I have a Phenom II X4 955 CPU OC'd to 965 speeds(3.4Ghz), and a Crossfire HD5850 setup. I haven't noticed any big performance gains in my games after I installed the second HD5850, and I was wondering if my CPU was bottlenecking the twin GPUs in some way.
I'm seeing about a 10%-15% increase in most of my games with Crossfire, and some others don't seem to run any better at all. I am aware that a source of bottleneck could be the motherboard, which has x8/x8 operation PCI-Express slots, but I doubt this could be bottlenecking the cards more than 5% at the most. The cards are literally stacked on top of the other, so I installed a 120mm fan right in front of the cards blowing air onto the cards towards the back of the PC, just as the rest of the PC airflow, intake from front, exhaust out back, no top, bottom or side vents, one direction only. There is about a 3 inch clearance from the case floor to the bottom card's fan(2nd card), but absolutely no room for the first card(primary) to draw any air directly, hence the fan blowing air from the front, to ensure the primary card gets some airflow. The whole point of me telling you this is that I am noticing a huge temperature difference between the cards when gaming. The primary card goes up to around 60-65 Celsius, while the second card, the one with 3 inch clearance, is running at 40 Celsius constantly. The primary card, the one running hotter, is an ASUS EAH5850 DirectCU, and the secondary, the one running cooler, is a PowerColor 5850 PCS+. I understand that the PCS+ probably has a better cooler, but I installed the ASUS as primary so I can use the ASUS Smartdoctor to tweak the voltage.
But the thing is, could it be possible that the rig is only really using ONE of the cards and barely even touching the second one? I do not believe a bit of airflow difference and fan size on the card cooler can yield 25 degrees of difference.
If the second HD5850 is really not being used much at all, could it be that the CPU just isn't strong enough to feed enough juice to both cards? I run my games at 1600x900 resolution, but that should still be enough to use up some GPU power in Crysis and Crysis Warhead, where I see about 5-10FPS increase with the second card installed. So my only conclusion is that the CPU isn't strong enough to make both cards work hard, or it's just that Crysis supports Crossfire like crap.
What do you think? Is that CPU the bottleneck here? I'm running 8GB of 1333MHz RAM by the way. I plan on upgrading to 2133MHz RAM along with the next gen "Bulldozer" AMD CPUs when it is released.