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AMD's dual-core Opteron processors


Because four is better than two
— 5:16 AM on April 21, 2005

MICROPROCESSORS ARE GETTING too hot, requiring too much power, and not delivering enough additional performance for it. That's the basic problem. The engine that's driven the microcomputer's incredible rise in capability over the past 30 years, Moore's Law, isn't quite out of steam yet, but some of its offshoots are on the ropes. CPU designers have nearly exhausted their collective bag of tricks to get more performance out of additional transistors on a chip by increasing parallelism at the instruction level. Speculative execution and deep pipelining are by now very standard features, and CPU designs are getting increasingly complex and hard to manage. When Gordon Moore's goose lays a golden egg and the number of transistors possible on a chip doubles, as it is supposed to do every 18 months, taking advantage of the windfall has proven increasingly difficult.

Cranking up clock speeds hasn't helped much, either, because of transistor leakage problems. Chips are sucking up large amounts of power and expending much of it as heat, and the problem grows more acute as clock speeds ramp up. The most widely noted example of these problems, by far, has been at the company Moore co-founded. The power, heat, and speed problems of the "Prescott" core inside of recent Pentium 4 processors prompted Intel into an impressive and very public change of direction over the course of the past year. The company has sworn off the quest for 4GHz, shied away from clock speed as a measure of performance, and utterly rewritten its CPU roadmap.

AMD has not been entirely immune to these problems, but it has sidestepped their worst effects by keeping clock speeds down. The original Opteron processor debuted two years ago today at speeds up to 2GHz. Two years later, the same processors are available at 2.6GHz—only a 600MHz increase, not much in the grand scheme.

Fortunately, both AMD and Intel seem to have settled on an answer that should allow them to take advantage of ballooning transistor counts to gain additional performance: thread-level parallelism. By dialing back clock speeds and putting multiple CPU cores on a chip, the theory goes, processor performance can rise as transistor counts do. This sort of parallelism will, of course, be familiar to those who know a thing or two about Opteron processors, which have commonly been employed in pairs as part of server or workstation systems.

In fact, AMD says that the Opteron was designed from the outset with dual-core implementations in mind. The folks there are also quick to remind anyone who will listen that AMD was first to tape out an x86-compatible dual-core design and first to demonstrate such a beast in public. Today, they aim to be the first manufacturer to deliver dual-core x86 processors for workstations and servers, just days after Intel officially announced its first dual-core desktop processors.

We've had a pair of dual-core Opteron processors on the test bench for some time now, and we're pleased to report some rather impressive results. AMD's dual-core design is something more than just a pair of CPUs glued together on a single piece of silicon, and this design choice yields a performance dividend. Keep reading to see how the new Opteron 275 stacks up against its Opteron predecessors and against Intel's latest "Nocona" Xeons. We also have a head-to-head battle of single-socket, dual-core workstation processors: the Opteron 175 versus the Pentium Extreme Edition 840.

The processors
On looks alone, one would be hard pressed to tell the difference between dual-core Opterons and their single-core counterparts.


A pair of Opteron 875 processors

They're cosmetically identical, save for the slightly revamped model numbering scheme. The three-tiered processor series convention remains intact. 100 series processors are for single-socket systems, the 200 series for dual, and the 800 series is intended for 4-socket systems or better. However, instead of incrementing the tail end of the model number by two as clock speeds ramp up, as the Opterons 246, 248, and 250 did, the dual-core models will come in increments of five. The first dual-core Opterons will arrive at clock speeds of 1.8, 2.0, and 2.2GHz as models x65, x70, and x75, respectively.

Prices will vary according to whether the chips are part of the 100, 200, or 800 series and according to clock speeds, but the general plan for pricing is fairly straightforward: it's almost as if AMD were introducing three new top-end speed grades at once. However, there is some overlap. For instance, the Opteron 252 is priced at $851, and the Opteron 265 will be priced the same. Consumers can choose whether they wish to purchase a dual-core processor at 1.8GHz or a single core at 2.6GHz for the same amount. The higher models will carry a premium, but AMD plans to bring the prices of dual-core Opterons down over time into the territory of the current single-core models.

The even better news for current owners of Opteron systems is that the dual-core Opterons will be pin-compatible with existing Socket 940 systems, capable of acting as drop-in replacements for current single-core models. The only requirement is that the motherboard must be able to support newer 90nm chips like the Opteron 252. If the board can do that, it should be able to handle the dual-core chips after a BIOS update, AMD claims. (Check with your motherboard maker to be sure.)

In order to pull off this impressive feat of backward compatibility, AMD had to make its dual-core parts fit into the same basic power and heat envelopes as its single-core processors. To do so, the company tweaked its fabrication process, using lower-leakage transistors that switch somewhat slower but waste less power, among other things. As a result, the Opteron 275 tops out at 2.2GHz, but it consumes no more power than the Opteron 252 at 2.6GHz.

This is one of the minor miracles of choosing thread-level parallelism over higher clock speeds. When we asked AMD CTO Fred Weber about how they managed to keep power and heat so low, he was coy about which specific optimizations AMD employed, but he offered some examples. When you're not optimizing for the absolute best linear performance, he noted, many things are possible, including everything from changing the oxide thickness and transistor voltages to resizing buffers and more extensive clock gating.

To further manage heat and power, dual-core Opterons will support AMD's PowerNow feature (also known as Cool'n'Quiet in the desktop world) that scales clock speeds and CPU voltages down at times of low CPU loads. This feature will function on a whole-chip basis; the CPU cores will not scale their clock speeds up and down independently.


A shot of the dual-core Opteron die. Source: AMD.

As for the chip itself, the dual-core Opteron will be manufactured on AMD's 90nm process with silicon-on-insulator (SOI) technology. The chips will include all of the latest enhancements AMD has made to the K8 core, including SSE3 support and an improved memory controller with broader compatibility, improved memory loading, and more efficient memory mapping.

A dual-core Opteron chip packs in about 233 million transistors, and its die size is a very healthy 199 mm2. The Intel Prescott/Nocona on which the Xeon is based is 112 mm2 with roughly 125 million transistors. (The newer version with 2MB of L2 cache has 133 million transistors.) So the dual-core Opteron is large, but it's also a very close match for Intel's "Smithfield" dual core, which weighs in at roughly 230 million transistors and 206 mm2, although estimates and methods of counting transistors can vary.