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Memory hierarchy of an AMD Bulldozer server. The number of levels in the memory hierarchy and the performance at each level has increased over time. The type of memory or storage components also change historically. [6] For example, the memory hierarchy of an Intel Haswell Mobile [7] processor circa 2013 is:
Cache hierarchy, or multi-level cache, is a memory architecture that uses a hierarchy of memory stores based on varying access speeds to cache data. Highly requested data is cached in high-speed access memory stores, allowing swifter access by central processing unit (CPU) cores.
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. [2]
Data locality is a typical memory reference feature of regular programs (though many irregular memory access patterns exist). It makes the hierarchical memory layout profitable. In computers, memory is divided into a hierarchy in order to speed up data accesses. The lower levels of the memory hierarchy tend to be slower, but larger.
The gap between processor speed and main memory speed has grown exponentially. Until 2001–05, CPU speed, as measured by clock frequency, grew annually by 55%, whereas memory speed only grew by 7%. [1] This problem is known as the memory wall. The motivation for a cache and its hierarchy is to bridge this speed gap and overcome the memory wall.
In practice, almost all computers use a storage hierarchy, [1]: 468–473 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 [a] technologies are referred to as "memory", while slower persistent technologies are referred to as "storage".
A model, called Concurrent-AMAT (C-AMAT), is introduced for more accurate analysis of current memory systems. More information on C-AMAT can be found in the external links section. AMAT's three parameters hit time (or hit latency), miss rate, and miss penalty provide a quick analysis of memory systems. Hit latency (H) is the time to hit in the ...
Consider an example of a two level cache hierarchy where L2 can be inclusive, exclusive or NINE of L1. Consider the case when L2 is inclusive of L1. Consider the case when L2 is inclusive of L1. Suppose there is a processor read request for block X.