4. AMC (Automatic Mostly-Copying)¶

AMC is a general-purpose automatically managed pool class. This is the most mature pool class in the MPS, intended for the majority of objects in the client program. Use this pool class unless you need a particular feature that it doesn’t provide.

“Mostly Copying” means that it uses copying garbage collection except for blocks that are pinned by ambiguous references.

It uses generational garbage collection. That is, it exploits assumptions about object lifetimes and inter-connection variously referred to as “the generational hypothesis”. In particular, the following tendencies will be efficiently exploited by an AMC pool:

most objects die young;
objects that don’t die young will live a long time.

4.1. AMC properties¶

Does not support allocation via mps_alloc() or deallocation via mps_free().
Supports allocation via allocation points. If an allocation point is created in an AMC pool, the call to mps_ap_create_k() takes no keyword arguments.
Supports allocation frames but does not use them to improve the efficiency of stack-like allocation.
Does not support segregated allocation caches.
Garbage collections are scheduled automatically. See Scheduling of collections.
Uses generational garbage collection: blocks are promoted from generation to generation in the pool’s chain.
Blocks may contain exact references to blocks in the same or other pools (but may not contain ambiguous references or weak references (1), and may not use remote references).
Allocations may be variable in size.
The alignment of blocks is configurable.
Blocks do not have dependent objects.
Blocks that are not reachable from a root are automatically reclaimed.
Blocks are scanned.
Blocks may be referenced by interior pointers (unless MPS_KEY_INTERIOR is set to FALSE, in which case only base pointers, or client pointers if the blocks have in-band headers, are supported).
Blocks may be protected by barriers (1).
Blocks may move.
Blocks may be registered for finalization.
Blocks must belong to an object format which provides scan, skip, forward, is-forwarded, and padding methods.
Blocks may have in-band headers.

4.2. AMC interface¶

#include "mpscamc.h"

mps_pool_class_t mps_class_amc(void)¶

Return the pool class for an AMC (Automatic Mostly-Copying) pool.

When creating an AMC pool, mps_pool_create_k() requires one keyword argument:

MPS_KEY_FORMAT (type mps_fmt_t) specifies the object format for the objects allocated in the pool. The format must provide a scan method, a skip method, a forward method, an is-forwarded method and a padding method.

It accepts three optional keyword arguments:

MPS_KEY_CHAIN (type mps_chain_t) specifies the generation chain for the pool. If not specified, the pool will use the arena’s default chain.
MPS_KEY_INTERIOR (type mps_bool_t, default TRUE) specifies whether ambiguous interior pointers to blocks in the pool keep objects alive. If this is FALSE, then only client pointers keep objects alive.
MPS_KEY_EXTEND_BY (type size_t, default 4096) is the minimum size of the memory segments that the pool requests from the arena. Larger segments reduce the per-segment overhead, but increase fragmentation and retention.

For example:

MPS_ARGS_BEGIN(args) {
    MPS_ARGS_ADD(args, MPS_KEY_FORMAT, fmt);
    res = mps_pool_create_k(&pool, arena, mps_class_amc(), args);
} MPS_ARGS_END(args);

Memory Management Reference

Table Of Contents

4. AMC (Automatic Mostly-Copying)¶

4.1. AMC properties¶

4.2. AMC interface¶