Seven GeForce GTX 660 Ti Cards: Exploring Memory Bandwidth

Testing For Memory Interface Limitations

We went back and took a closer look at the tests in our launch story, along with the benchmarks run by other sites. After sorting through the settings everyone used, we saw that it's not easy to properly test the limitations of a product's memory bus using real-world metrics. The variations between games, detail settings, and resolutions are so large that there really isn’t a way to definitively order Nvidia's GeForce GTX 660 Ti, AMD's Radeon HD 7950, and the Radeon HD 7870. Any recommendation based on a limited benchmark suite isn't wholly informative until you've covered many, many more titles than anyone could conceivably run leading up to a launch.

In short, every buyer is playing different games at different settings, making it impossible to pass one judgement that applies to everyone. This is particularly true with the GeForce GTX 660 Ti and AMD Radeon HD 7950.

Benchmark Selection and Setup

During the course of our exploration, we noticed the largest performance variations in games that weren't very hard on the GPU, allowing the cards to pump out high frame rates. Under those circumstances, even AMD's Radeon HD 7870 was sometimes able to take the lead. Nvidia's GeForce GTX 660 Ti seemed to run into the biggest problems in titles leveraging little or no tessellation, and the least-demanding effects. A strong GPU didn't matter, and the card was held back by its memory interface like a sports car that can’t put its horsepower down due to small tires.

In light of the 660 Ti's narrower 192-bit memory interface, we needed to figure out the best way to benchmark such a design decision. Some sites used very large textures at very high resolutions and maximum detail settings to demonstrate how the card runs out of steam in extreme situations. Unfortunately, low, unplayable frame rates make it hard to track accuracy. Those numbers aren't practical anyway.

We took a different route. First, we tried to get rid of manufacturer-specific advantages and disadvantages resulting from graphics drivers. After trying out a lot of games, we settled on Batman: Arkham City. We disabled tessellation, horizon-based ambient occlusion (HBAO), and multi-view soft shadows (MVSS) in order to not slow down the GPU too much. Running the game like this, without anti-aliasing, yielded the benchmark results we were expecting at both tested resolutions based on each card's specs. Nvidia's GeForce GTX 670 inched out AMD's Radeon HD 7950, followed by the GeForce GTX 660 Ti with the Radeon HD 7870 in the rear. Then, we set out to measure what happens when resolution is increased and anti-aliasing is enabled.

Choosing the Graphics Cards for the Comparison

We chose a reference GeForce GTX 670, HIS' 7870 IceQ 2 GB with a 1 GHz GPU clock, HIS' 7950 IceQ with an 800 MHz GPU clock, and two passively-cooled HIS 7750 iSilence 5s in CrossFire. Really, the Radeon HD 7750s shouldn't stand a chance. However, we're currently working on another story testing passive cards, and some of the benchmark results are pretty interesting. We decided to include them at 1920x1080.

We were interested in two things. First, how big is the difference between Nvidia's GeForce GTX 670 and 660 Ti. And second, where do the Radeon HD 7950 and 7870 fit in?

We used each card's reference clock rates for all tests, and we didn't apply the BIOS update for AMD's Radeon HD 7950. As you’ll see, the overclocking doesn’t really change anything when it comes to memory interface limitations. In fact, the performance gets hit even harder.

Also, we were particularly interested in how the 2 GB and 3 GB cards would compare. The only way 2 GB of memory can be handled over a 192-bit bus is with mixed-density ICs. It works like this: the three 64-bit controllers divide the total memory into 512 MB chunks, which are accessed at the full 192 bits. The remaining 512 MB is addressed by just one 64-bit controller in a completely separate transaction. Nvidia won't divulge anything else about its implementation for competitive reasons, but there is undoubtedly latency there the controllers have to contend with. It'd seemingly be easier to implement 3 GB using a trio of 1 GB chunks. Setting each card to Nvidia's reference clock rate provided us with a little insight.

Benchmark System and Settings

We overclocked our CPU to 5 GHz to avoid a CPU-imposed limitation (this required our Core i5-2500K, since the i7-2600K won't go that high; besides, Batman doesn't benefit from Hyper-Threading). In comparison, gaming performance at the processor's factory setting did suffer a little bit. This didn't change the order in which the cards finished, though, so we're confident that our results apply to non-overclocked platforms, too.

Our carefully-selected benchmark should tell us everything we need to know about how these graphics cards stack up against each other. We start out with separate benchmark settings and then combine all of the numbers to give an overview of the cards’ performance.