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In computer science, shared memory is memory that may be simultaneously accessed by a number of applications with an intent to offer communication among them or keep away from redundant copies. Shared memory is an efficient technique of passing information between programs. Depending on context, applications may run on a single processor or on a number of separate processors. Utilizing memory for communication inside a single program, e.g. amongst its multiple threads, MemoryWave Community is also referred to as shared memory. In laptop hardware, shared memory refers to a (typically giant) block of random access memory (RAM) that may be accessed by several different central processing units (CPUs) in a multiprocessor computer system. A shared memory system is relatively simple to program since all processors share a single view of data and the communication between processors could be as quick as memory accesses to the identical location. Trying to access close by memory places might cause false sharing. Shared memory computer systems can't scale very properly.

Such cache coherence protocols can, when they work properly, present extremely high-performance access to shared info between multiple processors. Then again, they will typically become overloaded and grow to be a bottleneck to performance. Technologies like crossbar switches, Omega networks, HyperTransport or front-facet bus can be utilized to dampen the bottleneck-effects. In case of a Heterogeneous System Architecture (processor structure that integrates different types of processors, similar to CPUs and GPUs, with shared memory), the memory management unit (MMU) of the CPU and the enter-output memory administration unit (IOMMU) of the GPU must share certain traits, like a common tackle space. The alternatives to shared memory are distributed memory and distributed shared memory, each having an analogous set of issues. CPUs and the underlying architecture is not cache coherent. IPC by shared memory is used for instance to switch images between the applying and the X server on Unix techniques, Memory Wave or inside the IStream object returned by CoMarshalInterThreadInterfaceInStream within the COM libraries below Windows.

Dynamic libraries are generally held in memory once and mapped to multiple processes, and solely pages that needed to be custom-made for the person course of (as a result of an emblem resolved in another way there) are duplicated, usually with a mechanism known as copy-on-write that transparently copies the web page when a write is tried, after which lets the write succeed on the private copy. Compared to a number of handle house operating programs, memory sharing -- especially of sharing procedures or pointer-based structures -- is simpler in single tackle area operating methods. POSIX gives a standardized API for utilizing shared memory, POSIX Shared Memory. POSIX interprocess communication (a part of the POSIX:XSI Extension) contains the shared-memory functions shmat, shmctl, shmdt and shmget. Unix System V offers an API for shared memory as effectively. This makes use of shmget from sys/shm.h. BSD techniques provide "anonymous mapped memory" which might be used by several processes. It stays within the system till explicitly removed by a course of.

This has a drawback in that if the method crashes and fails to scrub up shared memory it is going to keep until system shutdown; that limitation just isn't current in an Android-specific implementation dubbed ashmem. POSIX additionally gives the mmap API for mapping information into memory; a mapping can be shared, allowing the file's contents for use as shared memory. Linux distributions based mostly on the 2.6 kernel and later supply /dev/shm as shared memory in the form of a RAM disk, extra particularly as a world-writable directory (a listing during which every consumer of the system can create information) that's saved in memory. Both the RedHat and Debian primarily based distributions embody it by default. Assist for this sort of RAM disk is totally elective within the kernel configuration file. On Windows, one can use CreateFileMapping and MapViewOfFile functions to map a region of a file into memory in a number of processes. Qt offers the QSharedMemory class. Other programming languages may have their very own methods of utilizing these working facilities for related effect. For instance, Memory Wave PHP offers an API to create shared memory, just like POSIX functions. El-Rewini, Hesham; Abd-El-Barr, Mostafa (2005). Superior Laptop Structure and Parallel Processing. Jeffrey S. Chase; Henry M. Levy; Michael J. Feeley; and Edward D. Lazowska. Robbins, Kay A.; Robbins, Steven (2003). Unix techniques programming: communication, concurrency, and threads (2 ed.). Prentice Corridor PTR. p. Shared memory facility from the only Unix Specification. Christoph Rohland; Hugh Dickins; KOSAKI Motohiro. Creating Named Shared Memory from MSDN. Shared Memory Introduction, Ch. 12 from e-book by Richard Stevens "UNIX MemoryWave Community Programming, Volume 2, Second Edition: Interprocess Communications". SharedHashFile, An open supply, shared memory hash desk.