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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, which is particularly the case with CPUs in a multiprocessing system. In the illustration on the right, consider both the clients have a cached copy of a particular memory block from a previous read. Suppose the client on the bottom updates/changes that memory block, the client on the top could be left with an invalid cache of memory without any notification of the change. Cache coherence is intended to manage such conflicts by maintaining a coherent view of the data values in multiple caches. Overview In a shared memory multiprocessor system with a separate cache memory for each processor, it is possible to have many copies of shared data: one copy in the main memory and one in the local cache of each processor that requested it. When on ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cache Coherency Generic
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, Utah * Cache Peak (Idaho), a mountain in Castle Rocks State Park Other places * Cache, Aosta, a frazione in Italy * Cache Creek (other), several places Arts, entertainment and media * ''Caché'' (album), a 1993 album by Kirk Whalum * ''Caché'' (film), a 2005 film directed by Michael Haneke Science and technology * Cache (biology) or hoarding, a food storing behavior of animals * Cache (computing), a collection of data duplicating original values stored elsewhere on a computer, usually for easier access * InterSystems Caché, a database management system from InterSystems Other uses * Cache (archaeology), artifacts purposely buried in the ground * Geocaching, an outdoor treasure-hunting game which involves looking for co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
MESI Protocol
The MESI protocol is an Invalidate-based cache coherence protocol, and is one of the most common protocols that support write-back caches. It is also known as the Illinois protocol (due to its development at the University of Illinois at Urbana-Champaign). Write back caches can save a lot of bandwidth that is generally wasted on a write through cache. There is always a dirty state present in write back caches that indicates that the data in the cache is different from that in main memory. The Illinois Protocol requires a cache to cache transfer on a miss if the block resides in another cache. This protocol reduces the number of main memory transactions with respect to the MSI protocol. This marks a significant improvement in performance. States The letters in the acronym MESI represent four exclusive states that a cache line can be marked with (encoded using two additional bits): ;Modified (M): The cache line is present only in the current cache, and is ''dirty'' - it has been ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Memory Barrier
In computing, a memory barrier, also known as a membar, memory fence or fence instruction, is a type of barrier instruction that causes a central processing unit (CPU) or compiler to enforce an ordering constraint on memory operations issued before and after the barrier instruction. This typically means that operations issued prior to the barrier are guaranteed to be performed before operations issued after the barrier. Memory barriers are necessary because most modern CPUs employ performance optimizations that can result in out-of-order execution. This reordering of memory operations (loads and stores) normally goes unnoticed within a single thread of execution, but can cause unpredictable behavior in concurrent programs and device drivers unless carefully controlled. The exact nature of an ordering constraint is hardware dependent and defined by the architecture's memory ordering model. Some architectures provide multiple barriers for enforcing different ordering constrai ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Directory-based Coherence
Directory-based coherence is a mechanism to handle Cache coherence problem in Distributed shared memory (DSM) a.k.a. Non-Uniform Memory Access (NUMA). Another popular way is to use a special type of computer bus between all the nodes as a "shared bus" (a.k.a. System bus). Directory-based coherence uses a special directory to serve instead of the shared bus in the bus-based coherence protocols. Both of these designs use the corresponding medium (i.e. directory or bus) as tool to facilitate the communication between different nodes, and to guarantee that the coherence protocol is working properly along all the communicating nodes. In directory based cache coherence, this is done by using this directory to keep track of the status of all cache blocks, the status of each block includes in which cache coherence " state" that block is, and which nodes are sharing that block at that time, which can be used to eliminate the need to broadcast all the signals to all nodes, and only send i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
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 reading, writing, or updating memory will be predictable. Consistency models are used in distributed systems like distributed shared memory systems or distributed data stores (such as filesystems, databases, optimistic replication systems or web caching). Consistency is different from coherence, which occurs in systems that are cached or cache-less, and is consistency of data with respect to all processors. Coherence deals with maintaining a global order in which writes to a single location or single variable are seen by all processors. Consistency deals with the ordering of operations to multiple locations with respect to all processors. High level languages, such as C++ and Java, maintain the consistency contract by translating memory o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Arm Ltd
Arm is a British semiconductor and software design company based in Cambridge, England. Its primary business is in the design of ARM processors (CPUs). It also designs other chips, provides software development tools under the DS-5, RealView and Keil brands, and provides systems and platforms, system-on-a-chip (SoC) infrastructure and software. As a "holding" company, it also holds shares of other companies. Since 2016, it has been owned by Japanese conglomerate SoftBank Group. While ARM CPUs first appeared in the Acorn Archimedes, a desktop computer, today's systems include mostly embedded systems, including ARM CPUs used in virtually all smartphones. Systems such as iPhones and Android smartphones frequently include many chips, from many different providers, that include one or more licensed Arm cores, in addition to those in the main Arm-based processor. Arm's core designs are also used in chips that support all the most common network-related technologies. Process ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
