AMD Phenom II X4 940 & 920: A True Return to Competition
by Anand Lal Shimpi on January 8, 2009 12:00 AM EST- Posted in
- CPUs
Finally, Cool 'n' Quiet You Can Use
Modern day microprocessors have many operating frequencies they can choose from; these are called p-states. The original Phenom only had two p-states: full frequency and 1/2 frequency. A Phenom 9950 2.6GHz would either run at 2.6GHz or 1.3GHz. The original Phenom was the first quad-core x86 CPU to allow each core to operate at an independent p-state. All of Intel's quad-cores at that point required all four cores to run at the same p-state.
In theory, the AMD design made sense. If you were running a single threaded application, the core that your thread was active on would run at full speed, while the remaining three cores would run at a much lower speed. AMD included this functionality under the Cool 'n' Quiet umbrella. In practice however, Phenom's Cool 'n' Quiet was quite flawed. Vista has a nasty habit of bouncing threads around from one core to the next, which could result in the following phenomenon (no pun intended): when running a single-threaded application, the thread would run on a single core which would tell Vista that it needed to run at full speed. Vista would then move the thread to the next core, which was running at half-speed; now the thread is running on a core that's half the speed as the original core it started out on.
Phenom II fixes this by not allowing individual cores to run at clock speeds independently of one another; if one core must run at 3.0GHz, then all four cores will run at 3.0GHz. In practice this is a much better option as you don't run into the situations where Phenom performance is about half what it should be thanks to your applications running on cores that are operating at half speed. In the past you couldn't leave CnQ enabled on a Phenom system and watch an HD movie, but this is no longer true with Phenom II.
Honestly, AMD's initial Phenom approach is more elegant, but unfortunately the current task scheduling mechanism causes problems. The other issue is that Phenom wasn't switching core speeds quickly enough; ideally it shouldn't matter that a high-priority thread got bounced to a new core, as the new core should simply scale up to full speed in a fraction of a second. Regardless, Phenom II addresses the issues with Phenom CnQ performance not being where it should be.
The Phenom II now supports a maximum of four p-states, with a minimum clock speed of 800MHz. The states for each chip are defined below:
| Processor | Max P-State | P2 | P3 | Min P-State |
| AMD Phenom II X4 940 | 3.0GHz | 2.3GHz | 1.8GHz | 800MHz |
| AMD Phenom II X4 920 | 2.8GHz | 2.1GHz | 1.6GHz | 800MHz |
Intel still has the technological advantage with Core i7; while it too runs all of its cores at the same frequency, idle cores can be turned off completely thanks to the use of Intel's power gate transistors. While this would be nice to have with Phenom II, at least we finally have a working CnQ.
I ran SYSMark 2007 to demonstrate the performance impact of CnQ on Phenom and Phenom II:
| Processor | SYSMark 2007 Overall Score CnQ On |
SYSMark 2007 Overall CnQ Off |
% Increase When Disabling CnQ |
| AMD Phenom II X4 940 | 182 | 185 | 1.6% |
| AMD Phenom 9950BE | 136 | 157 | 15.4% |
Note that the performance on Phenom goes up by over 15% when I disable CnQ, while Phenom II shows less than a 2% gain. This is actually a best case scenario for the original Phenom, however; in my testing I've seen situations where performance is cut in half. Bottom line? The Cool'n'Quiet problems are now resolved, and Phenom II is starting to look recommendable.
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Proteusza - Thursday, January 8, 2009 - link
No, I said I hoped it could at least compete with a Core 2 Duo.if its too much to hope that a 2 year younger, 758 million transistor CPU could compete clock for clock with a first gen Core 2 Duo, then AMD has truly fallen to new lows. It has more transistors than i7, and yet it cant compete with a Core 2 Duo let alone i7. What happened to the sheer brilliance of the A64 days? It could beat the pants off any Pentium 4. Now the best AMD can do is barely acceptable performance at a higher clockspeed than Intel needs, all the while using a larger die than Intels.
