.. 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, answer these questions about your data:
1. Do you need the MPS to :term:`automatically <automatic memory
management>` :term:`reclaim` :term:`unreachable` blocks?
2. 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.)
3. 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>`, :term:`ambiguous
<ambiguous reference>` or :term:`weak <weak reference (1)>`?
Second, look up your answers in this table to find the recommended
pool class to use:
========== ====================== =========== ====================================
Automatic? Movable & protectable? References? Use this pool class
========== ====================== =========== ====================================
yes yes none :ref:`pool-amcz`
yes yes exact :ref:`pool-amc`
yes yes ambiguous nothing suitable
yes yes weak :ref:`pool-awl`
yes no none :ref:`pool-lo`
yes no exact :ref:`pool-ams`
yes no ambiguous nothing suitable
yes no weak nothing suitable
no *any* none :ref:`pool-mvff`
no *any* exact :ref:`pool-mvff` [1]_
no *any* ambiguous :ref:`pool-mvff` [1]_
no *any* weak :ref:`pool-mvff` [1]_
========== ====================== =========== ====================================
.. note::
.. [1] :ref:`pool-mvff` doesn't scan for references, but you can
work around this by registering your blocks as
:term:`roots` (with the appropriate :term:`rank`) just
after they are allocated, and deregistering them just
before freeing them.
.. 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)`.
============================================= ===== ===== ===== ===== ===== ===== ===== ===== ===== =====
Property AMC AMCZ AMS AWL LO MFS MV MVFF MVT SNC
============================================= ===== ===== ===== ===== ===== ===== ===== ===== ===== =====
Supports :c:func:`mps_alloc`? no no no no no yes yes yes no no
Supports :c:func:`mps_free`? no no no no no yes yes yes yes no
Supports allocation points? yes yes yes yes yes no no yes yes yes
Supports allocation frames? yes yes yes yes yes no no yes yes yes
Supports segregated allocation caches? no no no no no yes 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 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 var
Alignment? [5]_ conf conf conf conf conf [6]_ [6]_ [7]_ [6]_ 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 no
Blocks are manually managed? [10]_ no no no no no yes yes yes yes yes
Blocks are scanned? [11]_ yes no yes yes no no no no no yes
Blocks support base pointers only? [12]_ yes yes yes yes yes --- --- --- --- yes
Blocks support internal pointers? [12]_ no no no no no --- --- --- --- no
Blocks may be protected by barriers? yes no yes yes yes no no no no yes
Blocks may move? yes yes no no no no no no no no
Blocks may be finalized? yes yes yes yes yes no no no no no
Blocks must be formatted? [11]_ yes yes yes yes yes no 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-mv` and
:ref:`pool-mvt` pools is platform-dependent.
.. [7] :ref:`pool-mvff` pools have configurable alignment, but it
may not be smaller than the :term:`natural alignment` for
the :term:`platform` (see :c:macro:`MPS_PF_ALIGN`).
.. [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.
.. 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>`.