Low Power Server CPUs: the energy saving choice?
by Johan De Gelas on July 15, 2010 4:54 AM EST- Posted in
- IT Computing
How useful are low power server CPUs?
We were quite a bit surprised that the lower power CPU did not return any significant energy savings compared to the X5670. Intuition tells you that the best server CPUs like the X5670 only would pay off in a high performance environment (for example an HPC server). But human intuition is a bad guide when dealing with physics. Cold hard measurements are a much better way to make up your mind. And all our measurements point in the same direction: the fastest Xeon offers a superior performance/watt ratio in a typical virtualization consolidation scenario.
You could argue that the X5670 is quite a bit more expensive; a server equipped with a dual X5670 will indeed cost you about $900 more. We admit: our comparison is not completely accurate price wise… as always we work with the CPUs that we got in the lab. But a typical server with these CPUs, 64 GB and some accessories will set you back $9000 easily. The 2.8 GHz Xeon X5560 is hardly 4% slower than the X5670, and will probably show the same favorable performance/watt ratio. And if you place a X5560 2.8 GHz instead of a L5640 2.26 GHz, you only add $200 dollar to a $8k-$9k server. That is peanuts, lost in the noise of the TCO calculation. So the question is real: are the "Low power Xeons" (L-series) useless and should you immediately go for the X-series?
Defeated as it may be, the L5640 can still play one trump card: lower maximum currents. Over the period of our 70 minutes of testing, we decided to take a look at maximum power. To avoid that any extreme peaks would muddle up the picture, we used the 95th percentile.
Let us focus on the “balanced” power plan. The L5640 makes sure power never goes beyond 231 W, while the peak of the X5670 is almost 20% higher. As a result, a rack of low power Xeon will be able to keep the maximum current consumed lower. You could consider the low power Xeon L5640 a “power capped” version of Xeon X5670. In many datacenters you pay a hefty fine if you briefly need more than the amp limit you have agreed upon. So the low power Xeon might save you money by guaranteeing that you never need more than a certain amperage.
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cserwin - Thursday, July 15, 2010 - link
Some props for Johan, too, maybe... nice article.JohanAnandtech - Thursday, July 15, 2010 - link
Thanks! We have more data on "low power choices", but we decided to cut them up in several article to keep it readable.DavC - Thursday, July 15, 2010 - link
not sure whats going on with your electricity cost calcs on your first page. firstly your converting current unnessacarily from watts to amps (meaning your unnessacarily splitting into US and europe figures).basically here in the UK, 1kW which is what your your 4 PCs in your example consume, costs roughly 10p per hour. working on an average of 720 hours in a month, that would give a grand total of £72 a month to run those 4 PCs 24/7.
£72 to you US guys is around $110. And I cant imagine you're electricity is priced any dearer than ours.
giving a 4 year life cycle cost of $5280.
have I missed something obvious here or are you just out with the maths?
JohanAnandtech - Thursday, July 15, 2010 - link
You are calculating from the POV of a datacenter. I take the POV of a datacenter client, which has to pay per amp that he/she "reserves". AFAIK, datacenters almost always count with amps, not Watts.(also 10p per KWh seems low)
MrSpadge - Thursday, July 15, 2010 - link
With P=V*I at constant voltage power and amps are really just a different name for the same thing, i.e. equivalent. Personally I prefer W, because this is what matters in the end: it's what I pay for and what heats my room. Amps by themselves don't mean much (as long as you're not melting the wires), as voltages can easily be converted.Maybe the datacenter guys just like to juggle around smaller numbers? Maybe the should switch over to hecto watts instead? ;)
MrS
JohanAnandtech - Thursday, July 15, 2010 - link
I am surprised the electrical engineers have not jumped in yet :-). As you indicate yourself, the circuits/wires are made for a certain amount of amps, not watts. That is probably the reason datacenters specify the amount of power you get in watt.JohanAnandtech - Thursday, July 15, 2010 - link
I meant amps in that last sentence of course.knedle - Thursday, July 15, 2010 - link
Watts are universal, doesn't matter if you're in UK, or US - 220W is still 220W, but with ampers it's different. Since in the Europe voltage is higher than in the USA (EU=220V, US=110V), and P=U*I, you've got twice as much power for 1A, which means that in USA your server will use 2A, while the same server in UK will use only 1A...has407 - Friday, July 16, 2010 - link
No, not all Watts are the same.Watts in a decent datacenter come with power distribution, cooling, UPS, etc. Those typically add 3-4x to the power your server actually consumes. Add to that the amortized cost of the infrastructure and you're looking at 6-10x the cost of the power your server consumes directly.
Such is the fallacy of simplistic power/cost comparisons (and Johan, you should know better). Can we now dispense with the idiotic cost/KWH calculations?
Penti - Saturday, July 17, 2010 - link
A high-performance server probably can't be used on 1A 230V which is the cheapest options in some datacenters. However something like half a rack or 1/4 would probably have 10A/230V, more then enough for a small servercollection of 4 moderate servers. The big cost is cooling, normal racks might handle 4kW (up to 6kW over that then it's high density) of heat/power just. Then you need more expensive stuff. A cheap rack won't handle 40 250W servers in other regards. 6 kW power/cooling and 2x16A/230V shouldn't be that expensive. Any way you also pay for cooling (and UPS). Even cheap solutions normally charge per used kW here though. 4 2U is about 1/4 rack anyway. And like 15 amps is needed if in the states.