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Memory hierarchy
In computer architecture, the memory hierarchy separates computer storage into a hierarchy based on response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by their performance and controlling technologies. Memory hierarchy affects performance in computer architectural design, algorithm predictions, and lower level programming constructs involving locality of reference.
Designing for high performance requires considering the restrictions of the memory hierarchy, i.e. the size and capabilities of each component. Each of the various components can be viewed as part of a hierarchy of memories (m1, m2, ..., mn) in which each member mi is typically smaller and faster than the next highest member mi+1 of the hierarchy. To limit waiting by higher levels, a lower level will respond by filling a buffer and then signaling for activating the transfer.
There are four major storage levels.
This is a general memory hierarchy structuring. Many other structures are useful. For example, a paging algorithm may be considered as a level for virtual memory when designing a computer architecture, and one can include a level of nearline storage between online and offline storage.
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.
Some CPUs include additional levels of cache between L3 and memory. For example, the Haswell microarchitecture includes an L4 cache of 128 MB on mobile units.
The lower levels of the hierarchy – from mass storage downwards – are also known as tiered storage. The formal distinction between online, nearline, and offline storage is:
For example, always-on spinning disks are online, while spinning disks that spin down, such as massive arrays of idle disk (MAID), are nearline. Removable media such as tape cartridges that can be automatically loaded, as in a tape library, are nearline, while cartridges that must be manually loaded are offline.
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Memory hierarchy
In computer architecture, the memory hierarchy separates computer storage into a hierarchy based on response time. Since response time, complexity, and capacity are related, the levels may also be distinguished by their performance and controlling technologies. Memory hierarchy affects performance in computer architectural design, algorithm predictions, and lower level programming constructs involving locality of reference.
Designing for high performance requires considering the restrictions of the memory hierarchy, i.e. the size and capabilities of each component. Each of the various components can be viewed as part of a hierarchy of memories (m1, m2, ..., mn) in which each member mi is typically smaller and faster than the next highest member mi+1 of the hierarchy. To limit waiting by higher levels, a lower level will respond by filling a buffer and then signaling for activating the transfer.
There are four major storage levels.
This is a general memory hierarchy structuring. Many other structures are useful. For example, a paging algorithm may be considered as a level for virtual memory when designing a computer architecture, and one can include a level of nearline storage between online and offline storage.
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.
Some CPUs include additional levels of cache between L3 and memory. For example, the Haswell microarchitecture includes an L4 cache of 128 MB on mobile units.
The lower levels of the hierarchy – from mass storage downwards – are also known as tiered storage. The formal distinction between online, nearline, and offline storage is:
For example, always-on spinning disks are online, while spinning disks that spin down, such as massive arrays of idle disk (MAID), are nearline. Removable media such as tape cartridges that can be automatically loaded, as in a tape library, are nearline, while cartridges that must be manually loaded are offline.