.. Sources:
`<https://info.ravenbrook.com/project/mps/master/design/poolmvt/>`_
.. index::
single: MVT
single: pool class; MVT
.. _pool-mvt:
MVT (Manual Variable Temporal)
==============================
**MVT** :term:`manually manages <manual memory management>`
variable-sized, unformatted objects. It uses the :dfn:`temporal fit`
:term:`allocation policy`.
.. index::
pair: MVT; temporal fit
single: allocation policy; temporal fit
Temporal fit
------------
Temporal fit attempts to place consecutive allocations next to each
other. It relies on delaying re-use as long as possible to permit freed
blocks to :term:`coalesce`, thus maximizing the number of consecutive
allocations that can be co-located. Temporal fit permits a very fast
allocator and a deallocator competitive in speed with all other known
policies.
Temporal fit is intended to take advantage of knowledge of object
:term:`lifetimes`: either *a priori* knowledge, or knowledge acquired
by profiling. The best performance will be achieved by allocating
objects with similar expected death times together.
A simple policy can be implemented to take advantage of MVT. Object
size is typically well-correlated with object life-expectancy, and
birth time plus lifetime gives death time, so allocating objects of
similar size sequentially from the same pool instance should result in
objects allocated close to each other dying at about the same time.
An application that has several classes of objects of widely differing
life expectancy will best be served by creating a different MVT pool
instance for each life-expectancy class. A more sophisticated policy
can use either the programmer's knowledge of the expected lifetime of
an object, or any characteristic of objects that correlates with
lifetime, to choose an appropriate pool to allocate in.
Allocating objects with unknown or very different death times together
will pessimize the space performance of MVT.
.. index::
single: MVT; properties
MVT properties
--------------
* Does not support allocation via :c:func:`mps_alloc`.
* Supports allocation via :term:`allocation points` only. If an
allocation point is created in an MVT pool, the call to
:c:func:`mps_ap_create` takes no additional parameters.
* Supports deallocation via :c:func:`mps_free`.
* Supports :term:`allocation frames` but does not use them to improve
the efficiency of stack-like allocation.
* Does not support :term:`segregated allocation caches`.
* There are no garbage collections in this pool.
* Blocks may not contain :term:`references` to blocks in automatically
managed pools (unless these are registered as :term:`roots`).
* Allocations may be variable in size.
* The :term:`alignment` of blocks is not configurable: it is the
:term:`natural alignment` of the platform (see
:c:macro:`MPS_PF_ALIGN`).
* Blocks do not have :term:`dependent objects`.
* Blocks are not automatically :term:`reclaimed`.
* Blocks are not :term:`scanned <scan>`.
* Blocks are not protected by :term:`barriers (1)`.
* Blocks do not :term:`move <moving garbage collector>`.
* Blocks may not be registered for :term:`finalization`.
* Blocks must not belong to an :term:`object format`.
.. index::
single: MVT; interface
MVT interface
-------------
::
#include "mpscmvt.h"
.. c:function:: mps_class_t mps_class_mvt(void)
Return the :term:`pool class` for an MVT (Manual Variable
Temporal) :term:`pool`.
When creating an MVT pool, :c:func:`mps_pool_create_k` may take
five :term:`keyword arguments`:
* :c:macro:`MPS_KEY_MIN_SIZE` (type :c:type:`size_t`, default is
smallest general purpose alignment for the architecture) is the
predicted minimum size of blocks that will be allocated from the
pool.
* :c:macro:`MPS_KEY_MEAN_SIZE` (type :c:type:`size_t`, default 32) is the
predicted mean size of blocks that will be allocated from the
pool.
* :c:macro:`MPS_KEY_MAX_SIZE` (type :c:type:`size_t`, default 8192) is the
predicted maximum size of blocks that will be allocated from the
pool. Partial freeing is not supported for blocks larger than
this; doing so will result in the storage of the block never
being reused.
These three arguments are *hints* to the MPS: the pool will be
less efficient if they are wrong, but the only thing that will
break is the partial freeing of large blocks.
