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# 12. MVT (Manual Variable Temporal)

MVT manually manages variable-sized, unformatted objects. It uses the temporal fit allocation policy.

## 12.1. 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 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 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.

## 12.2. MVT properties

- Does not support allocation via `mps_alloc()`.
- Supports allocation via allocation points only. If an allocation point is created in an MVT pool, the call to `mps_ap_create()` takes no additional parameters.
- Supports deallocation via `mps_free()`.
- Supports allocation frames but does not use them to improve the efficiency of stack-like allocation.
- Does not support segregated allocation caches.
- There are no garbage collections in this pool.
- Blocks may not contain references to blocks in automatically managed pools (unless these are registered as roots).
- Allocations may be variable in size.
- The alignment of blocks is not configurable: it is the natural alignment of the platform (see `MPS_PF_ALIGN`).
- Blocks do not have dependent objects.
- Blocks are not automatically reclaimed.
- Blocks are not scanned.
- Blocks are not protected by barriers(1).
- Blocks do not move.
- Blocks may not be registered for finalization.
- Blocks must not belong to an object format.

## 12.3. MVT interface

    #include "mpscmvt.h"

mps_class_t `mps_class_mvt`(void)

  Return the pool class for an MVT (Manual Variable Temporal) pool.

  When creating an MVT pool, `mps_pool_create()` takes five extra arguments:

      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)

  `minimum_size`, `mean_size`, and `maximum_size` are the predicted minimum, mean, and maximum size of blocks expected to be allocated in the pool. Blocks smaller than `minimum_size` and larger than `maximum_size` may be allocated, but the pool is not guaranteed to manage them space-efficiently. Furthermore, partial freeing is not supported for blocks larger than `maximum_size`; doing so will result in the storage of the block never being reused. `mean_size` need not be an accurate mean, although the pool will manage `mean_size` blocks more efficiently if it is.

  `reserve_depth` is the expected hysteresis of the population of the pool. When blocks are freed, the pool will retain sufficient storage to allocate `reserve_depth` blocks of `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.

  `fragmentation_limit` is a percentage from 1 to 100 (inclusive). It sets an upper limit on the space overhead of MVT, 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 `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 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 100 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.

## 12.4. MVT introspection

    #include "mpscmvt.h"

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 bytes(1).

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 bytes(1). This is the sum of allocated space and free space.

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