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Shared memory

Shared memory

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In computing
Computing
Computing is usually defined as the activity of using and improving computer hardware and software. It is the computer-specific part of information technology...

, shared memory is memory
Random-access memory
Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...

 that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies. Depending on context, programs may run on a single processor or on multiple separate processors. Using memory for communication inside a single program, for example among its multiple threads
Thread (computer science)
In computer science, a thread of execution is the smallest unit of processing that can be scheduled by an operating system. The implementation of threads and processes differs from one operating system to another, but in most cases, a thread is contained inside a process...

, is generally not referred to as shared memory.

In hardware


In computer hardware, shared memory refers to a (typically) large block of random access memory (RAM) that can be accessed by several different central processing unit
Central processing unit
The central processing unit is the portion of a computer system that carries out the instructions of a computer program, to perform the basic arithmetical, logical, and input/output operations of the system. The CPU plays a role somewhat analogous to the brain in the computer. The term has been in...

s (CPUs) in a multiple-processor computer system
Multiprocessing
Multiprocessing is the use of two or more central processing units within a single computer system. The term also refers to the ability of a system to support more than one processor and/or the ability to allocate tasks between them...

.

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:
  • CPU-to-memory connection becomes a bottleneck. Shared memory computers cannot scale very well. Most of them have ten or fewer processors.
  • Cache coherence
    Cache coherence
    In computing, cache coherence refers to the consistency of data stored in local caches of a shared resource.When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data. This is particularly true of CPUs in a multiprocessing system...

    : Whenever one cache is updated with information that may be used by other processors, the change needs to be reflected to the other processors, otherwise the different processors will be working with incoherent data (see cache coherence
    Cache coherence
    In computing, cache coherence refers to the consistency of data stored in local caches of a shared resource.When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data. This is particularly true of CPUs in a multiprocessing system...

     and memory coherence
    Memory coherence
    Memory coherence is an issue that affects the design of computer systems in which two or more processors or cores share a common area of memory....

    ). Such coherence protocols can, when they work well, provide extremely high-performance access to shared information between multiple processors. On the other hand they can sometimes become overloaded and become a bottleneck to performance


Technologies like crossbar switches
Crossbar switch
In electronics, a crossbar switch is a switch connecting multiple inputs to multiple outputs in a matrix manner....

, Omega networks
Omega Network
An Omega network is a network configuration often used in parallel computing architectures. It is an indirect topology that relies on the perfect shuffle interconnection algorithm.-Connection Architecture:...

, HyperTransport
HyperTransport
HyperTransport , formerly known as Lightning Data Transport , is a technology for interconnection of computer processors. It is a bidirectional serial/parallel high-bandwidth, low-latency point-to-point link that was introduced on April 2, 2001...

 or Front-side bus can be used to dampen the bottleneck-effects.

The alternatives to shared memory are distributed memory
Distributed memory
In computer science, distributed memory refers to a multiple-processor computer system in which each processor has its own private memory. Computational tasks can only operate on local data, and if remote data is required, the computational task must communicate with one or more remote processors...

 and distributed shared memory
Distributed shared memory
Distributed Shared Memory , in Computer Architecture is a form of memory architecture where the memories can be addressed as one address space...

, each having a similar set of issues. See also Non-Uniform Memory Access
Non-Uniform Memory Access
Non-Uniform Memory Access is a computer memory design used in Multiprocessing, where the memory access time depends on the memory location relative to a processor...

.

In software


In computer software, shared memory is either
  • a method of inter-process communication
    Inter-process communication
    In computing, Inter-process communication is a set of methods for the exchange of data among multiple threads in one or more processes. Processes may be running on one or more computers connected by a network. IPC methods are divided into methods for message passing, synchronization, shared...

