In computer science, Memory Wave Routine shared memory is memory that may be concurrently accessed by multiple applications with an intent to provide communication among them or avoid redundant copies. Shared memory is an environment friendly technique of passing data between packages. Depending on context, programs might run on a single processor or on multiple separate processors. Utilizing memory for communication inside a single program, e.g. among its a number of threads, can be referred to as shared Memory Wave Routine. In computer hardware, shared memory refers to a (typically large) block of random access memory (RAM) that may be accessed by several different central processing items (CPUs) in a multiprocessor pc system. A shared memory system is relatively easy to program since all processors share a single view of knowledge and the communication between processors may be as fast as memory accesses to the same location. Attempting to access close by memory locations may cause false sharing. Shared memory computer systems cannot scale very effectively.

Such cache coherence protocols can, once they work effectively, provide extraordinarily excessive-performance access to shared data between a number of processors. Then again, they will sometimes develop into overloaded and grow to be a bottleneck to efficiency. Technologies like crossbar switches, Omega networks, HyperTransport or front-side bus can be utilized to dampen the bottleneck-results. In case of a Heterogeneous System Structure (processor structure that integrates various kinds of processors, resembling CPUs and GPUs, with shared Memory Wave), the memory management unit (MMU) of the CPU and the input-output memory administration unit (IOMMU) of the GPU should share certain traits, like a standard address house. The alternate options to shared memory are distributed memory and distributed shared memory, every having an identical set of issues. CPUs and the underlying structure shouldn't be cache coherent. IPC by shared memory is used for example to transfer photos between the application and the X server on Unix techniques, or inside the IStream object returned by CoMarshalInterThreadInterfaceInStream in the COM libraries below Home windows.

Dynamic libraries are usually held in memory once and mapped to a number of processes, and only pages that had to be custom-made for the individual process (as a result of an emblem resolved in another way there) are duplicated, often with a mechanism known as copy-on-write that transparently copies the web page when a write is attempted, after which lets the write succeed on the non-public copy. Compared to a number of handle space working systems, memory sharing -- especially of sharing procedures or pointer-primarily based constructions -- is less complicated in single address house working systems. POSIX provides a standardized API for utilizing shared memory, POSIX Shared Memory. POSIX interprocess communication (part of the POSIX:XSI Extension) includes the shared-memory capabilities shmat, shmctl, shmdt and shmget. Unix System V offers an API for shared memory as nicely. This makes use of shmget from sys/shm.h. BSD programs provide "anonymous mapped memory" which could be used by several processes. It stays within the system till explicitly eliminated by a process.

This has a downside in that if the method crashes and fails to clean up shared memory it can stay until system shutdown; that limitation just isn't current in an Android-particular implementation dubbed ashmem. POSIX additionally provides the mmap API for mapping information into memory; a mapping may be shared, permitting the file's contents to be used as shared memory. Linux distributions primarily based on the 2.6 kernel and later offer /dev/shm as shared memory within the type of a RAM disk, extra particularly as a world-writable directory (a listing by which each consumer of the system can create information) that is stored in memory. Both the RedHat and Debian based distributions embody it by default. Support for this sort of RAM disk is totally optionally available inside the kernel configuration file. On Home windows, one can use CreateFileMapping and MapViewOfFile capabilities to map a area of a file into memory in multiple processes. Qt provides the QSharedMemory class. Different programming languages could have their own ways of using these operating amenities for comparable effect. For instance, PHP supplies an API to create shared memory, just like POSIX capabilities. El-Rewini, Hesham; Abd-El-Barr, Mostafa (2005). Advanced Pc Structure and Parallel Processing. Jeffrey S. Chase; Henry M. Levy; Michael J. Feeley; and Edward D. Lazowska. Robbins, Kay A.; Robbins, Steven (2003). Unix methods programming: communication, concurrency, and threads (2 ed.). Prentice Corridor PTR. p. Shared memory facility from the one Unix Specification. Christoph Rohland; Hugh Dickins; KOSAKI Motohiro. Creating Named Shared Memory from MSDN. Shared Memory Introduction, Ch. 12 from guide by Richard Stevens "UNIX Network Programming, Quantity 2, Second Version: Interprocess Communications". SharedHashFile, An open supply, shared memory hash table.