X-Gene 1, Atom C2000 and Xeon E3: Exploring the Scale-Out Server World

The War of the SoCs: Performance/Watt

As we mentioned earlier, it is not that easy to determine the performance per watt of the different SoCs. Depending on the motherboard feature richness, performance per watt can vary a lot. We tested the Xeon E3-1240 v3 only on the feature-rich ASUS P9D, while the Atom C2750 is on a very efficient and simple HP m300 cartridge. Given the discrepancies, we cannot simply divide the performance by the power consumption and called it a day. No, we have to do a few calculations to get a good estimate of the performance/watt.

The current idle power of modern Intel CPUs is so low that it is almost irrelevant. All cores but one are put in a deep sleep (power gating), and the one that is still active runs at a very low clock and voltage. We have found that a Xeon E3-1200 v2's (Ivy Bridge) idle power is around 3W, perhaps even less... it is very hard and time consuming to measure correctly. We know from the mobile device reviews that the Haswell idle power is even lower. The Atom core is simpler, but the sleep states are slightly less advanced. Regardless, whether a CPU consumes 1.7W or 2.2W idling is not relevant for our calculation.

If we take the delta between idle power of a system and full load, and add about 3W idle, we're probably very close to the real power consumption of an Intel CPU. The only noise is the loss of the power supply (low because these are highly efficient ones) and the fact that the voltage regulators and DRAM consume a little more at higher load. Again, we are talking about very low numbers.

In the case of the Xeon E3, we also add about 3W for the Intel C224 chipset (0.7W idle, 4.1W TDP). For the X-Gene, we may assume that the idle power is a lot higher. When we calculated the power of the different components (8 DIMMs, disabled 10 GbE, etc.), we estimate that it is about 10W.

For the total system power, the power consumption of one node, we take the m300 numbers as measured. We subtract 7W from the m400 numbers as the m400 has four extra DIMM slots and a 10 GbE NIC. We add 9W to the SoC power of the Xeon E3 as we have found out that 12W is more or less the power that a Xeon E3 node consumes without the SoC.

Let's discuss our findings. The Xeon E3-1240 v3 consumes probably about 50W with a high web load and is nowhere near its TDP (80W). The Xeon E3-1265L v2 (45W TDP) and Xeon E3-1230 (25W TDP) consume probably slightly more than their advertised TDP. That is slightly worrying as an integer workload that raises the CPU load to about 85-90% is not the worst situation you can imagine.... a 100% FPU load will go far beyond the TDP numbers then. The Atom C2750 requires the least power.

We are not pretending that our calculations are 100% accurate, but they should be close enough. At the end of the day, a couple Watts more or less is not going to change our conclusion that the Xeon E3-1230L v3 and Xeon E3-1240 v3 are the most efficient processors for these workloads. The Xeon E3-1230L v3 wins because it will require less cooling and less electricity distribution infrastructure using the same dense servers.

The Atom wins if you are power limited but the power efficiency is a bit lower when it comes to serving up a web infrastructure. Lastly, the X-Gene 1 has some catching up to do. The X-Gene 2 promises to be 50% more efficient. The software optimization efforts could bridge the rest of the gap, but we don't have a crystal ball.