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* Offline storage is not immediately available, and requires some human intervention to bring online.
For example, always-on spinning disks are online, while spinning disks that spin down, such as massive
Most modern [[Central processing unit|CPUs]] are so fast that, for most program workloads, the [[wikt:bottleneck|bottleneck]] is the [[locality of reference]] of memory accesses and the efficiency of the [[CPU cache|caching]] and memory transfer between different levels of the hierarchy{{Citation needed|date=September 2009}}. As a result, the CPU spends much of its time idling, waiting for memory I/O to complete. This is sometimes called the ''space cost'', as a larger memory object is more likely to overflow a small and fast level and require use of a larger, slower level. The resulting load on memory use is known as ''pressure'' (respectively ''register pressure'', ''cache pressure'', and (main) ''memory pressure''). Terms for data being missing from a higher level and needing to be fetched from a lower level are, respectively: [[register spilling]] (due to [[register pressure]]: register to cache), [[cache miss]] (cache to main memory), and (hard) [[page fault]] (''real'' main memory to ''virtual'' memory, i.e. mass storage, commonly referred to as ''disk'' regardless of the actual mass storage technology used).
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