.. Sources:

    `<https://info.ravenbrook.com/project/mps/master/manual/wiki/pool_classes.html>`_
    `<https://info.ravenbrook.com/project/mps/master/design/poolams/>`_

.. index::
   single: AMS pool class
   single: pool class; AMS

.. _pool-ams:

AMS (Automatic Mark and Sweep)
==============================

**AMS** is an :term:`automatically managed <automatic memory
management>` but :term:`non-moving <non-moving garbage collector>`
:term:`pool class`. It should be used instead of :ref:`pool-amc` for
blocks that need to be automatically managed, but cannot be moved.

.. note::

    AMS is likely to be useful as a step in integrating a program with
    the MPS. It allows you to work on scanning (and investigate errors
    resulting from underscanning) without having to deal with objects
    moving as well. When you are confident that scanning is correct,
    you can switch to :ref:`pool-amc`.

    AMS is not currently suitable for production use. However, it
    could be developed into a solid mark-and-sweep pool. If you have a
    use case that needs this, :ref:`contact us <contact>`.


.. index::
   single: AMS pool class; properties

AMS properties
--------------

* Does not support allocation via :c:func:`mps_alloc` or deallocation
  via :c:func:`mps_free`.

* Supports allocation via :term:`allocation points`. If an allocation
  point is created in an AMS pool, the call to
  :c:func:`mps_ap_create_k` takes one optional keyword argument,
  :c:macro:`MPS_KEY_RANK`.

* Supports :term:`allocation frames` but does not use them to improve
  the efficiency of stack-like allocation.

* Does not support :term:`segregated allocation caches`.

* Garbage collections are scheduled automatically. See
  :ref:`topic-collection-schedule`.

* Does not use :term:`generational garbage collection`, so blocks are
  never promoted out of the generation in which they are allocated.

* Blocks may contain :term:`exact references` to blocks in the same or
  other pools, or :term:`ambiguous references` (unless the
  :c:macro:`MPS_KEY_AMS_SUPPORT_AMBIGUOUS` keyword argument is set to
  ``FALSE`` when creating the pool). Blocks may not contain
  :term:`weak references (1)`, and may not use :term:`remote
  references`.

* Allocations may be variable in size.

* The :term:`alignment` of blocks is configurable.

* Blocks do not have :term:`dependent objects`.

* Blocks that are not :term:`reachable` from a :term:`root` are
  automatically :term:`reclaimed`.

* Blocks are :term:`scanned <scan>`.

* Blocks may only be referenced by :term:`base pointers` (unless they
  have :term:`in-band headers`).

* Blocks are not protected by :term:`barriers (1)`.

* Blocks do not :term:`move <moving garbage collector>`.

* Blocks may be registered for :term:`finalization`.

* Blocks must belong to an :term:`object format` which provides
  :term:`scan <scan method>` and :term:`skip <skip method>` methods.

* Blocks may have :term:`in-band headers`.


.. index::
   single: AMS pool class; interface

AMS interface
-------------

::

   #include "mpscams.h"


.. c:function:: mps_pool_class_t mps_class_ams(void)

    Return the :term:`pool class` for an AMS (Automatic Mark & Sweep)
    :term:`pool`.

    When creating an AMS pool, :c:func:`mps_pool_create_k` requires
    one :term:`keyword argument`:

    * :c:macro:`MPS_KEY_FORMAT` (type :c:type:`mps_fmt_t`) specifies
      the :term:`object format` for the objects allocated in the pool.
      The format must provide a :term:`scan method` and a :term:`skip
      method`.

    It accepts three optional keyword arguments:

    * :c:macro:`MPS_KEY_CHAIN` (type :c:type:`mps_chain_t`) specifies
      the :term:`generation chain` for the pool. If not specified, the
      pool will use the arena's default chain.

    * :c:macro:`MPS_KEY_GEN` (type ``unsigned``) specifies the
      :term:`generation` in the chain into which new objects will be
      allocated. If you pass your own chain, then this defaults to
      ``0``, but if you didn't (and so use the arena's default chain),
      then an appropriate generation is used.

      Note that AWL does not use generational garbage collection, so
      blocks remain in this generation and are not promoted.

    * :c:macro:`MPS_KEY_AMS_SUPPORT_AMBIGUOUS` (type
      :c:type:`mps_bool_t`, default ``TRUE``) specifies whether
      references to blocks in the pool may be ambiguous.

    For example::

        MPS_ARGS_BEGIN(args) {
            MPS_ARGS_ADD(args, MPS_KEY_FORMAT, fmt);
            res = mps_pool_create_k(&pool, arena, mps_class_ams(), args);
        } MPS_ARGS_END(args);

    When creating an :term:`allocation point` on an AMS pool,
    :c:func:`mps_ap_create_k` accepts one optional keyword argument:

    * :c:macro:`MPS_KEY_RANK` (type :c:type:`mps_rank_t`, default
      :c:func:`mps_rank_exact`) specifies the :term:`rank` of references
      in objects allocated on this allocation point. It must be
      :c:func:`mps_rank_exact` (if the objects allocated on this
      allocation point will contain :term:`exact references`), or
      :c:func:`mps_rank_ambig` (if the objects may contain
      :term:`ambiguous references`).

    For example::

        MPS_ARGS_BEGIN(args) {
            MPS_ARGS_ADD(args, MPS_KEY_RANK, mps_rank_ambig());
            res = mps_ap_create_k(&ap, ams_pool, args);
        } MPS_ARGS_END(args);


.. c:function:: mps_pool_class_t mps_class_ams_debug(void)

    A :ref:`debugging <topic-debugging>` version of the AMS pool
    class.

    When creating a debugging AMS pool, :c:func:`mps_pool_create_k`
    accepts the following keyword arguments:
    :c:macro:`MPS_KEY_FORMAT`, :c:macro:`MPS_KEY_CHAIN`,
    :c:macro:`MPS_KEY_GEN`, and
    :c:macro:`MPS_KEY_AMS_SUPPORT_AMBIGUOUS` are as described above,
    and :c:macro:`MPS_KEY_POOL_DEBUG_OPTIONS` specifies the debugging
    options. See :c:type:`mps_pool_debug_option_s`.