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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 o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cache Coherency
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 one ... [...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] |
Bus (computing)
In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This expression covers all related hardware components (wire, optical fiber, etc.) and software, including communication protocols. Early computer buses were parallel electrical wires with multiple hardware connections, but the term is now used for any physical arrangement that provides the same logical function as a parallel electrical busbar. Modern computer buses can use both parallel and bit serial connections, and can be wired in either a multidrop (electrical parallel) or daisy chain topology, or connected by switched hubs, as in the case of Universal Serial Bus (USB). Background and nomenclature Computer systems generally consist of three main parts: * The central processing unit (CPU) that processes data, * The memory that holds the p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Computer Storage
Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a computer is what manipulates data by performing computations. In practice, almost all computers use a storage hierarchy, which puts fast but expensive and small storage options close to the CPU and slower but less expensive and larger options further away. Generally, the fast volatile technologies (which lose data when off power) are referred to as "memory", while slower persistent technologies are referred to as "storage". Even the first computer designs, Charles Babbage's Analytical Engine and Percy Ludgate's Analytical Machine, clearly distinguished between processing and memory (Babbage stored numbers as rotations of gears, while Ludgate stored numbers as displacements of rods in shuttles). This distinction was extended in the Von Neumann a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
James R
James is a common English language surname and given name: *James (name), the typically masculine first name James * James (surname), various people with the last name James James or James City may also refer to: People * King James (other), various kings named James * Saint James (other) * James (musician) * James, brother of Jesus Places Canada * James Bay, a large body of water * James, Ontario United Kingdom * James College, a college of the University of York United States * James, Georgia, an unincorporated community * James, Iowa, an unincorporated community * James City, North Carolina * James City County, Virginia ** James City (Virginia Company) ** James City Shire * James City, Pennsylvania * St. James City, Florida Arts, entertainment, and media * ''James'' (2005 film), a Bollywood film * ''James'' (2008 film), an Irish short film * ''James'' (2022 film), an Indian Kannada-language film * James the Red Engine, a character in ''Thomas the Tank En ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Symmetric Multiprocessing
Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all input and output devices, and are controlled by a single operating system instance that treats all processors equally, reserving none for special purposes. Most multiprocessor systems today use an SMP architecture. In the case of multi-core processors, the SMP architecture applies to the cores, treating them as separate processors. Professor John D. Kubiatowicz considers traditionally SMP systems to contain processors without caches. Culler and Pal-Singh in their 1998 book "Parallel Computer Architecture: A Hardware/Software Approach" mention: "The term SMP is widely used but causes a bit of confusion. ..The more precise description of what is intended by SMP is a shared memory multiprocessor where the cost of accessing a memory location ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Central Processing Unit
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions in the program. This contrasts with external components such as main memory and I/O circuitry, and specialized processors such as graphics processing units (GPUs). The form, design, and implementation of CPUs have changed over time, but their fundamental operation remains almost unchanged. Principal components of a CPU include the arithmetic–logic unit (ALU) that performs arithmetic and logic operations, processor registers that supply operands to the ALU and store the results of ALU operations, and a control unit that orchestrates the fetching (from memory), decoding and execution (of instructions) by directing the coordinated operations of the ALU, registers and other co ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
CPU Cache
A CPU cache is a hardware cache used by the central processing unit (CPU) of a computer to reduce the average cost (time or energy) to access data from the main memory. A cache is a smaller, faster memory, located closer to a processor core, which stores copies of the data from frequently used main memory locations. Most CPUs have a hierarchy of multiple cache levels (L1, L2, often L3, and rarely even L4), with different instruction-specific and data-specific caches at level 1. The cache memory is typically implemented with static random-access memory (SRAM), in modern CPUs by far the largest part of them by chip area, but SRAM is not always used for all levels (of I- or D-cache), or even any level, sometimes some latter or all levels are implemented with eDRAM. Other types of caches exist (that are not counted towards the "cache size" of the most important caches mentioned above), such as the translation lookaside buffer (TLB) which is part of the memory management unit (MMU) w ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Write-Once Transition Diagram , computer storage that can be written to once, but read from multiple times
{{disambig ...
Write once may refer to: * Write once, run anywhere, a slogan for the cross-platform benefits of Java * Write once, compile anywhere, a slogan for the cross-platform benefits of C * Write-once (cache coherency), a write-invalidate protocol in computer memory design * Write once read many Write once read many (WORM) describes a data storage device in which information, once written, cannot be modified. This write protection affords the assurance that the data cannot be tampered with once it is written to the device, excluding the p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Bus Sniffing
Bus snooping or bus sniffing is a scheme by which a coherency controller (snooper) in a cache (a snoopy cache) monitors or snoops the bus transactions, and its goal is to maintain a cache coherency in distributed shared memory systems. A cache containing a coherency controller (snooper) is called a snoopy cache. This scheme was introduced by Ravishankar and Goodman in 1983. How it works When specific data is shared by several caches and a processor modifies the value of the shared data, the change must be propagated to all the other caches which have a copy of the data. This change propagation prevents the system from violating cache coherency. The notification of data change can be done by bus snooping. All the snoopers monitor every transaction on a bus. If a transaction modifying a shared cache block appears on a bus, all the snoopers check whether their caches have the same copy of the shared block. If a cache has a copy of the shared block, the corresponding snooper performs ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
