Allow clients to implement large hash arrays without a collection-rehash feedback loop in the public MPS

code/mps.h

@@ -246,6 +246,9 @@ extern const struct mps_key_s _mps_key_FMT_PAD;
 extern const struct mps_key_s _mps_key_FMT_CLASS;
 #define MPS_KEY_FMT_CLASS   (&_mps_key_FMT_CLASS)
 #define MPS_KEY_FMT_CLASS_FIELD fmt_class
+extern const struct mps_key_s _mps_key_ap_hash_arrays;
+#define MPS_KEY_AP_HASH_ARRAYS (&_mps_key_ap_hash_arrays)
+#define MPS_KEY_AP_HASH_ARRAYS_FIELD b
 
 /* Maximum length of a keyword argument list. */
 #define MPS_ARGS_MAX          32

code/poolamc.c

@@ -513,7 +513,6 @@ static Res AMCBufInit(Buffer buffer, Pool pool, Bool isMutator, ArgList args)
   if (ArgPick(&arg, args, amcKeyAPHashArrays))
     forHashArrays = arg.val.b;
 
-  /* call next method */
   res = NextMethod(Buffer, amcBuf, init)(buffer, pool, isMutator, args);
   if(res != ResOK)
     return res;

manual/source/pool/amc.rst

@@ -139,3 +139,63 @@ AMC interface
             MPS_ARGS_ADD(args, MPS_KEY_FORMAT, fmt);
             res = mps_pool_create_k(&pool, arena, mps_class_amc(), args);
         } MPS_ARGS_END(args);
+
+    When creating an :term:`allocation point` on an AMC pool,
+    :c:func:`mps_ap_create_k` accepts one optional keyword argument:
+
+    * :c:macro:`MPS_KEY_AP_HASH_ARRAYS` (type :c:type:`mps_bool_t`,
+      defaulting to false) specifies (if true) that blocks allocated
+      from the allocation point do not contribute to the *new size* of
+      the :term:`nursery space` for the purposes of deciding whether
+      to start a collection of that generation. See
+      :ref:`pool-amc-hash-arrays`.
+
+
+.. index::
+   pair: AMC pool class; hash arrays
+
+.. _pool-amc-hash-arrays:
+
+Hash arrays
+-----------
+
+The :term:`location dependency` feature of the MPS allows the
+:term:`client program` to implement address-based hash tables in pools
+like AMC that use a :term:`moving memory manager`, re-hashing the
+tables when the addresses they contain might have moved.
+
+However, when a frequently-used hash table grows large enough, the
+following sequence of events may take place:
+
+1. The hash table discovers that its location dependency is stale.
+
+2. A new array is allocated to contain the re-hashed keys.
+
+3. The new array is large enough to push the *new size* of the
+   :term:`nursery space` (that is, the amount of newly allocated
+   memory since the last collection in the first :term:`generation` in
+   the :term:`generation chain` for the pool containing the array)
+   close to its capacity.
+
+4. A small amount of additional allocation causes the new size of the
+   nursery generation to exceed its capacity, which causes the MPS to
+   start a new collection of that generation. This in turn causes the
+   hash table to become stale again.
+
+When the hash table reaches this critical size, the client program may
+find that a large fraction of its time is being spent re-hashing the
+table.
+
+In order to avoid this happening, the MPS provides a mechanism for
+specifying that the newly allocated array does not contribute to the
+new size of the nursery space: this cuts off the vicious cycle at step
+3.
+
+To enable this mechanism, use the optional :c:macro:`MPS_KEY_AP_HASH_ARRAYS`
+keyword argument when creating an allocation point with
+:c:func:`mps_ap_create_k`. This interface is documented in the AMC Interface
+section of the :ref:`pool-amc` documentation above.
+
+See :ref:`topic-collection-schedule` for an explanation of the *new
+size* of a generation, and how the MPS uses this to determine when to
+start a collection of that generation.

manual/source/release.rst

@@ -47,6 +47,12 @@ New features
    :ref:`topic-scanning-protocol`. This allows the client program to
    safely update references in the visited objects.
 
+#. An :term:`allocation point` for a pool belonging to the class
+   :ref:`pool-amc` can now be configured so that allocations do not
+   provoke garbage collections, reducing the amount of re-hashing for
+   address-based hash tables using :term:`location dependency`. See
+   :ref:`pool-amc-hash-arrays`.
+
 
 Interface changes
 .................