Dual CPU Database Server Comparison

Analyses: IBM DB2 8.1.3 32 bit and 64 bit

Let us summarize what we have learned so far: L3-caches and Hyperthreading offer very limited performance increases. Hyperthreading is essentially free of charge, so we are happy with whatever little bonus we get. However, one can seriously question the wisdom of paying $800 more for a dual Xeon 3.2 GHz with 2 MB L3 instead of one with 1 MB L3-cache. Of course, it is a small amount when compared to license and labour costs, but still...

However, we must not be blind to the possible limitations of our benchmarks. The limited effect of the large caches made us think. Did we randomize a little too well? Often, some rows will be requested far more often than others. For example, if you have a forum, the most recent messages/threads will be accessed much more often than the oldest. If you have an e-commerce site, the items appearing on the front page or on major pages will be loaded much more often. While our real world queries do not access all rows equally, more research will be needed to see closer how our randomize function could mimick real world behavior.

Still, we don't expect spectacular improvements, as the benchmark scales well with CPU clock speed.

64 bit offers a very decent improvement (12% - 16%), although it might be less than what some reports speculated.

We certainly didn't expect Nocona to do so well. SQL databases access data in the memory sometimes randomly, courtesy of, for example, linked lists, and processing a single SQL database query requires parsing and checking the query, optimising it, and ordering results for output, which is very branch and integer intensive, similar to interpreting programming languages. The Nocona Xeon with its extremely long pipeline (and hence, high branch misprediction penalties), slightly higher latency caches (compared to Opteron and previous Xeons) and no L3-cache seemed like the worse architecture that one could think of for database serving.

The new Nocona Xeon surprised us in a positive way, and it outperformed the previous Xeons, even on a clock per clock basis. More in-depth research with CPU counters must give us the exact reasons why, but right now, we can only conclude that the faster memory bus (800 MHz versus 533 MHz) and the twice as large L1- and L2-caches outweigh the fact that it has no L3-cache and a longer pipeline than the previous Xeons. Maybe the improved branch predictor performs miracles, but we are not sure right now.