RealTime IT News

Memory: The Overlooked Power Issue

There has been considerably emphasis on the subject of power efficiency and microprocessors, mostly because Intel and AMD have been beating the drum of lower power consumption louder than Keith Moon in his prime.

However, there is an overlooked element inside that refrigerator-sized humming black plastic and metal box that is sucking up considerable amounts of power and generating heat: the memory.

Memory sticks are plastic and silicon and don't, for the most part, require a heat sink like the CPU. However, they draw a measurable amount of power, and in this new era of 64-bit computing, they are becoming the hidden electric bill.

The great limiting factor of 32-bit computing was its 4GB memory limit. Now that we are firmly in the era of 64-bit computing, that limitation has been smashed, consigned to the ash heap along with single core CPUs and the AGP bus. A 64-bit machine can easily handle 64GB, 128GB or more of memory, up to 16 petabytes.

With the mania around virtualization and people running five, 10 or 20 virtual servers on one box, that means a lot of memory is needed in these beasts. Instead of the days where a server had a pair of 2GB memory sticks, they now have as many as 32 memory sticks in them. Many four-socket motherboards have eight memory banks per socket.

A memory stick, or DIMM , can consume up to 12 watts. Multiplied by 32, and suddenly you've got what could be the single biggest power draw in the server next to storage, depending on the kind of memory you use. What has always been an afterthought in system purchases is going to become a major concern for the power constrained.

DDR, or dual data rate memory, is used in desktops, laptops and video cards. Currently, computers are using DDR2, the second generation of the technology, while DDR3, designed for faster speed and lower power, is under development. AMD uses DDR2 memory in its Opteron-based servers.

However, Intel  uses fully buffered DIMM, or FBDIMM, in its Xeon-based servers. FBDIMM has a chip smack in the middle of the stick called an Advanced Memory Buffer (AMB), which is not used in regular DDR2 sticks.

The AMB is a serial interface that increases the bandwidth of memory and makes it easier to put eight sticks in a bank without degradation of performance. In a bank of eight DIMM slots, memory would degrade in performance once you go beyond four sticks. It's also very good for accessing large amounts of sequential memory and offers error correction that DDR doesn't have.

The drawback, though, is its power draw. An FBDIMM can consume anywhere from five to eight watts more power than a DDR2 stick, which adds up quickly as you add more memory. The reason for the high power draw is the AMB never really has a chance to power down when under low workloads. A benchmarking lab called Neal Nelson Benchmark Laboratories ran a series of tests on power consumption.

It found that while idle, a dual processor, dual core Xeon server consumed 119.3 watts of power while a dual core, two-processor Opteron server drew only 66.7 watts, a 44 percent power savings for the Opteron-based machine. Both machines had 8GB of memory.

Under a full load of 500 simulated users, the Xeon system drew 145.5 watts while the Opteron drew 134.8 watts, giving the Opteron a 7.8 percent advantage. Intel declined to comment on the test results.

Another series of tests was done by the hobbyist site Anandtech, and it found that the FBDIMMs in a Xeon server consumed 862 percent more power than the DDR2 in an AMD server.