Core 2 Duo: That Can Hardly Be More Optimized

In this article I am to find out how well Core 2 Duo and K8 perform on the single-processed benchmarks, SPECint and SPECfp (i.e., no "rate" here), as some interesting observations come up again. A look at the facts is never short of revelation.

The systems are shown in the following table. K8int/K8fp denote K8 Opteron scores for integer and floating point benchmarks, respectively. Similarly, C2int/C2fp denote Core 2 Duo scores. Both SPEC CPU2000 and SPEC CPU2006 are compared. The main criteria in choosing these SPEC submissions are -

1. There are at least two or more processor speeds with all identical configurations otherwise.
2. They use 64-bit operating systems and compilers.
3. All have comparable memory across different architectures (DDR2-667 to be exact).

Unfortunately, I couldn't find a K8 and a C2D using the same operating system and compiler. Thus strictly speaking the absolute values in these tests are not comparable across system families. We will relax ourselves a bit here but keep this fact in mind.

Below are the SPEC CPU2000 results. All points are the "base" scores -

Below are the SPEC CPU2006 results. The points with a postfix 'B' character on their labels represent "base" scores; the other points represent the "peak" scores -

In terms of SPEC CPU2000, Core 2 Duo completely outclasses Opteron/K8 on both INT (50%) and FP (30%) scores. Interestingly, the vast advantage greatly reduces with respect to SPEC CPU2006, where the leads become less than 30% for INT and almost none for FP. One explanation is that Core 2 Duo, released 3 years after Opteron, was optimized by design for the benchmarks, at a time when only SPEC CPU2000 was available. Another explanation is the newer SPEC CPU2006 does not benefit from large L2 cache size (up to at least 4MB) as much and thus more favors K8's integrated memory controller. Yet another explanation is that the newer benchmark codes are more complex and thus less predictable by simple heuristics where Core 2 probably does better/more than K8.

No matter what are the reasons (probably a bit from all three and more), one message is clear: for single-processed integer codes, Core 2 Duo beats K8 Opteron hands-down. For floating point, it's a close match, and one should look at the type of program he runs to make a preference.

The really interesting observation lies on the "peak" versus "base" values of the benchmark results. For Core 2 Duo, peak offers just 4% boost on INT and 3% on FP. On the other hand, for K8 Opteron, peak offers 8% boost on INT and almost 30% on FP. It seems the microarchitecture of Core 2 Duo is so optimizing that there is little room for more software optimization, whereas K8 Opteron still can benefit from better compilation. This is certainly a plus for Core 2 Duo, because nobody likes to spend 2x time to compile an optimized executable.

Comparing the SPEC and SPEC_rate results, we clearly see that while Core 2 Duo has a much better core implementation, its memory architecture trails after K8 and drags down its throughput scalability. The FSB bottleneck can even be seen from the Core 2 Duo lines in the second graph above, where the two left-most point sets (with 1066MHz FSB) are much lower than the others (with 1333MHz FSB). Again, as I said, with Core 2 Duo, Intel goes back to its root to improve, market on, and profit from the personal/home (versus big server/high performance) computing.