Tracking down memory usage (leak?) in Pike program

I have a GUI Pike program that's been running for roughly a month, and
according to top(1), its RSS is five gigabytes. How can I track down
where that memory is being used?

Debug.memory_usage() returns this, which has some large values in
malloc stats, but not in array/object/string etc:

([ /* 50 elements */
  "array_bytes": 60025888,
  "ba_mixed_frame_bytes": 216,
  "call_out_bytes": 27424,
  "callable_bytes": 27896,
  "callback_bytes": 4328,
  "catch_context_bytes": 7488,
  "compat_cb_box_bytes": 216,
  "destroy_called_mark_bytes": 216,
  "free_block_bytes": 173104,
  "gc_rec_frame_bytes": 216,
  "malloc_block_bytes": 1150509056,
  "malloc_bytes": 1021242320,
  "mapping_bytes": 85012815,
  "marker_bytes": 216,
  "mc_marker_bytes": 216,
  "multiset_bytes": 114704,
  "node_s_bytes": 216,
  "num_arrays": 438612,
  "num_ba_mixed_frames": 0,
  "num_call_outs": 138,
  "num_callables": 255,
  "num_callbacks": 5,
  "num_catch_contexts": 2,
  "num_compat_cb_boxs": 0,
  "num_destroy_called_marks": 0,
  "num_free_blocks": 1,
  "num_gc_rec_frames": 0,
  "num_malloc": 24,
  "num_malloc_blocks": 8,
  "num_malloced_strings": 44288,
  "num_mappings": 430246,
  "num_markers": 0,
  "num_mc_markers": 0,
  "num_multisets": 143,
  "num_node_ss": 0,
  "num_objects": 2419654,
  "num_pike_frames": 889,
  "num_pike_list_nodes": 0,
  "num_pike_types": 10616,
  "num_programs": 2005,
  "num_short_strings": 43544,
  "num_static_strings": 1105,
  "num_strings": 91353,
  "num_substrings": 2416,
  "object_bytes": 245704408,
  "pike_frame_bytes": 114240,
  "pike_list_node_bytes": 216,
  "pike_type_bytes": 650568,
  "program_bytes": 4258088,
  "string_bytes": 11878328
])

Any advice?

ChrisA