| 词条 | Shared memory |
| 释义 |
In computer science, shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Shared memory is an efficient means of passing data between programs. Depending on context, programs may run on a single processor or on multiple separate processors. Using memory for communication inside a single program, e.g. among its multiple threads, is also referred to as shared memory. In hardwareIn computer hardware, shared memory refers to a (typically large) block of random access memory (RAM) that can be accessed by several different central processing units (CPUs) in a multiprocessor computer system. Shared memory systems may use:[1]
A shared memory system is relatively easy to program since all processors share a single view of data and the communication between processors can be as fast as memory accesses to a same location. The issue with shared memory systems is that many CPUs need fast access to memory and will likely cache memory, which has two complications:
Technologies like crossbar switches, Omega networks, HyperTransport or front-side bus can be used to dampen the bottleneck-effects. In case of a Heterogeneous System Architecture (processor architecture that integrates different types of processors, such as CPUs and GPUs, with shared memory), the memory management unit (MMU) of the CPU and the input–output memory management unit (IOMMU) of the GPU have to share certain characteristics, like a common address space. The alternatives to shared memory are distributed memory and distributed shared memory, each having a similar set of issues. In softwareIn computer software, shared memory is either
Since both processes can access the shared memory area like regular working memory, this is a very fast way of communication (as opposed to other mechanisms of IPC such as named pipes, Unix domain sockets or CORBA). On the other hand, it is less scalable, as for example the communicating processes must be running on the same machine (of other IPC methods, only Internet domain sockets—not Unix domain sockets—can use a computer network), and care must be taken to avoid issues if processes sharing memory are running on separate CPUs and the underlying architecture is not cache coherent. IPC by shared memory is used for example to transfer images between the application and the X server on Unix systems, or inside the IStream object returned by CoMarshalInterThreadInterfaceInStream in the COM libraries under Windows. Dynamic libraries are generally held in memory once and mapped to multiple processes, and only pages that had to be customized for the individual process (because a symbol resolved differently there) are duplicated, usually with a mechanism known as copy-on-write that transparently copies the page when a write is attempted, and then lets the write succeed on the private copy. Support on Unix-like systemsPOSIX provides a standardized API for using shared memory, POSIX Shared Memory. This uses the function The shared memory created by POSIX also provides the Linux distributions based on the 2.6 kernel and later offer /dev/shm as shared memory in the form of a RAM disk, more specifically as a world-writable directory (a directory in which every user of the system can create files) that is stored in memory. Both the RedHat and Debian based distributions include it by default. Support for this type of RAM disk is completely optional within the kernel configuration file.[5] Support on WindowsOn Windows, one can use Cross-platform supportSome C++ libraries provide a portable and object-oriented access to shared memory functionality. For example, Boost contains the Boost.Interprocess C++ Library[7] and Qt provides the QSharedMemory class.[8] Programming language supportThere is native support for shared memory also in programming languages besides C/C++. For example, PHP provides an API to create shared memory, similar to POSIX functions.[9] See also
References1. ^{{cite book| last1 = El-Rewini| first1 = Hesham| last2 = Abd-El-Barr| first2 = Mostafa| title = Advanced Computer Architecture and Parallel Processing| publisher = Wiley-Interscience| year = 2005| isbn = 978-0-471-46740-3| pages = 77–80}} 2. ^Documentation of shm_open from the Single Unix Specification 3. ^{{cite book| last1 = Robbins| first1 = Kay A.| last2 = Robbins| first2 = Steven| title = Unix systems programming: communication, concurrency, and threads| url = https://books.google.com/books?id=tdsZHyH9bQEC| accessdate = 2011-05-13| edition = 2| year = 2003| publisher = Prentice Hall PTR| isbn = 978-0-13-042411-2| page = 512| quote = The POSIX interprocess communication (IPC) is part of the POSIX:XSI Extension and has its origin in Unix System V interprocess communication.}} 4. ^Shared memory facility from the Single Unix Specification. 5. ^{{cite web| url = https://www.kernel.org/doc/Documentation/filesystems/tmpfs.txt| title = tmpfs.txt|author1=Christoph Rohland |author2=Hugh Dickins |author3=KOSAKI Motohiro | accessdate = 2010-03-16| publisher = kernel.org}} 6. ^Creating Named Shared Memory from MSDN. 7. ^Boost.Interprocess C++ Library 8. ^{{cite web|url=http://doc.qt.io/archives/qt-4.8/qsharedmemory.html|title=QSharedMemory Class Reference}} 9. ^Shared Memory Functions in PHP-API External links
6 : Computer architecture|Memory management|Inter-process communication|Concurrent computing|Parallel computing|Distributed computing architecture |
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