These Aren't the Sideports You're Looking For

Remember this diagram from the Radeon HD 4850/4870 review?

I do. It was one of the last block diagrams I drew for that article, and I did it at the very last minute and wasn't really happy with the final outcome. But it was necessary because of that little red box labeled CrossFire Sideport.

AMD made a huge deal out of making sure we knew about the CrossFire Sideport, promising that it meant something special for single-card, multi-GPU configurations. It also made sense that AMD would do something like this, after all the whole point of AMD's small-die strategy is to exploit the benefits of pairing multiple small GPUs. It's supposed to be more efficient than designing a single large GPU and if you're going to build your entire GPU strategy around it, you had better design your chips from the start to be used in multi-GPU environments - even more so than your competitors.

AMD wouldn't tell us much initially about the CrossFire Sideport other than it meant some very special things for CrossFire performance. We were intrigued but before we could ever get excited AMD let us know that its beloved Sideport didn't work. Here's how it would work if it were enabled:

The CrossFire Sideport is simply another high bandwidth link between the GPUs. Data can be sent between them via a PCIe switch on the board, or via the Sideport. The two aren't mutually exclusive, using the Sideport doubles the amount of GPU-to-GPU bandwidth on a single Radeon HD 4870 X2. So why disable it?

According to AMD the performance impact is negligible, while average frame rates don't see a gain every now and then you'll see a boost in minimum frame rates. There's also an issue where power consumption could go up enough that you'd run out of power on the two PCIe power connectors on the board. Board manufacturers also have to lay out the additional lanes on the graphics card connecting the two GPUs, which does increase board costs (although ever so slightly).

AMD decided that since there's relatively no performance increase yet there's an increase in power consumption and board costs that it would make more sense to leave the feature disabled.

The reference 4870 X2 design includes hardware support for the CrossFire Sideport, assuming AMD would ever want to enable it via a software update. However, there's no hardware requirement that the GPU-to-GPU connection is included on partner designs. My concern is that in an effort to reduce costs we'll see some X2s ship without the Sideport traces laid out on the PCB, and then if AMD happens to enable the feature in its drivers later on some X2 users will be left in the dark.

I pushed AMD for a firm commitment on how it was going to handle future support for Sideport and honestly, right now, it's looking like the feature will never get enabled. AMD should have never mentioned that it ever existed, especially if there was a good chance that it wouldn't be enabled. AMD (or more specifically ATI) does have a history of making a big deal of GPU features that never get used (Truform anyone?), so it's not too unexpected but still annoying.

The lack of anything special on the 4870 X2 to make the two GPUs work better together is bothersome. You would expect a company who has built its GPU philosophy on going after the high end market with multi-GPU configurations to have done something more than NVIDIA when it comes to actually shipping a multi-GPU card. AMD insists that a unified frame buffer is coming, it just needs to make economic sense first. The concern here is that NVIDIA could just as easily adopt AMD's small-die strategy going forward if AMD isn't investing more R&D dollars into enabling multi-GPU specific features than NVIDIA.

The lack of CrossFire Sideport support or any other AMD-only multi-GPU specific features reaffirms what we said in our Radeon HD 4800 launch article: AMD and NVIDIA don't really have different GPU strategies, they simply target different markets with their baseline GPU designs. NVIDIA aims at the $400 - $600 market while AMD shoots for the $200 - $300 market. And both companies have similar multi-GPU strategies, AMD simply needs to rely on its more.

Let's Talk Pricing General Performance at 2560x1600


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  • EglsFly - Tuesday, August 12, 2008 - link

    I agree!

    It is because of ATI/AMD's release of the 4000 series cards that Nvidia had to dramatically drop the price of its GPU's.

    AMD brought great performance to the masses at an affordable price point. Here they up the ante with an even higher performing solution and what do we get in return from Anandtech? A biased review full of negativity that it looks like it was written by somebody from Nvidia.
  • CyberHawk - Tuesday, August 12, 2008 - link

    That kind of message was I hoping for to get from review... a kind of didn't happen. Reply
  • BRDiger - Tuesday, August 12, 2008 - link

    I just wondered if you useed the 8.8 Catalysts... The testing rigs specs would be nice for comparison of the benchies... Reply
  • nubie - Wednesday, August 13, 2008 - link

    This is interesting, and thanks for the hints about a 1GB model, but guru3d ran an article two weeks ago on a 2GB 4850, so I believe that is trumped.

    I too was hoping for more enthusiasm, the 9800GTX is $200 and the just released GTX 260 is under $300?? Stop the presses nVidia is no longer on top!!

    AMD has wrested back the performance crown with a vengeance, and their mainstream products are totally playable in recent games.

    Meanwhile nVidia is trying to plug every price point with the 8800GS and 9600GSO and the 9600GT, not to mention the 9800GTX+, this is freaking ridiculous.

    You need to paint a more realistic picture, this is one of the rare times that mainstream games can be played for $170 while decimating the competition's products that cost $250, and the high end is owned by the same company with a working Dual chip card with the performance crown, and being a more efficient electricity user than the competition.

    If nVidia comes out with a GTX 260 x2 or a GTX 280 x2 I am going to look very carefully to see how glowing THAT review is.

    I want SLi and Crossfire to die. There is no reason to only allow 2 displays (or worse just one) on a multi-output machine. Worse still a machine with 2x PCI-E 16x slots (even in dual 8x mode) should be allowed to run any hardware that fits in them.

    This software hampering of a completely standard PCI-E interface is stupid and childish, they should just drop it.

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