System On A Chip
A system on a chip or system-on-chip (SoC ; pl. ''SoCs'' ) is an integrated circuit that integrates most or all components of a computer or other electronic system. These components almost always include a central processing unit (CPU), memory interfaces, on-chip input/output devices, input/output interfaces, and secondary storage interfaces, often alongside other components such as radio modems and a graphics processing unit (GPU) – all on a single substrate or microchip. It may contain digital, analog, mixed-signal, and often radio frequency signal processing functions (otherwise it is considered only an application processor). Higher-performance SoCs are often paired with dedicated and physically separate memory and secondary storage (such as LPDDR and eUFS or eMMC, respectively) chips, that may be layered on top of the SoC in what's known as a package on package (PoP) configuration, or be placed close to the SoC. Additionally, SoCs may use separate wireless m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
ARM Ltd
Arm is a British semiconductor and software design company based in Cambridge, England. Its primary business is in the design of ARM processors (CPUs). It also designs other chips, provides software development tools under the DS-5, RealView and Keil brands, and provides systems and platforms, system-on-a-chip (SoC) infrastructure and software. As a "holding" company, it also holds shares of other companies. Since 2016, it has been owned by Japanese conglomerate SoftBank Group. While ARM CPUs first appeared in the Acorn Archimedes, a desktop computer, today's systems include mostly embedded systems, including ARM CPUs used in virtually all smartphones. Systems such as iPhones and Android smartphones frequently include many chips, from many different providers, that include one or more licensed Arm cores, in addition to those in the main Arm-based processor. Arm's core designs are also used in chips that support all the most common network-related technologies. Process ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dragon Protocol
The Dragon Protocol is an update based cache coherence protocol used in multi-processor systems. Write propagation is performed by directly updating all the cached values across multiple processors. Update based protocols such as the Dragon protocol perform efficiently when a write to a cache block is followed by several reads made by other processors, since the updated cache block is readily available across caches associated with all the processors. States Each cache block resides in one of the four states: exclusive-clean, shared-clean, shared-modified and modify. * Exclusive-clean (E): This means that the cache block was first fetched by the current processor and has not been accessed by any other processor since. * Shared clean (Sc): This means that the cache block definitely exists in multiple processor’s caches, and that the current processor is not the last one to write the block. States E and Sc are maintained separately by the protocol to prevent read-write operations ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Firefly (cache Coherence Protocol)
The Firefly cache coherence protocol is the schema used in the DEC Firefly multiprocessor workstation, developed by DEC Systems Research Center. This protocol is a 3 State Write Update Cache Coherence Protocol. Unlike the Dragon protocol, the Firefly protocol updates the Main Memory as well as the Local caches on Write Update Bus Transition. Thus the Shared Clean and Shared Modified States present in case of Dragon Protocol, are not distinguished between in case of Firefly Protocol. States In this protocol, the following states can be assigned to each block: * Valid-Exclusive(V): The cache block is valid, clean and only resides in one cache. * Shared(S): The cache block is valid, clean and may reside in multiple caches. * Dirty(D): The block is the only copy of the memory and it is dirty i.e. its value has been modified since being brought from the memory. This is the only state that generates a write-back when the block is replaced in the cache. These states correspond to the '' ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Write-once (cache Coherence)
In cache coherency protocol literature, Write-Once was the first MESI protocol defined. It has the optimization of executing write-through on the first write and a write-back on all subsequent writes, reducing the overall bus traffic in consecutive writes to the computer memory. It was first described by James R. Goodman in (1983). Cache coherence protocols are an important issue in Symmetric multiprocessing systems, where each CPU maintains a cache of the memory. States In this protocol, each block in the local cache is in one of these four states: * Invalid: This block has an incoherent copy of the memory. * Valid: This block has a coherent copy of the memory. The data may be possibly shared, but its content is not modified. * Reserved: The block is the only copy of the memory, but it is still coherent. No write-back is needed if the block is replaced. * Dirty: The block is the only copy of the memory and it is incoherent. This copy was written one or more times. This is the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
MESIF Protocol
The MESIF protocol is a cache coherency and memory coherence protocol developed by Intel for cache coherent non-uniform memory architectures. The protocol consists of five states, Modified (M), Exclusive (E), Shared (S), Invalid (I) and Forward (F). The M, E, S and I states are the same as in the MESI protocol. The F state is a specialized form of the S state, and indicates that a cache should act as a designated responder for any requests for the given line. The protocol ensures that, if any cache holds a line in the S state, at most one (other) cache holds it in the F state. In a system of caches employing the MESI protocol, a cache line request that is received by multiple caches holding a line in the S state will be serviced inefficiently. It may either be satisfied from (slow) main memory, or ''all'' the sharing caches could respond, bombarding the requestor with redundant responses. In a system of caches employing the MESIF protocol, a cache line request will be responded ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
