Xeon 2.8GHz DP & 2.0GHz MP - Part I: Taking over the Enterprise

Performance: Asking the Right Questions

Now that you know all of the specs of the CPUs and the platforms, let's talk about performance. With so many different CPUs and platforms it is easy to confuse what's the best and as always, you're left with the feeling that the manufacturer is going to recommend the most expensive as being your heavenly option.

Before we start asking performance questions, let's look at the situation we have with the Xeon DP and MP systems from the perspective of a database server application.

On the one hand we have the Xeon DP processor that runs at very high clock speeds and has a fairly good sized on-die L2 cache (512KB). But, does the 40% clock speed advantage outweigh the presence of a large L3 cache on the Xeon MP?

As large as a 2MB L3 cache is, you're not going to be able to fit any reasonable database into that small of a cache; we'd need a 2GB cache for that. Instead, the presence of a large L3 cache helps reduce memory latency significantly. Remember that caching improves overall performance not by attempting to store everything in cache, but by taking what's used most frequently and putting it in a location much closer to the CPU. Along with putting more frequently used data in cache, a larger cache will allow the CPU to have more data that's adjacent to frequently used data in cache as well. Based on the principal of locality, we know that frequently used data tends to be surrounded in memory by data that will also be used in the near future; a large on-die L3 cache let's us store more of this data on the CPU as well. Since database servers are very memory and I/O intensive you would expect any reduction in overall data access latency to help performance tremendously, but again the question is whether that reduction in latency is enough to hide the clock speed penalty.

Then there's the question of going beyond two processors; with the Xeon DP you're stuck to a maximum of two CPUs, but with Xeon MP you can very easily expand to four. How much of a performance gain will you get from moving to four CPUs and is it worth the added cost and additional licensing fees?

On top of all of these concerns we have yet another variable to consider, Hyper-Threading. Does the presence of Hyper-Threading diminish the need for more than two CPUs? And how is Hyper-Threading performance impacted by the decrease in clock speed and increase in cache size offered by the Xeon MP? Remember that Hyper-Threading does increasingly better at higher clock speeds, and it is also most beneficial during times of idle execution. However the Xeon MP runs at lower clock speeds than the Xeon DP and its on-die L3 cache reduces the periods of idle execution, both working against Hyper-Threading.

These are the questions we hoped to answer when we started working on this review weeks ago, but as you're about to see, answering them was much more difficult than you'd expect.