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e counting 2 4 memory pools 3 systems with virtual memory 4 memory management in burroughs unisys mcp systems 2 5 memory management in os 360 and successors 6 see also 7 notes 8 references 9 bibliography 10 external links toggle the table of contents memory management 28 languages العربية català čeština deutsch español فارسی suomi français עברית hrvatski italiano 日本語 қазақша 한국어 bahasa melayu norsk bokmål polski português русский shqip српски srpski தமிழ் ไทย türkçe українська tiếng việt 粵語 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia computer memory management methodology memory allocation redirects here for memory allocation in the brain see neuronal memory allocation this article is about memory management in an address space for management of physical memory see memory management operating systems this article includes a list of general references but lacks sufficient corresponding inline citations please help improve this article by introducing more precise citations april 2014 learn how and when to remove this message operating systems common features process management interrupts memory management file system device drivers networking security input output v t e 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 1 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 2 and os 360 and successors 3 memory is managed by the operating system note 1 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 edit main article manual memory management an example of external fragmentation 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 note 2 of memory called the heap note 3 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 note 4 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 edit 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 1 implementations edit 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 edit main article memory pool 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 edit further information buddy memory allocation 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 edit main article slab allocation this memory allocation mechanism preallocates memory chunks suitable to fit objects of a certain type or size 5 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 edit main article stack based memory 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 6 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 7 gnulib provides an equivalent interface albeit instead of throwing an seh exception on overflow it delegates to malloc when an overlarge size is detected 8 a similar feature can be emulated using manual accounting and size checking such as in the uses of alloca_account in glibc 9 automated memory management edit the proper management of memory in an application is a difficult problem and several different strategies for handling memory management have been devised automatic management of call stack variables edit see also automatic variable and call stack in many programming language implementations the runtime environment for the program automatically allocates memory in the call stack for non static local variables of a subroutine called automatic variables when the subroutine is called and automatically releases that memory when the subroutine is exited special declarations may allow local variables to retain values between invocations of the procedure or may allow local variables to be accessed by other subroutines the automatic allocation of local variables makes recursion possible to a depth limited by available memory garbage collection edit main article garbage collection computer science garbage collection is a strategy for automatically detecting memory allocated to objects that are no longer usable in a program and returning that allocated memory to a pool of free memory locations this method is in contrast to manual memory management where a programmer explicitly codes memory requests and memory releases in the program while automatic garbage collection has the advantages of reducing programmer workload and preventing certain kinds of memory allocation bugs garbage collection does require memory resources of its own and can compete with the application program for processor time reference counting edit main article reference counting reference counting is a strategy for detecting that memory is no longer usable by a program by maintaining a counter for how many independent pointers point to the memory whenever a new pointer points to a piece of memory the programmer is supposed to increase the counter when the pointer changes where it points or when the pointer is no longer pointing to any area or has itself been freed the counter should decrease when the counter drops to zero the memory should be considered unused and freed some reference counting systems require programmer involvement and some are implemented automatically by the compiler a disadvantage of reference counting is that circular references can develop which cause a memory leak to occur this can be mitigated by either adding the concept of a weak reference a reference that does not participate in reference counting but is notified when the area it is pointing to is no longer valid or by combining reference counting and garbage collection together memory pools edit main article region based memory management a memory pool is a technique of automatically deallocating memory based on the state of the application such as the lifecycle of a request or transaction the idea is that many applications execute large chunks of code which may generate memory allocations but that there is a point in execution where all of those chunks are known to be no longer valid for example in a web service after each request the web service no longer needs any of the memory allocated during the execution of the request therefore rather than keeping track of whether or not memory is currently being referenced the memory is allocated according to the request or lifecycle stage with which it is associated when that request or stage has passed all associated memory is deallocated simultaneously systems with virtual memory edit main articles memory protection virtual memory and shared memory virtual memory is a method of decoupling the memory organization from the physical hardware the applications operate on memory via virtual addresses each attempt by the application to access a particular virtual memory address results in the virtual memory address being translated to an actual physical address 10 in this way the addition of virtual memory enables granular control over memory systems and methods of access in virtual memory systems the operating system limits how a process can access the memory this feature called memory protection can be used to disallow a process to read or write to memory that is not allocated to it preventing malicious or malfunctioning code in one program from interfering with the operation of another even though the memory allocated for specific processes is normally isolated processes sometimes need to be able to share information shared memory is one of the fastest techniques for inter process communication memory is usually classified by access rate into primary storage and secondary storage memory management systems among other operations also handle the moving of information between these two levels of memory memory management in burroughs unisys mcp systems 2 edit an operating system manages various resources in the computing system the memory subsystem is the system element for managing memory the memory subsystem combines the hardware memory resource and the mcp os software that manages the resource the memory subsystem manages the physical memory and the virtual memory of the system both part of the hardware resource the virtual memory extends physical memory by using extra space on a peripheral device usually disk the memory subsystem is responsible for moving code and data between main and virtual memory in a process known as overlaying burroughs was the first commercial implementation of virtual memory although developed at manchester university for the ferranti atlas computer and integrated virtual memory with the system design of the b5000 from the start in 1961 needing no external memory management unit mmu 11 48 the memory subsystem is responsible for mapping logical requests for memory blocks to physical portions of memory segments which are found in the list of free segments each allocated block is managed by means of a segment descriptor 12 a special control word containing relevant metadata about the segment including address length machine type and the p bit or presence bit which indicates whether the block is in main memory or needs to be loaded from the address given in the descriptor descriptors are essential in providing memory safety and security so that operations cannot overflow or underflow the referenced block commonly known as buffer overflow descriptors themselves are protected control words that cannot be manipulated except for specific elements of the mcp os enabled by the unsafe block directive in newp donald knuth describes a similar system in section 2 5 dynamic storage allocation of fundamental algorithms disputed discuss memory management in os 360 and successors edit ibm system 360 does not support virtual memory note 5 memory isolation of jobs is optionally accomplished using protection keys assigning storage for each job a different key 0 for the supervisor or 1 15 memory management in os 360 ...