Testing the latest x86 rack servers and low power server CPUs

Performance/Watt: Choosing the CPU

Let us make sense of the all the data we have seen so far. We start with the wretches and work our way up to the champions.

At the current pricing (July 2009), the Opteron 2384 through 2389 do not make any sense. You pay more than for a Xeon L5520 2.26GHz and you get - at best - the same performance while consuming 50W more running at full speed, and consuming 30W more in idle. AMD should really lower the pricing of the higher clocked Shanghais.

The "basic" range of Xeons (E5506, E5504, and E5502) does not make much sense either if you are looking for a good performance/watt ratio. These Xeons only support DDR3-800, QuickPath speed is lower (4.8GT/s instead of 5.86GT/s), and these CPUs have no support for Hyper-Threading and Turbo Boost. Combine that with the fact that these CPUs are "high leakage" parts and you'll understand that these CPUs should be avoided. Even Intel advises against buying these chips. Well, if you read between the lines….

Some like to portray the Xeon X55xx series as power guzzling CPUs. However, these are high performance CPUs that surprised us with modest power requirements. In virtualization environments, you will want to load up the machine with a lot more memory. The result is that the Xeon X5570 has the best performance/watt ratio of the industry, if you are looking for top performance and your application has little idle time (i.e. a 24/7 or worldwide server). The downside is that this is the most expensive two socket CPU. Is $1300 per CPU too much? Well, that depends on what you will be running on this CPU. Microsoft and Oracle licenses have this tendency to make even the most expensive CPUs look cheap by comparison.

The six-core Opterons make a lot more sense than their quad-core Opterons when you are looking for high performance and want to keep power in check. Just keep in mind that AMD's six-core is strong in two areas: decision support databases and virtualization. Our current virtualization test with 16 virtual CPUs even slows down the six-core a little; make sure the number of vCPUs can be divided by six and you will probably squeeze another 5% out of AMD's best Opteron. The six-core Opteron servers can also leverage the advantage of their cheaper infrastructure (DDR2, motherboard, already very mature platform) and are thus the choice of price sensitive buyers. The downside is that although the idle power consumption is excellent compared to the quad-core CPUs, it is still rather high compared to Intel platforms. We are looking forward to adding the six-core Opteron HE 2425HE 2.1GHz (55W ACP - 79 W TDP) to our tests, but maybe we should wait until AMD's low power platform comes available.

The Opteron EE is surely not for SMEs looking for a low power server: $700 per CPU is not what we would call a good deal. However, it performs decently and has a pretty low "full load" power consumption. If your application never runs idle and your data center is mostly power limited, the Opteron EE makes a strong case. In others words, the Opteron EE will mostly go to the "Facebooks, Netlogs, and Googles" of this world. They will get huge price reductions as they order in large volumes. The Achilles' heel of this low power CPU is the fact that it is not capable of shutting down cores completely and as a result consumes more than its archrival the L5520 when running idle.

If you are looking at the overall picture (price, performance/watt, and idle power), the Xeon L5520 2.26GHz is the best CPU of this test. Sure, it cannot beat its X5570 brother when it comes to "pure" performance/watt, but most applications run idle for long periods (nighttime for example). When running idle, the power consumption of the CPUs is as good as negligible and depends solely on the quality of your motherboard and PSU. Performance is also excellent, at (least at) the level of a quad-core Opteron at 2.9GHz. Only if performance is less important and your application is rarely running at idle might the slightly higher power consumption at full load sway the decision towards the Opteron EE.