A Three-Level Cache Hierarchy
The memory hierarchy of Conroe was extremely simple and Intel was able to concentrate on the performance of the shared L2 cache, which was the best solution for an architecture that was aimed mostly at dual-core implementations. But with Nehalem, the engineers started from scratch and came to the same conclusions as their competitors: a shared L2 cache was not suited to a native quad-core architecture. The different cores can too frequently flush data needed by another core and that surely would have involved too many problems in terms of internal buses and arbitration to provide all four cores with sufficient bandwidth while keeping latency sufficiently low. To solve the problem, the engineers provided each core with a Level 2 cache of its own. Since it’s dedicated to a single core and relatively small (256 KB), the engineers were able to endow it with very high performance; latency, in particular, has reportedly improved significantly over Penryn—from 15 cycles to approximately 10 cycles.
Then comes an enormous Level 3 cache memory (8 MB) for managing communications between cores. While at first glance Nehalem’s cache hierarchy reminds one of Barcelona, the operation of the Level 3 cache is very different from AMD’s—it’s inclusive of all lower levels of the cache hierarchy. That means that if a core tries to access a data item and it’s not present in the Level 3 cache, there’s no need to look in the other cores’ private caches—the data item won’t be there either. Conversely, if the data are present, four bits associated with each line of the cache memory (one bit per core) show whether or not the data are potentially present (potentially, but not with certainty) in the lower-level cache of another core, and which one.
This technique is effective for ensuring the coherency of the private caches because it limits the need for exchanges between cores. It has the disadvantage of wasting part of the cache memory with data that is already in other cache levels. That’s somewhat mitigated, however, by the fact that the L1 and L2 caches are relatively small compared to the L3 cache—all the data in the L1 and L2 caches takes up a maximum of 1.25 MB out of the 8 MB available. As on Barcelona, the Level 3 cache doesn’t operate at the same frequency as the rest of the chip. Consequently, latency of access to this level is variable, but it should be in the neighborhood of 40 cycles.
The only real disappointment with Nehalem’s new cache hierarchy is its L1 cache. The bandwidth of the instruction cache hasn’t been increased—it’s still 16 bytes per cycle compared to 32 on Barcelona. This could create a bottleneck in a server-oriented architecture since 64-bit instructions are larger than 32-bit ones, especially since Nehalem has one more decoder than Barcelona, which puts that much more pressure on the cache. As for the data cache, its latency has increased to four cycles compared to three on the Conroe, facilitating higher clock frequencies. To end on a positive note, though, the engineers at Intel have increased the number of Level 1 data cache misses that the architecture can process in parallel.
