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Posted 21 March 2010 - 10:42 AM
Another issue is the fundamental tradeoff between cache latency and hit rate. Larger caches have better hit rates but longer latency. To address this tradeoff, many computers use multiple levels of cache, with small fast caches backed up by larger slower caches.
Multi-level caches generally operate by checking the smallest Level 1 (L1) cache first; if it hits, the processor proceeds at high speed. If the smaller cache misses, the next larger cache (L2) is checked, and so on, before external memory is checked.
As the latency difference between main memory and the fastest cache has become larger, some processors have begun to utilize as many as three levels of on-chip cache. For example, the Alpha 21164 (1995) had a 96 KB on-die L3 cache; the IBM POWER4 (2001) had a 256 MB L3 cache off-chip, shared among several processors; the Itanium 2 (2003) had a 6 MB unified level 3 (L3) cache on-die; Intel's Xeon MP product code-named "Tulsa" (2006) features 16 MB of on-die L3 cache shared between two processor cores; the AMD Phenom II (2008) has up to 6 MB on-die unified L3 cache; and the Intel Core i7 (2008) has an 8 MB on-die unified L3 cache that is inclusive, shared by all cores. The benefits of an L3 cache depend on the application's access patterns.
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 is scheduled in software—typically by a compiler, as it allocates registers to hold values retrieved from main memory. (See especially loop nest optimization.) Register files sometimes also have hierarchy: The Cray-1 (circa 1976) had 8 address "A" and 8 scalar data "S" registers that were generally usable. There was also a set of 64 address "B" and 64 scalar data "T" registers that took longer to access, but were faster than main memory. The "B" and "T" registers were provided because the Cray-1 did not have a data cache. (The Cray-1 did, however, have an instruction cache.)
Edited by cryptodan, 21 March 2010 - 10:45 AM.
Posted 21 March 2010 - 10:48 AM
Edited by kitotz, 21 March 2010 - 10:54 AM.
Posted 21 March 2010 - 11:57 AM
Edited by dpunisher, 21 March 2010 - 11:57 AM.
I am a retired Ford tech. Next to Fords, any computer is a piece of cake. (The cake, its not a lie)
3770K @4.5, Corsair H100, GTX780, 16gig Samsung, Obsidian 700 (yes there is a 700)
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