In computer science, region-based memory management is a type of memory management in which each allocated object is assigned to a region. A region, also called a zone, arena, area, or memory context, is a collection of allocated objects that can be efficiently reallocated or deallocated all at once. Like stack allocation, regions facilitate allocation and deallocation of memory with low overhead; but they are more flexible, allowing objects to live longer than the stack frame in which they were allocated. In typical implementations, all objects in a region are allocated in a single contiguous range of memory addresses, similarly to how stack frames are typically allocated.
Example
As a simple example, consider the following C code which allocates and then deallocates a linked list data structure:
Region *r = createRegion(); ListNode *head = NULL; for (int i = 1; i <= 1000; i++) { ListNode* newNode = allocateFromRegion(r, sizeof(ListNode)); newNode->next = head; head = newNode; } // ... // (use list here) // ... destroyRegion(r);
Although it required many operations to construct the linked list, it can be quickly deallocated in a single operation by destroying the region in which the nodes were allocated. There is no need to traverse the list.
Implementation
Simple explicit regions are straightforward to implement; the following description is based on the work of Hanson. Each region is implemented as a linked list of large blocks of memory; each block should be large enough to serve many allocations. The current block maintains a pointer to the next free position in the block, and if the block is filled, a new one is allocated and added to the list. When the region is deallocated, the next-free-position pointer is reset to the beginning of the first block, and the list of blocks can be reused for the next allocated region. Alternatively, when a region is deallocated, its list of blocks can be appended to a global freelist from which other regions may later allocate new blocks. With either case of this simple scheme, it is not possible to deallocate individual objects in regions.
The overall cost per allocated byte of this scheme is very low; almost all allocations involve only a comparison and an update to the next-free-position pointer. Deallocating a region is a constant-time operation, and is done rarely. Unlike in typical garbage collection systems, there is no need to tag data with its type.
History and concepts
The basic concept of regions is very old, first appearing as early as 1967 in Douglas T. Ross's AED Free Storage Package, in which memory was partitioned into a hierarchy of zones; each zone had its own allocator, and a zone could be freed all-at-once, making zones usable as regions. In 1976, the PL/I standard included the AREA data type. In 1990, Hanson demonstrated that explicit regions in C (which he called arenas) could achieve time performance per allocated byte superior to even the fastest-known heap allocation mechanism. Explicit regions were instrumental in the design some early C-based software projects, including the Apache HTTP Server, which calls them pools, and the PostgreSQL database management system, which calls them memory contexts. Like traditional heap allocation, these schemes do not provide memory safety; it is possible for a programmer to access a region after it is deallocated through a dangling pointer, or to forget to deallocate a region, causing a memory leak.
Region inference
In 1988, researchers began investigating how to use regions for safe memory allocation by introducing the concept of region inference, where the creation and deallocation of regions, as well as the assignment of individual static allocation expressions to particular regions, is inserted by the compiler at compile-time. The compiler is able to do this in such a way that it can guarantee dangling pointers and leaks do not occur.
In an early work by Ruggieri and Murtagh, a region is created at the beginning of each function and deallocated at the end. They then use data flow analysis to determine a lifetime for each static allocation expression, and assign it to the youngest region that contains its entire lifetime.
In 1994, this work was generalized in a seminal work by Tofte and Talpin to support type polymorphism and higher-order functions in Standard ML, a functional programming language, using a different algorithm based on type inference and the theoretical concepts of polymorphic region types and the region calculus. Their work introduced an extension of the lambda calculus including regions, adding two constructs: