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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DerekWilson - Wednesday, June 25, 2008 - link
it looks like the witcher hits an artificial 72fps barrier ... not sure why as we are running 60hz displays, but that's our best guess. vsync is disabled, so it is likely a software issue.JarredWalton - Wednesday, June 25, 2008 - link
Again, try faster CPUs to verify whether you are game limited or if there is a different bottleneck. The Witcher has a lot of stuff going on graphically that might limit frame rates to 70-75 FPS without a 4GHz Core 2 Duo/Quad chip.chizow - Wednesday, June 25, 2008 - link
It looks like there seems to be a lot of this going on in the high-end, with GT200, multi-GPU and even RV770 chips hitting FPS caps. In some titles, are you guys using Vsync? I saw Assassin's Creed was frame capped, is there a way to remove the cap like there is with UE3.0 games? It just seems like a lot of the results are very flat as you move across resolutions, even at higher resolutions like 16x10 and 19x12.Another thing I noticed was that multi-GPU seems to avoid some of this frame capping but the single-GPUs all still hit a wall around the same FPS.
Anyways, 4870 looks to be a great part, wondering if there will be a 1GB variant and if it will have any impact on performance.
DerekWilson - Wednesday, June 25, 2008 - link
the only test i know where the multi-gpu cards get past a frame limit is oblivion.we always run with vsync disabled in games.
we tend not to try forcing it off in the driver as interestingly that decrease performance in situations where it isn't needed.
we do force off where we can, but assassins creed is limiting the frame rate in absentia of vsync.
not sure about higher memory variants ... gddr5 is still pretty new, and density might not be high enough to hit that. The 4870 does have 16 memory chips on it for its 256-bit memory bus, so space might be an issue too ...
JarredWalton - Wednesday, June 25, 2008 - link
Um, Derek... http://www.anandtech.com/video/showdoc.aspx?i=3320...">I think you're CPU/platform limited in Assassin's Creed. You'll certainly need something faster than 3.2GHz to get much above 63FPS in my experience. Try overclocking to 4.0GHz and see what happens.weevil - Wednesday, June 25, 2008 - link
I didnt see the heat or noise benchmarks?gwynethgh - Wednesday, June 25, 2008 - link
No info from Anandtech on heat or noise. The info on the 4870 is most needed as most reviews indicate the 4850 with the single slot design/cooler runs very hot. Does the two slot design pay off in better cooling, is it quiet?DerekWilson - Wednesday, June 25, 2008 - link
a quick not really well controlled tests shows the 4850 and 4870 to be on par in terms of heat ... but i can't really go more into it right now.the thing is quiet under normal operation but it spins up to a fairly decent level at about 84 degrees. at full speed (which can be heard when the system powers up or under ungodly load and ambient heat conditions) it sounds insanely loud.
legoman666 - Wednesday, June 25, 2008 - link
I don't see the AA comparisons. There is no info on the heat or noise either.DerekWilson - Wednesday, June 25, 2008 - link
the aa comparison page had a problem with nested quotes in some cases in combination with some google ads on firefox (though it worked in safari ie and opera) ...this has been fixed ...
for heat and noise our commentary is up, but we don't have any quantitative data here ... we just had so much else to pack into the review that we didn't quite get testing done here.