|PowerColor AX6990 4GBD5-M4D Video Card|
|Reviews - Featured Reviews: Video Cards|
|Written by Bruce Normann|
|Sunday, 20 March 2011|
Page 17 of 19
Overclocking and Power Consumption
I promised a look at overclocking performance, with the BIOS setting in position 1, and here it is. Even with the overclocking BIOS engaged, it ran perfectly without a hiccup. I didn't have a lot of luck pushing it very far past the 880 MHz level, but I didn't have a lot of time either, and overclocking is a sport that rewards patience over haste.
GPU Overclocking Results (1929 x 1200)
None of the resulting increases crosses 5%, and only one of them is in the 1% range. The next lowest is at 2%, and the HD6990 frame rates in Metro 2033 blow away the competition anyways, so it's a moot point. Overall the average improvement is 3%, which is not as good as you might expect for a 6% increase in core clocks. Someone will have to get these GPUs under water before we get a real sense of the scaling that can be achieved. It's a pity that AMD chose the lower-clocked memory modules to put on this card, the "-ROC" spec memory chips run a full 10% faster, at 1375 MHz, and could have kept up a little better once the Cayman bits get cooled down to sub-ambient temperatures.
When you look at the idle clock rates that AMD programmed into the BIOS for this GPU; they're a little higher than I'm used to seeing with the latest cards. This is definitely not the bleeding edge of what the industry is doing today, in terms of speed throttling to save energy. I was running Catalyst Control Center 2 at the time, but Overdrive was not enabled which is the usual culprit for messing with the factory power reduction schemes. I'm pretty sure the BIOS are designed this way to ensure absolute stability with this fairly complex system of systems. It's a fair trade, since the last thing you want is the BSOD when your computer is just idling. Trust me, there's been way too many instances of that exact behavior in the last few years; you just need to be aware that you are sacrificing the last few measures of power saving to get the increased performance of a single-slot CrossFireX solution.
I used a slightly different approach to power measurement this time, as I wanted to provide the best power solution I could for testing, so I used a slightly more complex PSU arrangement. I have a dedicated GPU power supply from Thermaltake that is 100% dedicated to supplying the PCI Express connectors on a modern graphics card. It's rated for 650 watts, and JonnyGuru tested it to slightly higher limits with good noise performance to boot, so I felt comfortable that it could easily supply the significant power requirements of the Radeon HD 6990. The performance of this card is just as much about power management as it is about the graphics performance. That is, the real design challenge was to get twice the performance of an HD 6970 in the space of a dual slot card and stay within the power and thermal constraints that a single card is limited to.
The PCI Express connector on the motherboard is officially limited to 75W of 12V DC power. Each of the two 8-position PCI Express power connectors on a modern graphics card is limited to 150W. So, the theoretical maximum that the Radeon HD 6990 is "allowed" to consume is 375W. I measured just the power being consumed by the PCI Express power connectors this time. This provides a more accurate reflection of the power that is going directly to the video card, as any CPU load is completely isolated from the dedicated GPU power supply. OTOH, the power that comes into the video card from the PCI Express 2.1 motherboard connector is not accounted for in the measurement. It's at times like this that I really miss the power measurement capability that ASUS built into their iTracker monitoring & control software.
To measure isolated video card power consumption, I used the Kill-A-Watt EZ (model P4460) power meter made by P3 International. A baseline test is taken without a video card installed inside our computer system, which is allowed to boot into Windows and rest idle at the login screen before power consumption is recorded. Once the baseline reading has been taken, the graphics card is installed and the system is again booted into Windows and left idle at the login screen. Our final loaded power consumption reading is taken with the video card running a stress test using FurMark. Below is a chart with the isolated video card power consumption (not system total) displayed in Watts for each specified test product:
* Results are accurate to within +/- 5W.
The PowerColor HD 6990 4GB GDDR5 Video Card pulled 35 (41-6) watts at idle and 248 (254-6) watts from the two 8-pin PCI-E power connectors when running full out with standard HD 6990 BIOS settings, using the test method outlined above. The results when running the enhanced BIOS settings were 35 (41-6) watts at idle and 285 (291-6) watts when running at full GPU load. Note: Don't compare these results with the chart, as a different test method was used.
I ran all these tests at stock voltages in the PowerColor HD 6990 BIOS, which GPU-Z reported as 0.900V at idle, 1.000V in 2D graphics mode, 1.120V in full 3D mode, and 1.175V in overclocked 3D mode. The core GPU voltage was increased by the overclocked BIOS configuration, but only the full power voltages were affected.
Next, I'll offer you some final thoughts, and my conclusions. On to the next page...