Update: Radeon R9 295X2 8 GB In CrossFire: Gaming At 4K

Radeon R9 295X2s, Working Together

Update: After this story went live in mid-April, AMD let us know that a number of our results did not match what the company was seeing in its own lab. Today’s update is the product of hundreds of benchmark runs designed to diagnose the behavior of two Radeon R9 295X2 cards in quad-CrossFire.According to AMD, the only officially-supported way to connect a 295X2 and a 4K display is through DisplayPort. The dual-HDMI method we’re forced to use (and described in detail here: Challenging FPS: Testing SLI And CrossFire Using Video Capture) is not considered valid. So, the first step was to run all of our tests using a DisplayPort cable and Fraps, comparing the outcome to FCAT-generated data.Using that information, we identified a number of games that behaved the same, regardless of test method, and a couple that suggested something else was the matter. And so we dug…All of the charts on the following pages were re-rendered. And much of what you’ll see confirms our original conclusions. But because we have so much more time invested into benchmarking and troubleshooting, all of the analysis is new. We also gathered additional information about AMD's XDMA engine. The Catalyst driver dynamically calculates PCIe bandwidth in real-time. If there isn't enough available, either because your platform cannot keep up or because the resolution you're driving is too throughput-intensive, compositing happens in software instead. Ideally, then, you want x16 slots running at x16 transfer rates, without PLX switches in front of them adding latency. AMD says its dual-GPU boards will still work on links narrower than 16 lanes and through motherboard switches, though stuttering/timing issues become more likely, as you'd expect.Does our revisit vindicate a pair of $1500 graphics cards working in parallel, or are you better off spending your budget elsewhere?

Twenty-four point eight billion transistors. Eleven-thousand two-hundred and sixty-four shaders. Sixteen gigabytes of GDDR5 memory. Twenty-three teraflops of compute power. One thousand watts of rated board power.

No matter which specification you use to describe two Radeon R9 295X2 cards in CrossFire, the number is undoubtedly unlike any other describing a gaming PC’s capabilities. And yet, late last Friday, a box from iBuyPower showed up at the lab with a tall, majestic Erebus PC sporting a pair of the dual-GPU, liquid-cooled flagships I reviewed in Radeon R9 295X2 8 GB Review: Project Hydra Gets Liquid Cooling. iBuyPower’s builders dutifully matched my test bed’s specifications, including a Core i7-4960X overclocked to 4.2 GHz, 32 GB of memory in a quad-channel configuration, and an even beefier 1350 W power supply. Despite the box’s brawn (and the haste with which I asked to have it built), every cable was tied down neatly, giving the lighting on AMD’s latest a neat, roomy enclosure with a side window to illuminate.

Also nifty, but something we don’t typically think much about as enthusiasts, is that iBuyPower shipped the exceedingly-heavy system with two liquid-cooled cards and it arrived working perfectly. Polyurethane foam works wonders.

And so, with years of experience under our belts suggesting that four GPUs working cooperatively typically don’t behave well at all, regardless of vendor, we set off an experiment to gauge whether history repeats itself, or if AMD’s best-built board breaks precedent in more ways than one.

Testing Two Radeon R9 295X2s

We have our reasons to believe that two Radeon R9 295X2s might behave different from quad-GPU solutions tested previously. To begin, there's AMD's new DMA engine that enables CrossFire without a bridge connector. There's also the fact that we're testing at 3840x2160 and using an overclocked Ivy Bridge-E-based platform, hopefully minimizing the poor scaling you'd expect from a platform-bound configuration. AMD also sent over a new beta driver late last week, which was supposed to be optimized for four-way CrossFire.

We're careful to make sure Frame Pacing is enabled, that Tessellation Mode is controlled by the applications, and v-sync is forced off in AMD's driver.