This keeps them in the game, but it means I wont bother buying one. Why should I?
coldpower27 - Thursday, January 8, 2009 - link
Those days are over, their success was also contigent with Intel stumbling a bit and they did that with P4, with Intel firing on all cylinders, AMD at acceptable is just where they are supposed to be.Denithor - Thursday, January 8, 2009 - link
It wasn't so much of a stumble, more like a face-plant into a cactus. Wearing shorts and a tshirt.Intel fell flat with Netburst and refused to give up on it for far too long (Willamette -> Northwood -> Prescott -> Cedar Mill). I mean, the early days of P4 were horrible - it was outperformed by lower-clocked P3 chips until the increased clockspeed was finally too high for architectural differences to negate.
Into this mix AMD tossed a grenade, the A64 - followed by the X2 on the same architecture. With its IMC and superior architecture there was no way Netburst could compete. Unfortunately, AMD hasn't really done anything since then to follow through. And even today's PII isn't going to change things dramatically for them, they're still playing second fiddle to Intel's products (which means they're forced into following Intel's lead in the pricing game).
JKflipflop98 - Thursday, January 8, 2009 - link
Damn it feels good to be a gangsta ;)Kob - Thursday, January 8, 2009 - link
Thanks for the meaningful comparison with such a wide range of processors. However, I wonder why the benchmarks are so much tilted toward the graphics/gaming world. I think that many in the SOHO world will benefit from test results of other common applications/fields such as VS Compilation, AutoCAD manipulation, Encryption, simple database indexing and even a Chess game.ThePooBurner - Thursday, January 8, 2009 - link
In the article you compare this to the 4800 series of GPUs. I actually see this as the 3800 series. It works out perfectly. The 2900 came along way late and didn't deliver, used to much power, didn't overclock well, and was just all around a looser of a card. Then the 3800 came along. Basically the same thing, but with a die shrink that allowed it to outstretch, just enough, it's predecessor. It was the first card where they got the mix right. After that came the 4800 with a big boost and even more competition. This is what i now see happening with the CPU line. The Phenom 1 was the 2900, and the Phenom II is the 3800. Getting the mix right and getting ready for the next big swing. But, as you point out, Intel isn't likely to sit back, and we can all agree that they are a much different competitor than Nvidia is.Denithor - Thursday, January 8, 2009 - link
...and just like the 3800 series, it falls just short of the target.Remember? The 3870 couldn't quite catch the 8800GT and the 3850 couldn't quite match the 9600GT. While they weren't bad cards, they unfortunately also didn't give AMD the muscle to set pricing where they wanted it, instead they had to put pricing in line with how nVidia priced their offerings.
Same is happening here, with AMD pricing their chips in line with Intel's Q9400/Q9300 processors. And they may have to drop those prices if Intel cuts the Q9550/Q9400 down another peg.
Griswold - Friday, January 9, 2009 - link
Rubbish theory. First of all, these cards were actually available whereas the 8800GT was in extreme short supply and thus much more expensive for many weeks, even into 2008, because it literally made everything else nvidia had to offer obsolete. I couldnt get one and settled for a 3870 for that reason.Secondly, the 9600GT? Do you realize how much later that card came to the game than the 3850? It hit the market near the end of february. Thats almost 3 months after the launch of the 38xx part.
The whole comparison is silly.
ThePooBurner - Friday, January 9, 2009 - link
The 3800 line wasn't ever meant to beat the 8800 line. It just wasn't in the cards. It's purpose was to get the reins back under control. Cut the power and get back to a decent power/performance ratio as well as get equal power to a previous generation in a smaller package to help improve margins. It was a stage setter. From the first time i read about it i knew that it was just a setup for something more, something "bigger and better" that was going to come next. And then the 4800 came along and delivered the goods. I get this same feeling reading about the Phenom II. It's setting the stage. Getting about the same power (a small bump, just like the 3870 over the 2900) in a smaller package, a better power/performance ratio, etc.. This is simply a stage setting for the next big thing. The next CPU from AMD after this one is going to deliver. I'm sure of it.Kougar - Thursday, January 8, 2009 - link
If you tried Everest and Sandra, what about CPU-Z's cache latency tool? It's not part of the CPU-Z package anymore, but they still offer it. Link: http://www.cpuid.com/download/latency.zip">http://www.cpuid.com/download/latency.zipI thought this tool was very accurate, or is this not the case? It even detected the disabled L3 cache on a Northwood that turned out to be a rebadeged Gallatin CPU.