使用者:LeafDuo/動態內存分配
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在計算機科學中，""動態記憶體分配""（也稱為""堆內存分配""）是一種在程序運行時分配存儲器空間的技術。
In computer science, dynamic memory allocation (also known as heap-based memory allocation) is the allocation of memory storage for use in a computer program during the runtime of that program. It can be seen also as a way of distributing ownership of limited memory resources among many pieces of data and code.
Dynamically allocated memory exists until it is released either explicitly by the programmer, or by the garbage collector. This is in contrast to static memory allocation, which has a fixed duration. It is said that an object so allocated has a dynamic lifetime.
Details
The task of fulfilling an allocation request consists of finding a block of unused memory of sufficient size.
- Problems during fulfilling allocation request
- Internal and external fragmentation.
- Reduction needs special care, thus making implementation more complex (see algorithm efficiency).
- Allocator's metadata can inflate the size of (individually) small allocations;
- Chunking attempts to reduce this effect.
- Internal and external fragmentation.
Usually, memory is allocated from a large pool of unused memory area called the heap (also called the free store). Since the precise location of the allocation is not known in advance, the memory is accessed indirectly, usually via a pointer reference. The precise algorithm used to organize the memory area and allocate and deallocate chunks is hidden behind an abstract interface and may use any of the methods described below.
Efficiency
The dynamic memory allocation algorithm actually used can impact performance significantly and 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)[1]
Implementations
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.
Buddy blocks
In this system, memory is allocated from a large block in memory that is a power of two in size. If the block is more than twice as large as desired, it is broken in two. One of the halves is selected, and the process repeats (checking the size again and splitting if needed) until the block is just large enough.
All the blocks of a particular size are kept in a sorted linked list or tree. When a block is freed, it is compared to its buddy. If they are both free, they are combined and placed in the next-largest size buddy-block list. (When a block is allocated, the allocator will start with the smallest sufficiently large block avoiding needlessly breaking blocks)
See also
External links
Further reading
- "Dynamic Storage Allocation: A Survey and Critical Review", Department of Computer Sciences University of Texas at Austin
References
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- Donald Knuth. Fundamental Algorithms, Third Edition. Addison-Wesley, 1997. ISBN 0-201-89683-4. Section 2.5: Dynamic Storage Allocation, pp. 435–456.
- Simple Memory Allocation Algorithms on OSDEV Community
- Wilson, P.R.; Johnstone, M.S.; Neely, M.; Boles, D. Dynamic Storage Allocation: A Survey and Critical Review. Memory Management: International Workshop, Iwmm'95, Kinross, Uk, September 27-29, 1995: Proceedings (Springer). 1995 [2008-01-06]. ISBN 9783540603689.
- Berger, E.D.; Zorn, B.G.; McKinley, K.S. Composing high-performance memory allocators. ACM SIGPLAN Notices. 2001, 36 (5): 114–124. doi:10.1145/381694.
- Berger, E.D.; Zorn, B.G.; McKinley, K.S. Reconsidering custom memory allocation. Proceedings of the 17th ACM SIGPLAN conference on Object-oriented programming, systems, languages, and applications. ACM Press New York, NY, USA: 1–12. 2002.