Overclocking Extravaganza: Radeon HD 4890 To The Max

When we first heard about the overclocking potential of the 4890 from AMD, we were a bit skeptical. At the same time, the numbers we were hearing were impressive and AMD doesn't have a history of talking up that sort of thing to us. There have already been some investigations around the web that do point to the 4890 as having some healthy overclocking potential, so we decided to try our hand at it and see what we could come up with.

We are testing review samples, which means that our parts may have more overclockability than off the shelf cards, but we can't attest to that at this point. What we do want to explore are the overclocking characteristics of the 4890 and how different adjustments may or may not affect performance. From what we are seeing around the web, many people are getting fairly close to the speeds we tested. Every part is different, but while clock speeds may vary, the general performance you can expect at any given point will not.

So what's so special about this AMD part that we are singling it out for overclocking anaysis? Well, the GPU has been massaged to allow for more headroom, some of which hasn't been exploited at stock clock speeds. This is the first time in a long time (or is it ever?) we are seeing multiple manufacturers bring out overclocked parts based on an AMD GPU at launch. With this as the flagship AMD GPU, we also want to see what kind of potential it has to compete with NVIDIA's top of the line GPU.

But it's more than just the chip. We also are also interested in how well the resources on the board are balanced. Core voltages and clock speeds must be selected along with framebuffer size and memory clock. These considerations must account for a target power, heat, noise and price. For high end parts, we see the emphasis on performance over other factors, but there will still be hard limits to work within.

Because of all this, balancing hardware specifications is very important. Memory bandwidth needs to be paired well with core speed in order to maximize performance. It doesn't do us as much good to have an infinitely fast core if we have slow memory that limits performance. We also aren't well served by really ridiculously fast memory if the core can't consume data quick enough. Using resources appropriately is key. And AMD did a good job balancing resources with the 4890.

Rather than just test the semi-official overclock (which is just a 50MHz core clock boost to 900MHz), we decided to test multiple core and memory overclocks (and one core + memory overclock) to better understand the performance characteristics of this beast. As expected, overclocking both core and memory saw the best results followed by only overclocking the core. Just boosting memory speed on its own didn't seem to have a significant impact on performance despite the large overclock that was possible.

So why not sell every chip at the "overclocked" speed? Well, it's all about yield. Our guess is that while the change that AMD made were certainly good enough to boost clock speed over the 4870 by a healthy margin that there were a good number of parts that couldn't be pushed up to 900MHz and AMD really didn't want to sell them as cheaper hardware. We haven't heard that endorsing the idea overclocked parts is really a policy change for AMD, so it might just be that previous layout, routing, and design choices provided for a narrower range of overclockability around the target clock frequency.

What ever the reason for it, we now have overclockable hardware from AMD. Our analysis starts with an in depth look at percent increase in performance, but if all you care about is raw performance data, we've got plenty of that in the second half. And with it comes a surprise in our conclusion we never expected.