     (IPC), i.e. a way of exchanging data between programs running at the same time. One process
    Process (computing)
    In computing, a process is an instance of a computer program that is being executed. It contains the program code and its current activity. Depending on the operating system , a process may be made up of multiple threads of execution that execute instructions concurrently.A computer program is a...

     will create an area in RAM
    Random-access memory
    Random access memory is a form of computer data storage. Today, it takes the form of integrated circuits that allow stored data to be accessed in any order with a worst case performance of constant time. Strictly speaking, modern types of DRAM are therefore not random access, as data is read in...

     which other processes can access, or
  • a method of conserving memory space by directing accesses to what would ordinarily be copies of a piece of data to a single instance instead, by using virtual memory
    Virtual memory
    In computing, virtual memory is a memory management technique developed for multitasking kernels. This technique virtualizes a computer architecture's various forms of computer data storage , allowing a program to be designed as though there is only one kind of memory, "virtual" memory, which...

     mappings or with explicit support of the program in question. This is most often used for shared libraries and for XIP
    Execute in place
    In computer science, execute in place is a method of executing programs directly from long term storage rather than copying it into RAM. It is an extension of using shared memory to reduce the total amount of memory required....

    .


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 pipe
Named pipe
In computing, a named pipe is an extension to the traditional pipe concept on Unix and Unix-like systems, and is one of the methods of inter-process communication. The concept is also found in Microsoft Windows, although the semantics differ substantially...

s, Unix domain socket
Unix domain socket
A Unix domain socket or IPC socket is a data communications endpoint for exchanging data between processes executing within the same host operating system. While similar in functionality to...

s or CORBA
Çorba
Chorba , ciorbă , shurpa , shorpo , or sorpa is one of various kinds of soup or stew found in national cuisines across Middle East...

). On the other hand, it is less powerful, as for example the communicating processes must be running on the same machine (whereas other IPC methods can use a computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....

), 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
Cache coherence
In computing, cache coherence refers to the consistency of data stored in local caches of a shared resource.When clients in a system maintain caches of a common memory resource, problems may arise with inconsistent data. This is particularly true of CPUs in a multiprocessing system...

.

IPC by shared memory is used for example to transfer images between the application and the X server
X Window System
The X window system is a computer software system and network protocol that provides a basis for graphical user interfaces and rich input device capability for networked computers...

 on Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...

 systems, or inside the IStream object returned by CoMarshalInterThreadInterfaceInStream in the COM libraries under Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

.

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 that transparently copies the page when a write is attempted, and then lets the write succeed on the private copy.

Specific implementations


POSIX
POSIX
POSIX , an acronym for "Portable Operating System Interface", is a family of standards specified by the IEEE for maintaining compatibility between operating systems...

 provides a standardized API for using shared memory, POSIX Shared Memory. This uses the function shm_open from sys/mman.h. POSIX interprocess communication (part of the POSIX:XSI Extension) includes the shared-memory functions shmat, shmctl, shmdt and shmget.

Unix System 5 provides an API for shared memory as well. This uses shmget from sys/shm.h.

BSD systems provide "anonymous mapped memory" which can be used by several processes.

Recent 2.6 Linux kernel builds have started to offer /dev/shm as shared memory in the form of a RAM disk
RAM disk
A RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...

, more specifically as a world-writable directory that is stored in memory. /dev/shm support is completely optional within the kernel configuration file
Configuration file
In computing, configuration files, or config files configure the initial settings for some computer programs. They are used for user applications, server processes and operating system settings. The files are often written in ASCII and line-oriented, with lines terminated by a newline or carriage...

. Both the Fedora
Fedora (operating system)
Fedora is a RPM-based, general purpose collection of software, including an operating system based on the Linux kernel, developed by the community-supported Fedora Project and sponsored by Red Hat...

 and Ubuntu
Ubuntu (operating system)
Ubuntu is a computer operating system based on the Debian Linux distribution and distributed as free and open source software. It is named after the Southern African philosophy of Ubuntu...

 distributions include it by default.

See also

  • Distributed shared memory
    Distributed shared memory
    Distributed Shared Memory , in Computer Architecture is a form of memory architecture where the memories can be addressed as one address space...

  • Global variable
    Global variable
    In computer programming, a global variable is a variable that is accessible in every scope . Interaction mechanisms with global variables are called global environment mechanisms...

  • Nano-threads
    Nano-threads
    In computer science nano-threads are highly optimized lightweight threads designed for use on shared memory multiprocessors . The Nano-threads specification was written in 1997....

  • Shared Memory Architecture
    Shared Memory Architecture
    In computer architecture, Shared Memory Architecture refers to a design where the graphics chip does not have its own dedicated memory, and instead shares the main system RAM with the CPU and other components....

    , where the graphics chip uses main system RAM

External links