Memory management (also dynamic memory management, dynamic storage allocation, or dynamic memory allocation) is a form of resource management applied to computer memory. The essential requirement of memory management is to provide ways to dynamically allocate portions of memory to programs at their request, and free it for reuse when no longer needed. This is critical to any advanced computer system where more than a single process might be underway (multitasking) at any time. Several methods have been devised that increase the effectiveness of memory management. Virtual memory systems separate the memory addresses used by a process from actual physical addresses, allowing separation of processes and increasing the size of the virtual address space beyond the available amount of RAM using paging or swapping to secondary storage. The quality of the virtual memory manager can have an extensive effect on overall system performance. The system allows a computer to appear as if it may have more memory available than physically present, thereby allowing multiple processes to share it. In some operating systems, e.g. Burroughs/Unisys MCP, and OS/360 and successors, memory is managed by the operating system. In other operating systems, e.g. Unix-like operating systems, memory is managed at the application level. Memory management within an address space is generally categorized as either manual memory management or automatic memory management.
Manual memory management
The task of fulfilling an allocation request consists of locating a block of unused memory of sufficient size. Memory requests are satisfied by allocating portions from a large pool of memory called the heap or free store. At any given time, some parts of the heap are in use, while some are "free" (unused) and thus available for future allocations. In the C language, the function which allocates memory from the heap is called malloc and the function which takes previously allocated memory and marks it as "free" (to be used by future allocations) is called free. Several issues complicate the implementation, such as external fragmentation, which arises when there are many small gaps between allocated memory blocks, which invalidates their use for an allocation request. The allocator's metadata can also inflate the size of (individually) small allocations. This is often managed by chunking. The memory management system must track outstanding allocations to ensure that they do not overlap and that no memory is ever "lost" (i.e. that there are no "memory leaks").
Efficiency The specific dynamic memory allocation algorithm implemented can impact performance significantly. A study conducted in 1994 by Digital Equipment Corporation illustrates the overheads involved for a variety of allocators. The lowest average instruction path length required to allocate a single memory slot was 52 (as measured with an instruction level profiler on a variety of software).
Implementations Since the precise location of the allocation is not known in advance, the memory is accessed indirectly, usually through a pointer reference. The specific algorithm used to organize the memory area and allocate and deallocate chunks is interlinked with the kernel, and may use any of the following methods:
Fixed-size blocks allocation
Fixed-size blocks allocation, also called memory pool allocation, uses a free list of fixed-size blocks of memory (often all of the same size). This works well for simple embedded systems where no large objects need to be allocated but suffers from fragmentation especially with long memory addresses. However, due to the significantly reduced overhead, this method can substantially improve performance for objects that need frequent allocation and deallocation, and so it is often used in video games.
Buddy blocks
In this system, memory is allocated into several pools of memory instead of just one, where each pool represents blocks of memory of a certain power of two in size, or blocks of some other convenient size progression. All blocks of a particular size are kept in a sorted linked list or tree and all new blocks that are formed during allocation are added to their respective memory pools for later use. If a smaller size is requested than is available, the smallest available size is selected and split. When a block is split, it is divided into two smaller blocks, and each smaller block becomes a unique "buddy" to the other. One of the resulting parts is selected, and the process repeats until the request is complete. When a block is allocated, the allocator will start with the smallest sufficiently large block to avoid needlessly breaking blocks. When a block is freed, it is compared to its buddy. If they are both free, they are combined and placed in the correspondingly larger-sized buddy-block list.
Slab allocation
This memory allocation mechanism preallocates memory chunks suitable to fit objects of a certain type or size. These chunks are called caches and the allocator only has to keep track of a list of free cache slots. Constructing an object will use any one of the free cache slots and destructing an object will add a slot back to the free cache slot list. This technique alleviates memory fragmentation and is efficient as there is no need to search for a suitable portion of memory, as any open slot will suffice.
Stack allocation
Many Unix-like systems as well as Microsoft Windows implement a function called alloca for dynamically allocating stack memory in a way similar to the heap-based malloc. A compiler typically translates it to inlined instructions manipulating the stack pointer. Although there is no need of manually freeing memory allocated this way, as it is automatically freed when the function that called alloca returns, there exists a risk of overflow. And since alloca is an ad hoc expansion seen in many systems but never in POSIX or the C standard, its behavior in case of a stack overflow is undefined. A safer version of alloca called _malloca, which reports errors, exists on Microsoft Windows. It requires the use of _freea. gnulib provides an equivalent interface, albeit instead of throwing an SEH exception on overflow, it delegates to malloc when an overlarge size is detected. A similar feature can be emulated using manual accounting and size-checking, such as in the uses of alloca_account in glibc.
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