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FX Vs. Core i7: Exploring CPU Bottlenecks And AMD CrossFire

Battlefield 3, Frame By Frame

Though we usually talk about average frames per second, an even more important measure of playability is milliseconds per frame. That's because frames that take a relatively long time to render can be quite jarring. In theory, a 91 FPS rate could include a single 100-ms frame and ninety 10-ms frames, and that one 100-ms frame would be what kills your experience. 

This can happen on a single-GPU card. However, the complexities of synchronizing multiple GPUs make them more common in CrossFire and SLI configurations. We covered this micro-stutter effect in Micro-Stuttering And GPU Scaling In CrossFire And SLI, and have plans to cover this phenomenon in more depth in the next couple of months.

Since an evenly-spread 20 FPS rate would consist of 20 50-ms frames, we’re using 50 ms as the cut-off for actual playability in today’s analysis. Many gamers get annoyed with frame intervals far shorter (say, 30 ms), but that isn't as likely to get you killed as it is to simply bug you.

The performance of our Radeon HD 7970s in CrossFire appears fairly similar on our AMD- and Intel-based platforms when we run at 1920x1080. Our system based on the FX-8350 encounters a couple of higher spikes, but the worst of these we see only reaches up to 40 ms.

It's worth noting that we're using Fraps to take these measurements (currently the only solution, short of capturing the output with a PCI Express-based frame grabber). Consequently, we're not representing the entire rendering pipeline. After comparing our recorded results to actual gameplay, however, we're confident that the most egregious performance interruptions are being illustrated. Moreover, we're not comparing SLI to CrossFire, so the frame-time spikes are truly attributable to each platform.

Frame times simultaneously appear more variable (the bulk of the graph is wider) and with lower variability (the largest spikes are smaller) at 4800x900. Both platforms seldom cross the 30 ms barrier, and the AMD-based machine only spikes to 40 ms once.

You'll probably want to stop at 4800x900 or dial detail settings back to the Medium preset if 30-ms and greater frame times bother you. Ultra-quality details at this super-high resolution appear barely playable.

Thomas Soderstrom
Thomas Soderstrom is a Senior Staff Editor at Tom's Hardware US. He tests and reviews cases, cooling, memory and motherboards.