *)o)->p);
-	    if (!t) {
-	      /* The thread is already collected */
-	      continue;
-	    } else if (SAME_OBJ(t->mref, m->mrefs[i]))
-	      o = (Scheme_Object *)t;
-	    else {
-	      /* The main custodian for this thread is someone else */
-	      continue;
-	    }
-	  }
-
-	  cf(o);
-	}
-      }
-    }
-
-    m = CUSTODIAN_FAM(m->global_prev);
-  }
-}
-
 static void do_close_managed(Scheme_Custodian *m)
 /* The trick is that we may need to kill the thread
    that is running us. If so, delay it to the very
@@ -1483,30 +1456,6 @@ static Scheme_Object *extract_thread(Scheme_Object *o)
   return (Scheme_Object *)WEAKIFIED(((Scheme_Thread_Custodian_Hop *)o)->p);
 }
 
-static void pause_place(Scheme_Object *o)
-{
-#ifdef MZ_USE_PLACES
-  scheme_pause_one_place((Scheme_Place *)o);
-#endif
-}
-
-void scheme_pause_all_places()
-{
-  for_each_managed(scheme_place_type, pause_place);
-}
-
-static void resume_place(Scheme_Object *o)
-{
-#ifdef MZ_USE_PLACES
-  scheme_resume_one_place((Scheme_Place *)o);
-#endif
-}
-
-void scheme_resume_all_places()
-{
-  for_each_managed(scheme_place_type, resume_place);
-}
-
 void scheme_init_custodian_extractors()
 {
   if (!extractors) {
@@ -2053,6 +2002,9 @@ static Scheme_Thread *make_thread(Scheme_Config *config,
     process->prev = NULL;
     process->next = NULL;
 
+    gc_prep_thread_chain = process;
+    scheme_current_thread->gc_prep_chain = process;
+
     process->suspend_break = 1; /* until start-up finished */
 
     process->error_buf = NULL;
@@ -2444,7 +2396,6 @@ static void do_swap_thread()
   }
 #endif
 
-
   if (!swap_no_setjmp && SETJMP(scheme_current_thread)) {
     /* We're back! */
     /* See also initial swap in in start_child() */
@@ -2537,6 +2488,10 @@ static void do_swap_thread()
 #endif
 
     scheme_current_thread = new_thread;
+    if (!new_thread->gc_prep_chain) {
+      new_thread->gc_prep_chain = gc_prep_thread_chain;
+      gc_prep_thread_chain = new_thread;
+    }
 
     /* Fixup current pointers in thread sets */
     if (!scheme_current_thread->return_marks_to) {
@@ -3240,6 +3195,9 @@ Scheme_Object *scheme_call_as_nested_thread(int argc, Scheme_Object *argv[], voi
   scheme_first_thread->prev = np;
   scheme_first_thread = np;
 
+  np->gc_prep_chain = gc_prep_thread_chain;
+  gc_prep_thread_chain = np;
+
   np->t_set_parent = p->t_set_parent;
   schedule_in_set((Scheme_Object *)np, np->t_set_parent);
 
@@ -8095,6 +8053,8 @@ static void prepare_thread_for_GC(Scheme_Object *t)
 {
   Scheme_Thread *p = (Scheme_Thread *)t;
 
+  if (!p->running) return;
+
   /* zero ununsed part of env stack in each thread */
 
   if (!p->nestee) {
@@ -8267,7 +8227,20 @@ static void get_ready_for_GC()
   }
 #endif
 
-  for_each_managed(scheme_thread_type, prepare_thread_for_GC);
+  /* Prepare each thread that has run: */
+  if (gc_prep_thread_chain) {
+    Scheme_Thread *p, *next;
+    p = gc_prep_thread_chain;
+    while (p != p->gc_prep_chain) {
+      prepare_thread_for_GC((Scheme_Object *)p);
+      next = p->gc_prep_chain;
+      p->gc_prep_chain = NULL;
+      p = next;
+    }
+    prepare_thread_for_GC((Scheme_Object *)p);
+    p->gc_prep_chain = NULL;
+    gc_prep_thread_chain = NULL;
+  }
 
 #ifdef MZ_PRECISE_GC
   scheme_flush_stack_copy_cache();
@@ -8318,6 +8291,9 @@ static void done_with_GC()
   end_this_gc_real_time = scheme_get_inexact_milliseconds();
   scheme_total_gc_time += (end_this_gc_time - start_this_gc_time);
 
+  gc_prep_thread_chain = scheme_current_thread;
+  scheme_current_thread->gc_prep_chain = scheme_current_thread;
+
   run_gc_callbacks(0);
 
 #ifdef MZ_USE_FUTURES