.. index::
single: pool class; choosing
.. _pool-choose:
Choosing a pool class
=====================
This section contains a simple procedure for choosing a :term:`pool
class` based on the properties of the data you plan to store in
it. The MPS works well if you can segregate your data into a variety
of pools, choosing the most appropriate pool class for each.
.. note::
Pool classes can differ in many ways not considered here: speed,
vulnerability to fragmentation, control overhead, and so on. This
procedure gives you a decent recommendation, but an expert in the
MPS might be able to make a better recommendation. And if no pool
class in the open source MPS exactly matches your needs, then it
is possible to develop new pool classes. See :ref:`pool-writing`.
First, do you need the MPS to :term:`automatically <automatic memory
management>` :term:`reclaim` :term:`unreachable` blocks? If so, you
need an automatically managed (garbage collected) pool class and you
should consult :ref:`pool-choose-automatic` below.
Otherwise, you need a manually managed pool class and you should
consult :ref:`pool-choose-manual` below.
.. _pool-choose-automatic:
Choosing an automatic pool class
--------------------------------
Answer these questions about your data:
#. Is it acceptable for the MPS to :term:`move <moving memory
manager>` blocks in memory and to place :term:`barriers (1)` on
blocks? (For example, it might not be acceptable to move a block if
it has been passed to :term:`foreign code` that remembered its
location.)
#. Do your blocks contain :term:`references` to blocks stored in
automatically managed pools (including references to other blocks
in the same pool, if it's automatically managed)? And if so, are
these references :term:`exact <exact reference>` or :term:`weak
<weak reference (1)>`?
Second, look up your answers in this table to find the recommended
pool class to use:
====================== =========== ===================
Movable & protectable? References? Use this pool class
====================== =========== ===================
yes none :ref:`pool-amcz`
yes exact :ref:`pool-amc`
yes weak :ref:`pool-awl`
no none :ref:`pool-lo`
no exact :ref:`pool-ams`
no weak nothing suitable
====================== =========== ===================
.. _pool-choose-manual:
Choosing a manual pool class
----------------------------
Answer these questions about your data:
#. Are the blocks fixed in size? If so, use :ref:`pool-mfs`.
#. Are the lifetimes of blocks predictable? If so, use
:ref:`pool-mvt`, and arrange that objects that are predicted to die
at about the same time are allocated from the same
:term:`allocation point`.
#. Otherwise, use :ref:`pool-mvff`.
.. Sources:
`<https://info.ravenbrook.com/project/mps/doc/2002-06-18/obsolete-mminfo/mmdoc/doc/mps/guide/pool-classes/>`_
.. index::
single: pool class; table of properties
.. _pool-properties:
Pool class properties
=====================
This table summarizes the properties of each :term:`pool class`
provided by the open source MPS. For "block" properties, "yes" means
that the property holds for *all* blocks allocated from the pool. An
entry "---" indicates that a property makes no sense for a pool class:
for example, if blocks in a pool may not contain :term:`references`,
it makes no sense to ask whether they may contain :term:`weak
references (1)`.
.. csv-table::
:header: "Property", ":ref:`AMC <pool-amc>`", ":ref:`AMCZ <pool-amcz>`", ":ref:`AMS <pool-ams>`", ":ref:`AWL <pool-awl>`", ":ref:`LO <pool-lo>`", ":ref:`MFS <pool-mfs>`", ":ref:`MVFF <pool-mvff>`", ":ref:`MVT <pool-mvt>`", ":ref:`SNC <pool-snc>`"
:widths: 6, 1, 1, 1, 1, 1, 1, 1, 1, 1
Supports :c:func:`mps_alloc`?, no, no, no, no, no, yes, yes, no, no
Supports :c:func:`mps_free`?, no, no, no, no, no, yes, yes, yes, no
Supports allocation points?, yes, yes, yes, yes, yes, no, yes, yes, yes
Manages memory using allocation frames?, no, no, no, no, no, no, no, no, yes
Supports segregated allocation caches?, no, no, no, no, no, yes, yes, no, no
Timing of collections? [2]_, auto, auto, auto, auto, auto, ---, ---, ---, ---
May contain references? [3]_, yes, no, yes, yes, no, no, no, no, yes
May contain exact references? [4]_, yes, ---, yes, yes, ---, ---, ---, ---, yes
May contain ambiguous references? [4]_, no, ---, no, no, ---, ---, ---, ---, no
May contain weak references? [4]_, no, ---, no, yes, ---, ---, ---, ---, no
Allocations fixed or variable in size?, var, var, var, var, var, fixed, var, var, var
Alignment? [5]_, conf, conf, conf, conf, conf, [6]_, [7]_, [7]_, conf
