diff --git a/bdw-attrs.h b/bdw-attrs.h
index 960e543b0..e7a08100d 100644
--- a/bdw-attrs.h
+++ b/bdw-attrs.h
@@ -40,13 +40,13 @@ static inline int gc_allocator_needs_clear(void) {
   return 0;
 }
 
-static inline enum gc_write_barrier_kind gc_small_write_barrier_kind(void) {
+static inline enum gc_write_barrier_kind gc_write_barrier_kind(size_t) {
   return GC_WRITE_BARRIER_NONE;
 }
-static inline size_t gc_small_write_barrier_card_table_alignment(void) {
+static inline size_t gc_write_barrier_card_table_alignment(void) {
   GC_CRASH();
 }
-static inline size_t gc_small_write_barrier_card_size(void) {
+static inline size_t gc_write_barrier_card_size(void) {
   GC_CRASH();
 }
 
diff --git a/bdw.c b/bdw.c
index 29958f133..cf17f19e7 100644
--- a/bdw.c
+++ b/bdw.c
@@ -115,6 +115,10 @@ void gc_collect(struct gc_mutator *mut) {
   GC_gcollect();
 }
 
+void gc_write_barrier_extern(struct gc_ref obj, size_t obj_size,
+                             struct gc_edge edge, struct gc_ref new_val) {
+}
+
 // In BDW-GC, we can't hook into the mark phase to call
 // gc_trace_ephemerons_for_object, so the advertised ephemeron strategy
 // doesn't really work.  The primitives that we have are mark functions,
diff --git a/gc-api.h b/gc-api.h
index 5f0d3c1aa..6cf783703 100644
--- a/gc-api.h
+++ b/gc-api.h
@@ -177,18 +177,30 @@ static inline void gc_small_write_barrier(struct gc_ref obj, struct gc_edge edge
                                           struct gc_ref new_val) GC_ALWAYS_INLINE;
 static inline void gc_small_write_barrier(struct gc_ref obj, struct gc_edge edge,
                                           struct gc_ref new_val) {
-  switch (gc_small_write_barrier_kind()) {
+}
+
+GC_API_ void gc_write_barrier_extern(struct gc_ref obj, size_t obj_size,
+                                     struct gc_edge edge, struct gc_ref new_val) GC_NEVER_INLINE;
+
+static inline void gc_write_barrier(struct gc_ref obj, size_t obj_size,
+                                    struct gc_edge edge, struct gc_ref new_val) GC_ALWAYS_INLINE;
+static inline void gc_write_barrier(struct gc_ref obj, size_t obj_size,
+                                    struct gc_edge edge, struct gc_ref new_val) {
+  switch (gc_write_barrier_kind(obj_size)) {
   case GC_WRITE_BARRIER_NONE:
     return;
   case GC_WRITE_BARRIER_CARD: {
-    size_t card_table_alignment = gc_small_write_barrier_card_table_alignment();
-    size_t card_size = gc_small_write_barrier_card_size();
+    size_t card_table_alignment = gc_write_barrier_card_table_alignment();
+    size_t card_size = gc_write_barrier_card_size();
     uintptr_t addr = gc_ref_value(obj);
     uintptr_t base = addr & ~(card_table_alignment - 1);
     uintptr_t card = (addr & (card_table_alignment - 1)) / card_size;
     atomic_store_explicit((uint8_t*)(base + card), 1, memory_order_relaxed);
     return;
   }
+  case GC_WRITE_BARRIER_EXTERN:
+    gc_write_barrier_extern(obj, obj_size, edge, new_val);
+    return;
   default:
     GC_CRASH();
   }
diff --git a/gc-attrs.h b/gc-attrs.h
index 17ff2add5..60d8e3351 100644
--- a/gc-attrs.h
+++ b/gc-attrs.h
@@ -29,11 +29,12 @@ static inline int gc_allocator_needs_clear(void) GC_ALWAYS_INLINE;
 
 enum gc_write_barrier_kind {
   GC_WRITE_BARRIER_NONE,
-  GC_WRITE_BARRIER_CARD
+  GC_WRITE_BARRIER_CARD,
+  GC_WRITE_BARRIER_EXTERN
 };
 
