Memory coherence is an issue that affects the design of
computer system
A computer is a machine that can be programmed to carry out sequences of arithmetic or logical operations ( computation) automatically. Modern digital electronic computers can perform generic sets of operations known as programs. These prog ...
s in which two or more
processor
Processor may refer to:
Computing Hardware
* Processor (computing)
**Central processing unit (CPU), the hardware within a computer that executes a program
*** Microprocessor, a central processing unit contained on a single integrated circuit (I ...
s or
cores share a common area of
memory
Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
.
In a
uniprocessor system (whereby, in today's terms, there exists only one core), there is only one
processing element
This glossary of computer hardware terms is a list of definitions of terms and concepts related to computer hardware, i.e. the physical and structural components of computers, architectural issues, and peripheral devices.
A
...
doing all the work and therefore only one processing element that can read or write from/to a given memory location. As a result, when a value is changed, all subsequent read operations of the corresponding memory location will see the updated value, even if it is
cache
Cache, caching, or caché may refer to:
Places United States
* Cache, Idaho, an unincorporated community
* Cache, Illinois, an unincorporated community
* Cache, Oklahoma, a city in Comanche County
* Cache, Utah, Cache County, Utah
* Cache County ...
d.
Conversely, in
multiprocessor
Multiprocessing is the use of two or more central processing units (CPUs) within a single computer system. The term also refers to the ability of a system to support more than one processor or the ability to allocate tasks between them. There ar ...
(or
multicore
A multi-core processor is a microprocessor on a single integrated circuit with two or more separate processing units, called cores, each of which reads and executes program instructions. The instructions are ordinary CPU instructions (such a ...
) systems, there are two or more processing elements working at the same time, and so it is possible that they simultaneously access the same memory location. Provided none of them changes the data in this location, they can share it indefinitely and cache it as they please. But as soon as one updates the location, the others might work on an out-of-date copy that, e.g., resides in their local cache. Consequently, some scheme is required to notify all the processing elements of changes to shared values; such a scheme is known as a memory coherence protocol, and if such a protocol is employed the system is said to have a coherent memory.
The exact nature and meaning of the memory coherency is determined by the
consistency model
In computer science, a consistency model specifies a contract between the programmer and a system, wherein the system guarantees that if the programmer follows the rules for operations on memory, memory will be consistent and the results of read ...
that the coherence protocol implements. In order to write correct concurrent programs, programmers must be aware of the exact consistency model that is employed by their systems.
When implemented in hardware, the coherency protocol can, for example, be
directory-based or
snooping-based (also called ''sniffing''). Specific protocols include the
MSI protocol
In computing, the MSI protocol - a basic cache-coherence protocol - operates in multiprocessor systems. As with other cache coherency protocols, the letters of the protocol name identify the possible states in which a cache line can be.
Overv ...
and its derivatives
MESI,
MOSI and
MOESI
In Roman literature of the early 1st century CE, the Moesi ( or ; grc, Μοισοί, ''Moisoí'' or Μυσοί, ''Mysoí''; lat, Moesi or ''Moesae'') appear as a Paleo-Balkan people who lived in the region around the River Timok to the south ...
.
See also
*
Cache coherence
In computer architecture, cache coherence is the uniformity of shared resource data that ends up stored in multiple local caches. When clients in a system maintain caches of a common memory resource, problems may arise with incoherent data, wh ...
*
Distributed shared memory
In computer science, distributed shared memory (DSM) is a form of memory architecture where physically separated memories can be addressed as a single shared address space. The term "shared" does not mean that there is a single centralized memor ...
*
Race condition
A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is Sequential logic, dependent on the sequence or timing of other uncontrollable events. It becomes a software ...
References
{{DEFAULTSORT:Memory Coherence
Computer memory
Parallel computing