Dependent objects? [8]_, no, ---, no, yes, ---, ---, ---, ---, no
May use remote references? [9]_, no, ---, no, no, ---, ---, ---, ---, no
Blocks are automatically managed? [10]_, yes, yes, yes, yes, yes, no, no, no, no
Blocks are promoted between generations, yes, yes, no, no, no, ---, ---, ---, ---
Blocks are manually managed? [10]_, no, no, no, no, no, yes, yes, yes, yes
Blocks are scanned? [11]_, yes, no, yes, yes, no, no, no, no, yes
Blocks support base pointers only? [12]_, no, no, yes, yes, yes, ---, ---, ---, yes
Blocks support internal pointers? [12]_, yes, yes, no, no, no, ---, ---, ---, no
Blocks may be protected by barriers?, yes, no, yes, yes, yes, no, no, no, yes
Blocks may move?, yes, yes, no, no, no, no, no, no, no
Blocks may be finalized?, yes, yes, yes, yes, yes, no, no, no, no
Blocks must be formatted? [11]_, yes, yes, yes, yes, yes, no, no, no, yes
Blocks may use :term:`in-band headers`?, yes, yes, yes, yes, yes, ---, ---, ---, no
.. note::
.. [2] "Timing of collections" is "auto" if :term:`garbage collection`
is under the control of the MPS, which decides when collection
should take place and performs it :term:`automatically
<automatic memory management>` and :term:`incrementally
<incremental garbage collection>`.
.. [3] The references in question are references to blocks in
:term:`automatically managed <automatic memory management>`
:term:`pools`.
.. [4] Pools "may contain :term:`ambiguous <ambiguous reference>` /
:term:`exact <exact reference>` / :term:`weak <weak
reference (1)>` references" if the references that the client
program fixes during scanning may include references of the
indicated :term:`rank`.
.. [5] "Alignment" is "conf" if the client program may specify
:term:`alignment` for each pool.
.. [6] The alignment of blocks allocated from :ref:`pool-mfs`
pools is the platform's :term:`natural alignment`,
:c:macro:`MPS_PF_ALIGN`.
.. [7] :ref:`pool-mvt` and :ref:`pool-mvff` pools have
configurable alignment, but it may not be smaller than
``sizeof(void *)``.
.. [8] In pools with this property, each object may specify an
:term:`dependent object` which the client program
guarantees will be accessible during the scanning of the
first object. This may be used in the implementation of
:term:`weak hash tables`.
.. [9] "Remote references" are references that are stored outside the
block to which they logically belong (for example, in some kind
of auxiliary table). A pool containing remote references cannot
rely on a :term:`write barrier` to detect changed references.
.. [10] Blocks are "automatically managed" if they may be
automatically discarded when the MPS determines that they
are unreachable; they are "manually managed" if they can be
discarded when the :term:`client program` requests it. Note
that these properties are not mutually exclusive, although
the MPS does not provide a pool class that satisfies both.
.. [11] Blocks "are scanned" if the MPS :term:`scans` them for
references; blocks "must be formatted" if they are
described to the MPS by an :term:`object format`. At
present, the MPS only knows how to scan blocks using the
:term:`scan method` from an object format, but the MPS
design does not preclude pools that scan unformatted
blocks.
.. [12] A block "supports internal pointers" if a pointer to any
location within the block is considered to be a reference
to the block. It "supports base pointers only" if only a
pointer to the base of the block (or, if the block belongs
to an object format with :term:`in-band headers`, a pointer
just past the end of the header) is considered to be a
reference to the block.
Pools that support internal pointers can be switched to
base pointers only, by setting the optional keyword
argument :c:macro:`MPS_KEY_INTERIOR` to ``FALSE`` when
calling :c:func:`mps_pool_create_k`.
.. index::
single: pool class; writing
.. _pool-writing:
Writing a new pool class
========================
If none of the pool classes supplied with the MPS are quite right for
your application, don't despair: the MPS is designed to be extensible
with new pool classes, and designed so that the properties of pools
are as orthogonal as possible. So if you need a pool containing
objects that are scannable but unformatted, or movable objects which
are manually managed, or a pool all of whose objects are roots, there
is no technical reason why it should not be possible to write it.
If you'd be interested in our developing new pool classes for your
requirements, or if you've started writing a new pool class
yourself, :ref:`we'd love to hear from you <contact>`.