.. 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 ` :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 ` 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 ` or :term:`weak `? 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: ``_ .. 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 `", ":ref:`AMCZ `", ":ref:`AMS `", ":ref:`AWL `", ":ref:`LO `", ":ref:`MFS `", ":ref:`MVFF `", ":ref:`MVT `", ":ref:`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 ` and :term:`incrementally `. .. [3] The references in question are references to blocks in :term:`automatically managed ` :term:`pools`. .. [4] Pools "may contain :term:`ambiguous ` / :term:`exact ` / :term:`weak ` 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 `.