* :c:macro:`MPS_KEY_MVT_RESERVE_DEPTH` (type
:c:type:`mps_count_t`) is the expected hysteresis of the
population of the pool. When blocks are freed, the pool will
retain sufficient storage to allocate this many blocks of the
mean size for near term allocations (rather than immediately
making that storage available to other pools).
If a pool has a stable population, or one which only grows over
the lifetime of the pool, or one which grows steadily and then
shrinks steadily, use a reserve depth of 0.
It is always safe to use a reserve depth of 0, but if the
population typically fluctuates in a range (for example, the
client program repeatedly creates and destroys a subset of
blocks in a loop), it is more efficient for the pool to retain
enough storage to satisfy that fluctuation. For example, if a
pool has an object population that typically fluctuates between
8,000 and 10,000, use a reserve depth of 2,000.
The reserve will not normally be available to other pools for
allocation, even when it is not used by the pool. If this is
undesirable, a reserve depth of 0 may be used for a pool whose
object population does vary, at a slight cost in efficiency. The
reserve does not guarantee any particular amount of allocation.
* :c:macro:`MPS_KEY_MVT_FRAG_LIMIT` (type :c:type:`mps_count_t`)
is a double from 0.0 to 1.0 (inclusive). It sets an upper
limit on the space overhead of an MVT pool, in case block death
times and allocations do not correlate well. If the free space
managed by the pool as a ratio of all the space managed by the
pool exceeds the fragmentation limit, the pool falls back to a
first fit allocation policy, exploiting space more efficiently
at a cost in time efficiency. A fragmentation limit of 0.0 would
cause the pool to operate as a first-fit pool, at a significant
cost in time efficiency: therefore this is not permitted.
A fragmentation limit of 1.0 causes the pool to always use
temporal fit (unless resources are exhausted). If the objects
allocated in the pool have similar lifetime expectancies, this
mode will have the best time- and space-efficiency. If the
objects have widely varying lifetime expectancies, this mode
will be time-efficient, but may be space-inefficient. An
intermediate setting can be used to limit the space-inefficiency
of temporal fit due to varying object life expectancies.
For example::
MPS_ARGS_BEGIN(args) {
MPS_ARGS_ADD(ARGS, MPS_KEY_MIN_SIZE, 4);
MPS_ARGS_ADD(ARGS, MPS_KEY_MEAN_SIZE, 32);
MPS_ARGS_ADD(ARGS, MPS_KEY_MAX_SIZE, 1024);
MPS_ARGS_ADD(ARGS, MPS_KEY_MVT_RESERVE_DEPTH, 256);
MPS_ARGS_ADD(ARGS, MPS_KEY_MVT_FRAG_LIMIT, 0.5);
MPS_ARGS_DONE(args);
res = mps_pool_create_k(&pool, arena, mps_class_mvt(), args);
} MPS_ARGS_END(args);
.. deprecated:: starting with version 1.112.
When using :c:func:`mps_pool_create`, pass the arguments like
this::
mps_res_t mps_pool_create(mps_pool_t *pool_o, mps_arena_t arena,
mps_class_t mps_class_mvt(),
size_t minimum_size,
size_t mean_size,
size_t maximum_size,
mps_count_t reserve_depth,
mps_count_t fragmentation_limit)
.. note::
The fragmentation_limit is a percentage from 0 to 100
inclusive when passed to :c:func:`mps_pool_create`, not a
double from 0.0 to 1.0 as in :c:func:`mps_pool_create_k`.
.. index::
pair: MVT; introspection
MVT introspection
-----------------
::
#include "mpscmvt.h"
.. c:function:: size_t mps_mvt_free_size(mps_pool_t pool)
Return the total amount of free space in an MVT pool.
``pool`` is the MVT pool.
Returns the total free space in the pool, in :term:`bytes (1)`.
.. c:function:: size_t mps_mvt_size(mps_pool_t pool)
Return the total size of an MVT pool.
``pool`` is the MVT pool.
Returns the total size of the pool, in :term:`bytes (1)`. This
is the sum of allocated space and free space.