-static inline enum gc_write_barrier_kind gc_small_write_barrier_kind(void) GC_ALWAYS_INLINE;
-static inline size_t gc_small_write_barrier_card_table_alignment(void) GC_ALWAYS_INLINE;
-static inline size_t gc_small_write_barrier_card_size(void) GC_ALWAYS_INLINE;
+static inline enum gc_write_barrier_kind gc_write_barrier_kind(size_t obj_size) GC_ALWAYS_INLINE;
+static inline size_t gc_write_barrier_card_table_alignment(void) GC_ALWAYS_INLINE;
+static inline size_t gc_write_barrier_card_size(void) GC_ALWAYS_INLINE;
 
 #endif // GC_ATTRS_H
diff --git a/gc-embedder-api.h b/gc-embedder-api.h
index 8ae45ef61..6e39f05ea 100644
--- a/gc-embedder-api.h
+++ b/gc-embedder-api.h
@@ -41,6 +41,14 @@ GC_EMBEDDER_API inline void gc_trace_heap_roots(struct gc_heap_roots *roots,
                                                 struct gc_heap *heap,
                                                 void *trace_data);
 
+// Some heap objects have space for a "remembered" bit, indicating they
+// are in the remembered set.  Large or potentially large objects
+// (e.g. a vector whose size is a run-time property) must have a
+// remembered set bit.  Small objects may or may not have such a bit.
+GC_EMBEDDER_API inline void gc_object_set_remembered(struct gc_ref ref);
+GC_EMBEDDER_API inline int gc_object_is_remembered_nonatomic(struct gc_ref ref);
+GC_EMBEDDER_API inline void gc_object_clear_remembered_nonatomic(struct gc_ref ref);
+
 GC_EMBEDDER_API inline uintptr_t gc_object_forwarded_nonatomic(struct gc_ref ref);
 GC_EMBEDDER_API inline void gc_object_forward_nonatomic(struct gc_ref ref,
                                                         struct gc_ref new_ref);
diff --git a/large-object-space.h b/large-object-space.h
index de41dea60..9d8d0d06a 100644
--- a/large-object-space.h
+++ b/large-object-space.h
@@ -58,6 +58,49 @@ static size_t large_object_space_npages(struct large_object_space *space,
   return (bytes + space->page_size - 1) >> space->page_size_log2;
 }
 
+static void large_object_space_clear_one_remembered(uintptr_t addr,
+                                                    void *unused) {
+  struct gc_ref ref = gc_ref(addr);
+  if (gc_object_is_remembered_nonatomic(ref))
+    gc_object_clear_remembered_nonatomic(ref);
+}
+
+static void
+large_object_space_clear_remembered_set(struct large_object_space *space) {
+  if (!GC_GENERATIONAL)
+    return;
+  address_set_for_each(&space->to_space,
+                       large_object_space_clear_one_remembered, NULL);
+}
+
+struct large_object_space_trace_remembered_data {
+  void (*trace)(struct gc_ref, struct gc_heap*);
+  struct gc_heap *heap;
+};
+
+static void large_object_space_trace_one_remembered(uintptr_t addr,
+                                                    void *data) {
+  struct gc_ref ref = gc_ref(addr);
+  if (gc_object_is_remembered_nonatomic(ref)) {
+    gc_object_clear_remembered_nonatomic(ref);
+    struct large_object_space_trace_remembered_data *vdata = data;
+    vdata->trace(ref, vdata->heap);
+  }
+}
+
+static void
+large_object_space_trace_remembered_set(struct large_object_space *space,
+                                        void (*trace)(struct gc_ref,
+                                                      struct gc_heap*),
+                                        struct gc_heap *heap) {
+  struct large_object_space_trace_remembered_data vdata = { trace, heap };
+
+  if (!GC_GENERATIONAL)
+    return;
+  address_set_for_each(&space->to_space,
+                       large_object_space_trace_one_remembered, &vdata);
+}
+
 static void large_object_space_start_gc(struct large_object_space *space,
                                         int is_minor_gc) {
   if (is_minor_gc)
diff --git a/mt-gcbench.c b/mt-gcbench.c
index f72dac66e..e7e7d8a58 100644
--- a/mt-gcbench.c
+++ b/mt-gcbench.c
@@ -144,9 +144,9 @@ static void allocate_garbage(struct thread *t) {
 }
 
