Christoph Rohland; Hugh Dickins; KOSAKI Motohiro

In computer science, shared memory is memory that could be simultaneously accessed by multiple applications with an intent to offer communication amongst them or keep away from redundant copies. Shared memory is an environment friendly technique of passing knowledge between applications. Depending on context, packages may run on a single processor or on a number of separate processors. Using memory for communication inside a single program, e.g. among its a number of threads, is also known as shared memory. In computer hardware, shared memory refers to a (usually large) block of random entry memory (RAM) that may be accessed by a number of totally different central processing models (CPUs) in a multiprocessor computer system. A shared memory system is comparatively straightforward to program since all processors share a single view of knowledge and the communication between processors can be as quick as memory accesses to the identical location. Attempting to access nearby memory places might cause false sharing. Shared memory computers can not scale very well.



Such cache coherence protocols can, after they work well, present extremely excessive-performance entry to shared data between a number of processors. Alternatively, they will generally become overloaded and change into a bottleneck to performance. Applied sciences like crossbar switches, Omega networks, HyperTransport or entrance-aspect bus can be utilized to dampen the bottleneck-results. In case of a Heterogeneous System Structure (processor structure that integrates various kinds of processors, akin to CPUs and GPUs, with shared memory), the memory administration unit (MMU) of the CPU and the enter-output memory management unit (IOMMU) of the GPU need to share certain characteristics, like a typical deal with space. The options to shared memory are distributed memory and distributed shared memory, every having an identical set of issues. CPUs and the underlying structure isn't cache coherent. IPC by shared memory is used for example to transfer photographs between the applying and the X server on Unix methods, or inside the IStream object returned by CoMarshalInterThreadInterfaceInStream in the COM libraries under Home windows.



Dynamic libraries are generally held in memory as soon as and mapped to a number of processes, and solely pages that needed to be personalized for the person process (as a result of an emblem resolved in a different way there) are duplicated, normally with a mechanism often called copy-on-write that transparently copies the page when a write is tried, and then lets the write succeed on the non-public copy. In comparison with a number of tackle area working programs, memory sharing -- particularly of sharing procedures or pointer-based mostly structures -- is less complicated in single tackle house working programs. POSIX gives a standardized API for utilizing shared memory, POSIX Shared Memory. POSIX interprocess communication (a part of the POSIX:XSI Extension) consists of the shared-memory functions shmat, shmctl, shmdt and shmget. Unix System V provides an API for shared memory as nicely. This uses shmget from sys/shm.h. BSD techniques provide "anonymous mapped memory" which will be used by several processes. It stays within the system until explicitly eliminated by a course of.



This has a downside in that if the process crashes and fails to wash up shared memory it'll stay until system shutdown; that limitation is not present in an Android-specific implementation dubbed ashmem. POSIX also provides the mmap API for mapping information into Memory Wave System; a mapping may be shared, allowing the file's contents for use as shared memory. Linux distributions primarily based on the 2.6 kernel and later offer /dev/shm as shared memory in the type of a RAM disk, more specifically as a world-writable listing (a listing by which each person of the system can create recordsdata) that's stored in memory. Each the RedHat and Debian primarily based distributions include it by default. Support for this type of RAM disk is totally non-compulsory inside the kernel configuration file. On Home windows, one can use CreateFileMapping and MapViewOfFile functions to map a region of a file into memory in a number of processes. Qt supplies the QSharedMemory class. Different programming languages may have their very own ways of utilizing these working amenities for comparable effect. For Memory Wave example, PHP offers an API to create shared memory, just like POSIX features. 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: Memory Wave System communication, concurrency, and Memory Wave threads (2 ed.). Prentice Corridor PTR. p. Shared memory facility from the single Unix Specification. Christoph Rohland; Hugh Dickins; KOSAKI Motohiro. Creating Named Shared Memory from MSDN. Shared Memory Introduction, Ch. 12 from ebook by Richard Stevens "UNIX Network Programming, Volume 2, Second Version: Interprocess Communications". SharedHashFile, An open supply, shared memory hash table.