David Kanter dissects Haswell

Intel's upcoming Haswell processor is likely to be as big an advance over today's Sandy and Ivy Bridge CPUs as the "Bridge sisters" were over the prior-gen Nehalem chips. That's huge progress, but how could Intel achieve such gains once again?

The answer comes down to three broad areas of change: a nicely improved CPU microarchitecture, design work more carefully tailored to 22-nm fabrication process, and quite likely an improved system architecture. We don't yet have details on the Haswell system architecture, but Intel has disclosed quite a bit about Haswell otherwise. As usual, David Kanter at Real World Tech has put together a very informative write-up covering Haswell's microarchitecture, with some nods to coming changes in the other areas, as well. The article is replete with custom diagrams of the microarchitecture's high-level layout, comparing Haswell directly to Sandy Bridge, and is very much worth taking the time to read carefully.

The scope of changes coming with Haswell may be difficult to absorb if you've been focusing on the breathless talk about 10W variants of the processor. As Kanter notes:

Turning to the microarchitecture, the Haswell core has a modestly larger out-of-order window, with a substantial increase in dispatch ports and execution resources. Together with the ISA extensions, the theoretical FLOPs and integer operations per core have doubled. More significantly, the bandwidth for the cache hierarchy, including the L1D and L2 has doubled, while reducing utilization bottlenecks. Compared to Nehalem, the Haswell core offers 4× the peak FLOPs, 3× the cache bandwidth, and nearly 2× the re-ordering window.

He estimates Haswell will achieve 10% higher performance on existing software—we take that to be a per-clock improvement, with higher frequencies also possible—versus Sandy Bridge. The potential ramps up from there when software takes advantage of the additional FLOPS and integer throughput available with new instructions like fused multiply-add and hardware lock elision.

Interestingly, Kanter doesn't expect the performance gap between Intel and AMD to widen with the coming clash between Haswell and Steamroller. His reason? "Realistically, the performance gap should narrow given the scope of opportunities for AMD to improve, but Haswell will continue to have significant advantages." Hmm. Time will tell if he's right about that.

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