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Low-Power Server CPUs Don’t Save Energy

Low Power Server CPUs: the energy saving choice? – AnandTech. Must reading for IT datacenter planners.

This Anandtech research concludes:

1. Low power L-series Xeons save power but do not save energy in a typical Hyper-v consolidation ratio. Power is “capped”, but the total energy consumed for a certain task is (more or less) the same.

2. High power X-series Xeons offer a much better performance per watt ratio, but at the expense of brief power peaks. They do not necessarily need more energy in the long run than the lower power versions, and offer much better response times if your application is CPU bound.

The workload used for the tests consists of ‘“one tile” of the vApus benchmark on each of tested servers. Such a tile consists of a OLAP database (4 vCPUs), an OLTP database (4 vCPUs) and two web VMs (2 vCPUs). So in total we have 12 virtual CPUs. These 12 virtual CPUs are much less than what a typical high-end dual CPU server can offer.’ While this may not represent anybody’s actual workload, it is a good test for a single-socket, 1U commodity server running virtualized modern web-oriented workloads.

The microprocessors are from the recent Westmere generation of Intel server chips.  The high power test used the X5670 6-core/12 thread and the low power test used the L5640. Using the Balanced Performance setting in Windows Server, the high-power Xeon uses about 2% more energy than the low-power chip to complete the benchmark, a difference that can be ignored by most datacenters. But the winning traits for the high-power server Xeon is 24% better throughput and response times. Throughput and response times are frequent SLA variables, so the high-power X5670 ends up being highly attractive. Put simply, as Intel has been preaching, the faster X5670 gets the work done quicker and goes to idle, where power consumption is reduced.

The low-power server chips cap power use, allowing a datacenter to avoid excess electricity penalties. However, power usage caps are available today in software such as VMware’s vSphere. So even the sole differentiator of the L-series low-power chips may not be a real-world benefit in most virtualized datacenters.

My walkaway from the Anandtech research is that it pays to use fast multi-core Intel Xeon processors, and that overall energy use is not a significant detractor to using them.


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