28 Facts About CPU cache

CPU cache is a hardware cache used by the central processing unit of a computer to reduce the average cost to access data from the main memory.

The cache memory is typically implemented with static random-access memory, in modern CPUs by far the largest part of them by chip area, but SRAM is not always used for all levels, or even any level, sometimes some latter or all levels are implemented with eDRAM.

The first CPUs that used a cache had only one level of cache; unlike later level 1 cache, it was not split into L1d and L1i.

The L2 CPU cache is usually not split and acts as a common repository for the already split L1 CPU cache.

L4 CPU cache is currently uncommon, and is generally on dynamic random-access memory, rather than on static random-access memory, on a separate die or chip.

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Alternatively, in a write-back or copy-back CPU cache, writes are not immediately mirrored to the main memory, and the CPU cache instead tracks which locations have been written over, marking them as dirty.

The CPU cache hit rate and the CPU cache miss rate play an important role in determining this performance.

Time taken to fetch one cache line from memory matters because the CPU will run out of things to do while waiting for the cache line.

One benefit of this scheme is that the tags stored in the CPU cache do not have to include that part of the main memory address which is implied by the CPU cache memory's index.

Since the CPU cache tags have fewer bits, they require fewer transistors, take less space on the processor circuit board or on the microprocessor chip, and can be read and compared faster.

An effective memory address which goes along with the CPU cache line is split into the tag, the index and the block offset.

An instruction CPU cache requires only one flag bit per CPU cache row entry: a valid bit.

The first hardware CPU cache used in a computer system was not actually a data or instruction CPU cache, but rather a TLB.

Alternatively, if CPU cache entries are allowed on pages not mapped by the TLB, then those entries will have to be flushed when the access rights on those pages are changed in the page table.

Also, during miss processing, the alternate ways of the CPU cache line indexed have to be probed for virtual aliases and any matches evicted.

Since virtual hints have fewer bits than virtual tags distinguishing them from one another, a virtually hinted CPU cache suffers more conflict misses than a virtually tagged CPU cache.

Cache entry replacement policy is determined by a CPU cache algorithm selected to be implemented by the processor designers.

The victim CPU cache is usually fully associative, and is intended to reduce the number of conflict misses.

The main disadvantage of the trace CPU cache, leading to its power inefficiency, is the hardware complexity required for its heuristic deciding on caching and reusing dynamically created instruction traces.

Smart CPU cache is a level 2 or level 3 caching method for multiple execution cores, developed by Intel.

Furthermore, the shared CPU cache makes it faster to share memory among different execution cores.

Finally, at the other end of the memory hierarchy, the CPU register file itself can be considered the smallest, fastest cache in the system, with the special characteristic that it was scheduled in software—typically by a compiler, as it allocates registers to hold values retrieved from main memory for, as an example, loop nest optimization.

Typically, sharing the L1 CPU cache is undesirable because the resulting increase in latency would make each core run considerably slower than a single-core chip.

An associative CPU cache is more complicated, because some form of tag must be read to determine which entry of the CPU cache to select.

An N-way set-associative level-1 CPU cache usually reads all N possible tags and N data in parallel, and then chooses the data associated with the matching tag.

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Early history of CPU cache technology is closely tied to the invention and use of virtual memory.

Early CPU cache designs focused entirely on the direct cost of CPU cache and RAM and average execution speed.

Multi-ported CPU cache is a CPU cache which can serve more than one request at a time.