AMD Radeon HD 4870 X2 Review

The R700 board is literally made up of two RV770s with a PCI Express switch connecting the two. The clock speeds are identical to the Radeon HD 4870, and memory size per GPU has been doubled to 1GB, which could help in hi res cases with AA enabled. In other words, R700 should perform very much like a pair of 4870s in CrossFire. Or should it?
Building a Better CrossFire

When AMD began talking about no longer building high end hardware using single monolithic GPUs a few weeks back, we let them know that improving CrossFire support would be incredibly important going forward. AMD told us that they are putting a lot into that but also that they have some exciting technology up their sleeves with R700 to help out as well. Unfortunately, we haven’t gotten as much detailed information on how it works, but the new technology is GPU to GPU communication.

Until now, CrossFire has done zero GPU to GPU or framebuffer to framebuffer communication. As with the first iteration, each card fully renders the parts of the screen for which it is responsible (be it a whole frame in AFR, the top or bottom half of a screen, or alternating tiles). These results are sent to a combiner where the digital signals are merged and output to the screen. This is the only communication that takes place in CrossFire at the moment. R700 will change that by allowing GPUs to communicate.
RV770 has a CrossFire X Sideport…we assume that the two RV770s on a single R700 board somehow connect Sideports and make fast. AMD hasn’t told us how yet.

It is not clear how extensive this communication will be, what information will be shared, or how much bandwidth requirements are increased because of this feature. And while it is a step in the right direction, the holy grail of single-card multi-GPU solutions will be a shared framebuffer. Currently both GPUs need a copy of all textures, geometry, etc., and this is a huge waste of resources. While the R700 has 2GB of RAM on board, it will still be limited in many of the same ways a 1GB RV770 would be as each GPU only has access to half the RAM on the card. Of course, since we don’t have a 1GB RV770 yet, this card could show some advantages over the single 4870 regardless of CrossFire.

Final Words

The two new additions to the test suite, Age of Conan and Race Driver GRID certainly beef up the portfolio of advantages AMD has in the current generation. Not only that, but we got quite an interesting surprise with GRID. Yes, even with R700 the menu screens were slow, jerky and painful at 2560×1600, but gameplay with R700 was much improved. We will still see the upper limit at twice the performance of a single RV770, but the fact that R700 looks to offer at least the potential for much better scaling at the high end (where the card will be marketed) than two card solutions is an advantage, even if it only comes from the additional RAM. We will need to look at CrossFire again when the 1GB 4870 is available to get a more apples to apples comparison here.

We don’t have any solid evidence in this article of the GPU to GPU communication hardware making a real difference in scaling, but we will be interested to see if that changes by the time R700 hits store shelves. We can say that, for those who want to game at the extremely high end, 4870 X2 with it’s 2GB of on board RAM will be a more consistent solution than 2x 512MB 4870 cards in CrossFire, as evidenced by our Race Driver GRID test.

But as we’ve said many times before, the success of CrossFire is in the consistency of it’s performance. We absolutely need to see AMD put everything they have into making all games past, present, and future scale well with CrossFire. There needs to not be even the inkling of a question that CrossFire might not improve the performance of a game. Until then it is very risky for AMD to put all of its hope for the high end business into a multi-GPU solution.

Maybe they’ve done it. Maybe their driver team, by the time 4870 X2 launches will have improved driver support to where it needs to be. Maybe the changes to the R700 hardware will be enough to fill in the gaps and bring performance up closer to the theoretical limit for many more games. The information we have here shows the incredible potential R700 has, but it will be absolutely necessary to wait until launch day to see if the execution behind the hardware has been enough to realize this potential across the board.

This isn’t a case where the quality of the hardware, but rather the quality of the driver and programming style of modern game developers will be key in delivering value. For the launch of this card we will be looking at as large a cross section of games as we can, as the importance of broad testing has never been more clear than with AMD’s first part after their declared strategy of using single card multi-GPU solutions to compete in the high end space. We will need to see more examples of improved single card multi-GPU performance over CrossFire, as well as improvements in CrossFire performance in general if AMD is to be taken seriously going forward in the high end space.

The 4870 X2 will be AMD’s proving ground. This preview shows what might be, what could be … but we must wait for final hardware and final drivers before we can honestly evaluate the card for what it is. Let us hope AMD knows how important having pervasive compatibility really is for this launch.