Object Movement def compact heap = [ ... ] # many slots left = 0 right = heap.length - 1 while left < right left_slot = heap[left] right_slot = heap[right] if is_empty?(left_slot) && !is_empty?(right_slot) && can_move?(right_slot) swap(left, right) heap[right] = T_MOVED.new(left) # leave forwarding address end while !is_empty?(heap[left]) left += 1 end while is_empty?(heap[right]) || !can_move?(heap[right]) right -= 1 end end end Pointers Met? Copy / Forward Advance "free" Retreat "scan"
Reference Updating def update_references heap.each do |slot| next if is_empty?(slot) || is_moved?(slot) slot.references.each_with_index do |child, i| if is_moved?(child) slot.set_reference(i, child.new_location) end end end end How are references stored?
Reference Updating static void gc_ref_update_array(rb_objspace_t * objspace, VALUE v) { long i, len; if (FL_TEST(v, ELTS_SHARED)) return; len = RARRAY_LEN(v); if (len > 0) { VALUE *ptr = (VALUE *)RARRAY_CONST_PTR_TRANSIENT(v); for(i = 0; i < len; i++) { UPDATE_IF_MOVED(objspace, ptr[i]); } } } static void gc_ref_update_object(rb_objspace_t * objspace, VALUE v) { uint32_t i, len = ROBJECT_NUMIV(v); VALUE *ptr = ROBJECT_IVPTR(v); for (i = 0; i < len; i++) { UPDATE_IF_MOVED(objspace, ptr[i]); } } static int hash_replace_ref(st_data_t *key, st_data_t *value, st_data_t argp, int existing)
Yajl typedef struct { VALUE builderStack; VALUE parse_complete_callback; int nestedArrayLevel; int nestedHashLevel; int objectsFound; int symbolizeKeys; yajl_handle parser; } yajl_parser_wrapper; C Code (yajl_ext.h) malloc(yajl_parser_wrapper) Ruby Object T_DATA Ruby Object Ruby Object builderStack parse_complete_callback
"No Pin" Marking void yajl_parser_wrapper_mark(void * wrapper) { yajl_parser_wrapper * w = wrapper; if (w) { rb_gc_mark(w->builderStack); rb_gc_mark(w->parse_complete_callback); } } No Compaction Support void yajl_parser_wrapper_mark(void * wrapper) { yajl_parser_wrapper * w = wrapper; if (w) { rb_gc_mark_movable(w->builderStack); rb_gc_mark_movable(w->parse_complete_callback); } } With Compaction Support
Object ID After Move x = Object.new x.object_id GC.compact x.object_id 1 2 3 4 X Heap Object ID Table Memory Location Object ID 4 4 Updated Object ID Table Memory Location Object ID 1 4
Object ID Collisions x = Object.new x.object_id GC.compact x.object_id y = Object.new y.object_id 1 2 3 4 X Heap Object ID Table Memory Location Object ID 4 4 Updated Object ID Table Memory Location Object ID 1 4 y.object_id => ??? x.object_id => 4 Y
Collision Resolution $seen_object_id = {} $location_to_object_id = {} class Object def object_id id = memory_location while $seen_object_id[id] id += 1 end $seen_object_id[id] = id $location_to_object_id[memory_location] = id end end
GC Cleanup $seen_object_id = {} $location_to_object_id = {} def free_obj(obj) if $location_to_object_id[obj.memory_location] id = $location_to_object_id.delete(obj.memory_location) $seen_object_id.delete(id) end end
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