 static void set_field(Node *obj, Node **field, Node *val) {
-  gc_small_write_barrier(gc_ref_from_heap_object(obj),
-                         gc_edge(field),
-                         gc_ref_from_heap_object(val));
+  gc_write_barrier(gc_ref_from_heap_object(obj), sizeof(Node),
+                   gc_edge(field),
+                   gc_ref_from_heap_object(val));
   *field = val;
 }
 
diff --git a/semi-attrs.h b/semi-attrs.h
index e6b429178..3bf9584b8 100644
--- a/semi-attrs.h
+++ b/semi-attrs.h
@@ -42,13 +42,13 @@ static inline uint8_t gc_allocator_alloc_table_end_pattern(void) {
   GC_CRASH();
 }
 
-static inline enum gc_write_barrier_kind gc_small_write_barrier_kind(void) {
+static inline enum gc_write_barrier_kind gc_write_barrier_kind(size_t) {
   return GC_WRITE_BARRIER_NONE;
 }
-static inline size_t gc_small_write_barrier_card_table_alignment(void) {
+static inline size_t gc_write_barrier_card_table_alignment(void) {
   GC_CRASH();
 }
-static inline size_t gc_small_write_barrier_card_size(void) {
+static inline size_t gc_write_barrier_card_size(void) {
   GC_CRASH();
 }
 
diff --git a/semi.c b/semi.c
index 5d85ec8c6..11b74ec5f 100644
--- a/semi.c
+++ b/semi.c
@@ -375,6 +375,10 @@ void gc_collect(struct gc_mutator *mut) {
   collect(mut, 0);
 }
 
+void gc_write_barrier_extern(struct gc_ref obj, size_t obj_size,
+                             struct gc_edge edge, struct gc_ref new_val) {
+}
+
 static void collect_for_large_alloc(struct gc_mutator *mut, size_t npages) {
   collect_for_alloc(mut, npages * mutator_semi_space(mut)->page_size);
 }
diff --git a/simple-gc-embedder.h b/simple-gc-embedder.h
index 70fd5c7a8..14fb142e7 100644
--- a/simple-gc-embedder.h
+++ b/simple-gc-embedder.h
@@ -86,6 +86,25 @@ static inline void gc_object_forward_nonatomic(struct gc_ref ref,
   *tag_word(ref) = gc_ref_value(new_ref);
 }
 
+static inline void gc_object_set_remembered(struct gc_ref ref) {
+  uintptr_t *loc = tag_word(ref);
+  uintptr_t tag = *loc;
+  while (!(tag & gcobj_remembered_bit))
+    atomic_compare_exchange_weak(loc, &tag, tag | gcobj_remembered_bit);
+}
+
+static inline int gc_object_is_remembered_nonatomic(struct gc_ref ref) {
+  uintptr_t *loc = tag_word(ref);
+  uintptr_t tag = *loc;
+  return tag & gcobj_remembered_bit;
+}
+
+static inline void gc_object_clear_remembered_nonatomic(struct gc_ref ref) {
+  uintptr_t *loc = tag_word(ref);
+  uintptr_t tag = *loc;
+  *loc = tag & ~(uintptr_t)gcobj_remembered_bit;
+}
+
 static inline struct gc_atomic_forward
 gc_atomic_forward_begin(struct gc_ref ref) {
   uintptr_t tag = atomic_load_explicit(tag_word(ref), memory_order_acquire);
diff --git a/simple-tagging-scheme.h b/simple-tagging-scheme.h
index b6b8a924c..aa0b707e4 100644
--- a/simple-tagging-scheme.h
+++ b/simple-tagging-scheme.h
@@ -7,9 +7,11 @@ struct gc_header {
   uintptr_t tag;
 };
 
