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Llano in overtime: 1866MHz RAM & IGP-only power

We dig a little deeper to see what turns up
— 11:51 PM on July 17, 2011

Between our reviews of the mobile and desktop versions of the chip, we've spent a tremendous amount of time recently with AMD's new Llano APU. That doesn't mean we haven't had some lingering questions about it, though. We always have more questions about new technologies than we have time to devote to them, seems like.

One of our remaining questions about Llano has to do with its memory support. Uniquely, the desktop versions of Llano officially support DDR3 memory at clock speeds of 1600 and 1866MHz, higher frequencies than the 1333MHz memory currently widely available on the market, with the potential for higher performance.

Now, that official endorsement from AMD comes along with some big, honkin' caveats. Among them: those speeds are only supported "in a single DIMM-per-channel configuration." So you'll have to forgo half of the DIMM slots on most desktop Llano motherboards in order to use those higher speeds. Also, only "true 1.5V, JDEC-spec memory" gets official support.

In other words, AMD's stamp of approval for the use of higher-frequency RAM with Llano is essentially forward-looking. In preparation for our review, we pinged the memory mavens at Corsair about the availability of JEDEC-approved DRAMs rated for these new frequencies. We were told JEDEC has indeed ratified the spec for those parts. However, Corsair couldn't give us any firm date for the market availability of such DRAMs. All we know at present is that they're on the way. AMD's advice to reviewers concurs with this estimate, stating, "that kind of memory will likely not be widely available until a later time" and conceding that most reviewers "will likely end up testing with 1333MHz DDR3 appropriate for the price tag of this platform."

That's not to say one can't find DIMMs on the market, made with current DRAMs, that will operate at 1600 or 1866MHz. Firms like Corsair perform the invaluable service of sorting and binning DRAMs and packaging up the best chips into enthusiast-class modules. Such modules can sometimes perform amazing feats that would make a JEDEC committee gasp in unison, drop the monocles from their eye holes, and whisper-talk in exasperated tones about how scandalized they are. In fact, among enthusiast brands, DIMMs rated for 1600MHz are fairly common these days and don't carry much of a price premium over 1333MHz modules. Many of them will even operate at 1.5V. One can find modules rated for 1866MHz, as well, although they tend to come with a fairly substantial price premium attached.

The complexities of this situation forced us to make some choices when formulating our review of the desktop Llano APUs. We took a multi-pronged approach. We started by testing with 1333MHz memory in our CPU benchmark suite, since that's the type of memory most likely to be used in value-focused systems based on this APU. Also, the architecture of Llano's CPU cores is largely familiar, and given what we knew about it, we didn't expect major performance gains from incremental improvements in memory bandwidth. We added 1600MHz memory to the mix in our integrated graphics testing, because that's where we expected additional memory bandwidth to make the biggest impact. We also requested some 1866MHz-capable DIMMs from Corsair, but those unfortunately didn't arrive in time for us to test them and include the results in our initial review.

For reasons science is only beginning to understand, the omissions of higher-speed DIMMs in the CPU suite and of 1866MHz memory in the graphics tests led to a minor outcry from some quarters. Little did the complainers know that deep in the underground expanse of Damage Labs, we were taking the wraps off of these puppies:

These two Corsair Vengeance DIMMs are 4GB each and are rated for operation at a clock speed of 1866MHz at only 1.5V with 9-10-9-27 timings. That's pretty sweet, but at $94 for a pair, they'll set you back about 30 bucks more than the cheapest 1600MHz modules at Newegg.

We dropped these DIMMs into our A8-3850 APU-based test system and configured them to use Corsair's recommended timings. That's a bit of a step back from the 8-8-8-20 timings we'd used before with 1333MHz and 1600MHz DIMMs, but with two crucial differences: these modules are running at 1866MHz, and because we've dropped from four DIMMs to two, we're able to lower the DRAM command rate from 2T to 1T. The end result should be a higher bandwidth, lower latency memory subsystem than anything we've tested with Llano to date.

The question is: what difference does it make? To find out, we ran a broad selection of benchmarks from our CPU and IGP test suites, which you'll find in the following pages. If you want more info on our test configurations and methods, be sure to see our original A8-3850 review for that.