This is a snapshot of the frame latency picture; it's the point below which 99% of all frames have been rendered. We're simply excluding the last 1% of frames, many of them potential outliers, to get a sense of overall smoothness.
Again, the Intel processors perform well. All but one of them render the great majority of frames in under 23 milliseconds, which translates to a steady-state frame rate of just under 50 FPS. There is some reshuffling in the move from FPS average to a latency-sensitive metricthe "big iron" Core i7-3820 with its large cache and quad memory channels moves up the ranks, for instancebut the changes are what one might expect, given the hardware in question.
Meanwhile, the AMD FX processors suffer in this comparison. The FX-8150, which is ostensibly AMD's top-of-the-line desktop processor, trails two older Phenom IIs and the FX-4170. The FX-6200 falls behind the A8-3850, a budget APU based on AMD's prior CPU microarchitecture. The absolute numbers aren't stellar, either. The FX processors are cranking out 99% of the frames in 33 milliseconds or so, which translates to a steady rate of 30 FPSmuch lower than even the slower Intel processors.
What's the problem? The broader latency curve suggests some answers.
The "tail" of the curve for the AMD processors is telling. Although the FX chips keep pace with the Phenom II X6 1100T in the first 95% or so of frames rendered, their frame times spike upward to meet the slower A8-3850 budget APU and Phenom II X4 850 in the last ~5% of frames. In the most crucial function of gaming performance, latency avoidance, the more expensive FX processors essentially perform like low-end CPUs.
Why? I think the answer is suggested by the relatively strong performance of the FX-4170 compared to the FX-6200 and FX-8150. As we noted, the FX-4170 actually has the highest base and Turbo clock speeds of the FX lineup. That means it likely has the highest per-thread performance of any FX chip, and that appears to translate into better latency mitigation. (I also suspect the FX-4170 spends more time operating near its peak Turbo speed, since it only has to fit two "modules" and four cores into the same 125W thermal envelope as the higher-end FX chips.)
Looks to me like the FX CPUs have an Amdahl's Law problem. Even though they have a relatively large amount of cores for their given product segments, their per-thread performance is fairly weak. The Bulldozer architecture combines relatively low instruction throughput per cycle with clock speeds that aren't as high as AMD probably anticipated. That adds up to modest per-thread performanceand even with lots of cores on hand, the execution speed of a single thread can limit an application's throughput.
Thus, the FX-6200 and FX-8150 processors aren't as well-suited to our Skyrim test scenario as their predecessors in the Phenom II lineup. Only the FX-4170 outperforms the CPU it replaces, the Phenom II X4 850, whose lack of L3 cache and modest 3.3GHz clock frequency aren't doing it any favors.
