The Radeon HD 4850 & 4870: AMD Wins at $199 and $299
by Anand Lal Shimpi & Derek Wilson on June 25, 2008 12:00 AM EST- Posted in
- GPUs
One, er, Hub to Rule them All?
With R500 AMD introduced its first ring bus, a high speed, high bandwidth bus designed to move tons of data between consumers of memory bandwidth and the memory controllers themselves. The R600 GPU saw an updated version of the ring bus, capable of moving 100GB/s of data internally:
On R600 the ring bus consisted of two 512-bit links for true bi-directional operation (data could be sent either way along the bus) and delivered a total of 100GB/s of internal bandwidth. The ring bus was a monster and it was something that AMD was incredibly proud of, however in the quest for better performance per watt, AMD had to rid itself of the ring and replace it with a more conventional switched hub architecture:
With the ring bus data needed to be forwarded from one ring stop to the next and all clients got access to the full bandwidth, regardless of whether or not they needed it. For relatively low bandwidth data (e.g. UVD2 and display controller data), the ring bus was a horrible waste of power.
With the RV770 all that exists is a simple switched hub, which means that sending data to the display controller, PCIe and UVD2 (AMD's video decode engine) traffic are now far less costly from a power standpoint. Another side effect of ditching the ring bus is a reduction in latency since data is sent point to point rather than around a ring. With the move to a hub, AMD increased their internal bus width to 2kbits wide (which is huge). Maximum bandwidth has increased to 192GB/s (in 4870) but this depends on clock speeds.
With nearly double the internal bandwidth and a point to point communication system, latency between memory clients should be decreased, and huge amounts of data can move between parts of the chip. Certainly getting enough data on to the GPU to feed 800 execution units is a major undertaking and AMD needed to make a lot of things wider to accommodate this.
The CrossFire Sideport
Although AMD isn't talking about it now, the CrossFire Sideport is a new feature of the RV770 architecture that isn't in use on the RV770 at all. In future, single-card, multi-GPU solutions (*cough* R700) this interface will be used to communicate between adjacent GPUs - in theory allowing for better scaling with CrossFire. We'll be able to test this shortly as AMD is quickly readying its dual-GPU RV770 card under the R700 codename.
One thing is for sure, anything AMD can do to assist in providing more reliable consistent scaling with CrossFire will go a long way to help them move past some of the road blocks they currently have with respect to competing in the high end space. We're excited to see if this really makes a difference, as currently CrossFire is performed the same way it always has been: by combining the output of the rendered framebuffer of two cards. Adding some sort of real GPU-to-GPU communication might help sort out some of their issues.
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natty1 - Thursday, June 26, 2008 - link
There's no good reason to pull that garbage. People assume they are seeing raw numbers when they read these reviews.DerekWilson - Sunday, June 29, 2008 - link
i don't understand what you mean by raw numbers ... these are the numbers we got in our tests ...we can't do crossfire on the nvidia board we tested and we can't do sli on the intel board we tested ...
we do have another option (skulltrail) but people seemed not to like that we went there ... and it was a pain in the ass to test with. plus fb-dimm performance leaves something to be desired.
in any case, without testing every solution in two different platforms we did the best we could in the time we had. it might be interesting to look at testing single card performance in two different platforms for all cards, but that will have to be a separate article and would be way to tough to do for a launch.
Denithor - Wednesday, June 25, 2008 - link
In Bioshock in the multiGPU section the SLI 9800GTX+ seems to fall down on the job. In all other benches this SLI beats out the GTX 280 easily, here it fails miserably. While even the SLI 8800GT beats the GTX 280. Methinks something's wrong here.jamstan - Wednesday, June 25, 2008 - link
Egg's got them for 309.99. I'm gonna run 2 4870s in CF. I planned on using a P45 board but I am wondering if the P45s X8 per card will bottleneck the bandwidth and if I should go with an X48 board instead? When I research CF all I seem to find is "losing any bandwidth at X8 versus X16 is "debateable". What I'm thinking is that 8 pipelines can handle 4GBs so if I look at the 4870s 3.6 Gbs of memory bandwidth then X8 should be able to handle the 4870 without any performance hits. It that correct or am I all wet?jamstan - Friday, June 27, 2008 - link
I contacted ATI and they said I was correct. A P45 board only running X8 per card in CF will bottleneck the massive DDR5 bandwidth of the 4870s. If you're gonna CF 2 4870s use an X38 or X48 board.SVM79 - Wednesday, June 25, 2008 - link
I created an account just to say how awesome this article was. It was really nice to see all the technical details laid out and compared to the competition. I was lucky to get in on that $150 hd4850 price at best buy last week and I am hoping the future drivers with improve performance even more. Please keep up the good work on these articles!!!DerekWilson - Sunday, June 29, 2008 - link
Wow, Anand and I are honored.We absolutely appreciate the feedback we've gotten from all of you guys (even the bad stuff cause it helps us refine our future articles).
of course we enjoy the good stuff more :-)
thanks again, everyone.
D3SI - Wednesday, June 25, 2008 - link
Long time reader, first time postergreat article, very informative
looks like the 4870 is the card to get, cant be beat at that price
and yes a lot of posters are reading way too much into it "you're biased waaa waaa boo hoo"
just get the facts from the article (thats what the charts and graphs are for) and then make your decision, if you cant do simple math and come to the conclusion yourself that the $300 card is a better buy than the $650 then you deserve to get ripped off.
joeschleprock - Wednesday, June 25, 2008 - link
nVidia just got their pussy smoked.kelectron - Wednesday, June 25, 2008 - link
a very important comparison is missing. for those who want to go in for a multi-GPU setup, the 260 SLI vs 4870 CF is a very important consideration since SLI scaling has always been better than CF, and the 260 scales very very well.in that case, if nvidia responds by reducing the price on the 260, the 260 SLI could be the real winner here. but sadly there were no 260 SLI benches.
please give us a 260 SLI vs 4870 CF review.