-// Alloc kind is in bits 1-7, for live objects.
-static const uintptr_t gcobj_alloc_kind_mask = 0x7f;
-static const uintptr_t gcobj_alloc_kind_shift = 1;
+// Alloc kind is in bits 2-7, for live objects.
+static const uintptr_t gcobj_alloc_kind_mask = 0x3f;
+static const uintptr_t gcobj_alloc_kind_shift = 2;
+static const uintptr_t gcobj_remembered_mask = 0x2;
+static const uintptr_t gcobj_remembered_bit = 0x2;
 static const uintptr_t gcobj_forwarded_mask = 0x1;
 static const uintptr_t gcobj_not_forwarded_bit = 0x1;
 static const uintptr_t gcobj_busy = 0;
diff --git a/whippet-attrs.h b/whippet-attrs.h
index bfecc44db..b26d79ad3 100644
--- a/whippet-attrs.h
+++ b/whippet-attrs.h
@@ -40,16 +40,19 @@ static inline int gc_allocator_needs_clear(void) {
   return 0;
 }
 
-static inline enum gc_write_barrier_kind gc_small_write_barrier_kind(void) {
-  if (GC_GENERATIONAL)
-    return GC_WRITE_BARRIER_CARD;
+static inline enum gc_write_barrier_kind gc_write_barrier_kind(size_t obj_size) {
+  if (GC_GENERATIONAL) {
+    if (obj_size <= gc_allocator_large_threshold())
+      return GC_WRITE_BARRIER_CARD;
+    return GC_WRITE_BARRIER_EXTERN;
+  }
   return GC_WRITE_BARRIER_NONE;
 }
-static inline size_t gc_small_write_barrier_card_table_alignment(void) {
+static inline size_t gc_write_barrier_card_table_alignment(void) {
   GC_ASSERT(GC_GENERATIONAL);
   return 4 * 1024 * 1024;
 }
-static inline size_t gc_small_write_barrier_card_size(void) {
+static inline size_t gc_write_barrier_card_size(void) {
   GC_ASSERT(GC_GENERATIONAL);
   return 256;
 }
diff --git a/whippet.c b/whippet.c
index 5d692b728..4771e37e9 100644
--- a/whippet.c
+++ b/whippet.c
@@ -1333,6 +1333,10 @@ static void trace_global_conservative_roots(struct gc_heap *heap) {
       (mark_and_globally_enqueue_heap_conservative_roots, heap, NULL);
 }
 
+static void enqueue_generational_root(struct gc_ref ref, struct gc_heap *heap) {
+  tracer_enqueue_root(&heap->tracer, ref);
+}
+
 // Note that it's quite possible (and even likely) that any given remset
 // byte doesn't hold any roots, if all stores were to nursery objects.
 STATIC_ASSERT_EQ(GRANULES_PER_REMSET_BYTE % 8, 0);
@@ -1352,7 +1356,7 @@ static void mark_space_trace_card(struct mark_space *space,
       size_t granule = granule_base + granule_offset;
       uintptr_t addr = first_addr_in_slab + granule * GRANULE_SIZE;
       GC_ASSERT(metadata_byte_for_addr(addr) == &slab->metadata[granule]);
-      tracer_enqueue_root(&heap->tracer, gc_ref(addr));
+      enqueue_generational_root(gc_ref(addr), heap);
     }
   }
 }
@@ -1385,12 +1389,22 @@ static void mark_space_clear_remembered_set(struct mark_space *space) {
   }
 }
 
+void gc_write_barrier_extern(struct gc_ref obj, size_t obj_size,
+                             struct gc_edge edge, struct gc_ref new_val) {
+  GC_ASSERT(size > gc_allocator_large_threshold());
+  gc_object_set_remembered(obj);
+}
+
 static void trace_generational_roots(struct gc_heap *heap) {
   // TODO: Add lospace nursery.
   if (atomic_load(&heap->gc_kind) & GC_KIND_FLAG_MINOR) {
     mark_space_trace_remembered_set(heap_mark_space(heap), heap);
+    large_object_space_trace_remembered_set(heap_large_object_space(heap),
+                                            enqueue_generational_root,
+                                            heap);
   } else {
     mark_space_clear_remembered_set(heap_mark_space(heap));
+    large_object_space_clear_remembered_set(heap_large_object_space(heap));
   }
 }