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AMD's FX-8350 processor reviewed


Piledriver digs in
— 11:00 PM on October 22, 2012

AMD has been going through some difficult times lately, with management changes, layoffs, and a steady drip of talent drain, as a host of familiar faces have fled for greener pastures. These things have come against a backdrop of mounting financial losses and tough questions about the company's future and direction.

Much of the turmoil can be traced back to one big, fateful event: the difficult, disappointing birth of the all-new CPU microarchitecture known as Bulldozer. As techie types, we're perhaps overestimating the role technology plays in these matters. Still, Bulldozer was viewed by many as AMD's next great hope, its first from-the-ground-up new x86 CPU architecture in over a decade. When the FX processors not only failed to catch up to the competition from Intel but also struggled to beat AMD's own prior generation in performance and power efficiency, some unpleasant fallout was inevitable.

Once the first chips were out the door, AMD's engineering task became clear: to do as much as possible to improve the Bulldozer microarchitecture as quickly as possible. Alongside the FX processors, the firm announced a plan that included a series of updates to its CPU cores over the next few years, with promised increases in performance and power efficiency. The first of those incremental updates was dubbed Piledriver, a modest refresh that first hit the market this past spring aboard the Trinity APU. Now, roughly a year after the first FX chips arrived, revamped FX processors based on Piledriver are making their debut, more or less on schedule. That is, all things considered, a very positive sign.

The question now is whether it's enough. Are these CPUs good enough to carve out some space in the market against some tremendously formidable competition? You may be surprised by the answer.


Vishera ain't just a river in Russia


A picture of the Vishera die. Source: AMD.

The chip that's the subject of our attention today is code-named Vishera, and it's the direct successor to the silicon that powered the prior-gen FX processors, which was known as Orochi. Vishera and Orochi share almost everything—both are manufactured on GlobalFoundries' 32-nm SOI fabrication process, both have 8MB of L3 cache, and both are essentially eight-core CPUs. The one big difference is the transition from Bulldozer to Piledriver cores—or, to put it more precisely, from Bulldozer to Piledriver modules. These "modules" are a fundamental structure in AMD's latest architectures, and they house two "tightly coupled" integer cores that share certain resources, including a front-end, L2 cache, and floating-point unit. Thus, AMD bills a four-module FX processor as an eight-core CPU, and we can't entirely object to that label.

Code name Key
products
Cores/
modules
Threads Last-level
cache size
Process node
(Nanometers)
Estimated
transistors
(Millions)
Die
area
(mm²)
Lynnfield Core i5, i7 4 8 8 MB 45 774 296
Gulftown Core i7-9xx 6 12 12 MB 32 1168 248
Sandy Bridge Core i5, i7 4 8 8 MB 32 995 216
Sandy Bridge-E Core-i7-39xx 8 16 20 MB 32 2270 435
Ivy Bridge Core i5, i7 4 8 8 MB 22 1400 160
Deneb Phenom II 4 4 6 MB 45 758 258
Thuban Phenom II X6 6 6 6 MB 45 904 346
Llano A8, A6, A4 4 4 1 MB x 4 32 1450 228
Trinity A10, A8, A6 2 4 2 MB x 2 32 1303 246
Orochi/Zambezi FX 4 8 8 MB 32 1200 315
Vishera FX 4 8 8 MB 32 1200 315

We covered the enhancements made to the Piledriver modules in more detail here, but the highlights are pretty straightforward. Piledriver includes a collection of small tweaks to individual parts of the module intended to increase instruction throughput. The changes range from the CPU's front end through the cores and into the cache subsystem, and no single change contributes much more than a 1% increase in throughput. All together, the gains are maybe on the order of 6%, perhaps less, so we're not looking at a vast improvement. Still, Piledriver includes other modifications. The FPU supports the three-operand version of the fused multiply-add instruction, a key part of the AVX specification that will also be supported in Intel's upcoming Haswell chips. This change puts AMD and Intel on the same page going forward. (Support for the FMA4 instruction from Bulldozer is retained, at least for now.) More crucially, Piledriver has been optimized to reach higher clock speeds at lower voltages, a tweak that paid off nicely for the mobile Trinity chip. As you'll see, it has benefited the desktop FX processors, as well.

The lineup


Model Modules Threads Base core
clock speed
Max Turbo
clock speed
North
bridge
speed
L3 cache TDP Price
FX-8350 4 8 4.0 GHz 4.2 GHz 2.2 GHz 8 MB 125 W $195
FX-8320 4 8 3.5 GHz 4.0 GHz 2.2 GHz 8 MB 125 W $169
FX-6300 3 6 3.5 GHz 4.1 GHz 2.0 GHz 8 MB 95 W $132
FX-4300 2 4 3.8 GHz 4.0 GHz 2.0 GHz 4 MB 95 W $122

The new lineup of FX chips is detailed above. Today's headliner is the FX-8350, the only one of the four new Vishera-based parts AMD has supplied us for review. The FX-8350 shares the same power envelope (125W) and Turbo peak (4.2GHz) as the chip it supplants, the FX-8150. The most notable difference is the base clock; the FX-8350's is a nosebleed-inducing 4GHz, up from its predecessor's 3.6GHz.

The FX-8350's higher base frequency should boost performance, especially in widely multithreaded workloads. Still, if you're like me, you're looking at the 200MHz gap between the base and Turbo peak clock speeds and wondering why it isn't larger. The whole idea of these dynamic clocking schemes, after all, is to take advantage of the additional thermal headroom made available when not all cores are busy. Vishera can gate off power to inactive modules, granting more space for those that remain active. Higher voltages and frequencies are then usually possible within the same thermal envelope. There's a 500MHz gap between the base and peak clocks for the FX-8320. Why doesn't the FX-8350 offer a similar increase in peak clock frequency?

Our best guess is that too few of these chips will tolerate frequencies above 4.2GHz well enough, consistently enough, and at low enough voltages to allow AMD to ship a product in volume with a higher Turbo peak. If so, that's a shame, because low performance in lightly threaded workloads is arguably this CPU architecture's biggest weakness. Higher Turbo frequencies could do a lot to remedy that problem.

That said, the FX-8350's price is quite nice. The $195 sticker positions it between a couple of Intel's Ivy Bridge-based offerings, the Core i5-3470 at $185 and the Core i5-3570K at $225. Both are true quad-core, four-threaded processors. Of those two, only the i5-3570K has an unlocked upper multiplier for easy overclocking, whereas all of the FX parts are unlocked. On the other hand, the Intel processors have peak power ratings of 77W, vastly lower than the FX-8350's 125W TDP.

Speaking of smaller power envelopes, the two lower-end FX models take advantage of Piledriver's power enhancements by dropping to a more modest 95W. The chips they replace, the FX-6200 and FX-4170, are both 125W parts. The new models even sacrifice a bit of clock speed to get there. For instance, the FX-6300 is clocked at 3.5/4.1GHz, while the older FX-6200 runs at 3.8/4.1GHz. AMD tells us it expects the performance of these two parts to be similar, since the per-clock performance gains in Piledriver should make up some of the difference.

The lowest-end FX processor, the FX-4300, overlaps almost entirely with the A10-5800K desktop Trinity that we reviewed earlier this month. Both list for $122. The 5800K has a 200MHz higher Turbo peak, five more watts of max power draw, and integrated graphics. The FX-4300 instead has 4MB of L3 cache, which Trinity lacks. Then again, the A-series APUs have integrated PCIe connectivity and drop into their own brand-new socket, while the new FX series uses the same Socket AM3+ infrastructure as the prior models, so they're really aimed at different platforms.