Don Guinn-3 wrote: > > [...] >> This touches on a question I have been wondering about. Is the cache >> shared > among multiple cores or does each one have its own cache? I would be > surprised if it is shared as that could result in a big performance hit if > there are lots of cores. If the cache is private to each core then somehow > the data has to be moved from one cache to another. Main memory? > Your question is that of the cache coherency, namely how hardware handles writes of a core to a memory location that is also cached by another core. While the trend is towards having cache-coherent multicore systems. It simplifies programming while complicating hardware/microcode to take care that the writes are updated without accessing the main memory. The issue coherent vs incoherent cache(s) is as far as I know not completely settled, since incoherent caches are easier to implement and they do provide benefits in situations when cores access independent memory locations, which of course complicates programming and may lead to serious performance hits in situations as the one you describe, but then it might be much easier to scale up to a large number of cores (say 128 or some such number?).
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