- Jan 20, 2004
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Catalyst 8.1 correctly identifies the cards. We could see one HD3870 and one HD3850 card and we could access them separately in the Overdrive section. You can overclock them and the driver allowed us to change GPU and memory clocks on each card regardless from the other card's settings. However, this didn't give us much in terms of performance, but we're still pleased to see that you can mix these R670 based cards and set the clocks independantly.
Originally posted by: betasub
apoppin: Are you using Catalyst 8.1 for your 2900 crossfire? Unlinking the GPU clocks should allow you to find how much extra you can gain from the second card before you become totally CPU or PCIe4x limited.
Originally posted by: betasub
It must be nice to be getting a decent boost from a cheap, secondary "support" card (compared to the XT!). In fact my local price for the 256bit pro has dropped so low, the 512bit is now >50% more expensive. Sapphire must have a container-load of R600 GPUs to sell, but they're being priced to sell to the budget market using 256bit memory, instead of the original expensive 512bit memory. At this rate I could sell my 512bit pro and pick up two 256bit cards... :Q
apoppin, I'm looking for a comparison of similarly clocked R600s with 256 vs 512bit memory. I know you are busy with your crossfire benchmarks, but if you know of any online comparison pls could point me to the URL.
This will probably be the first time in history that the part with narrower memory controller (256-bit) will beat wider part (512-bit), at least in some tests. We were told to pay special attention to titles that are heavy on longer shader code. Expect that Radeon HD 2900XT will speed pass through RV670 in all tests that require heavy bandwidth load, such as FullHD (1920x1200) or XHD (2560x1600) variants. Bear in mind that AMD will push for multi-GPU configurations for these users, and if you opt for a single-slot board, it should be no problem to put 2-3-4 boards in the same system.
In addition to its DirectX 10.1 support, the HD 3800 series offers several other features that differentiate it from its R600-based brethren. The new core retains the R600's 512-bit ring bus, but substitutes a higher-efficiency 256-bit memory bus over the old 512-bit configuration. ATI maintains that the increased efficiency of the new memory bus will generally compensate for bandwidth loss.
The HD 3800 series has also been updated to compute double-precision floating point data, as opposed to the single-point precision the R600 was capable of.
Originally posted by: Killrose
I have a Gigabyte AM2 board that supports crossfire but only has (1)16x and (1)4x PCi-e slots. I wonder what the penalty would be in that scenario? Do both slots become 4x only?
Originally posted by: apoppin
Originally posted by: Killrose
I have a Gigabyte AM2 board that supports crossfire but only has (1)16x and (1)4x PCi-e slots. I wonder what the penalty would be in that scenario? Do both slots become 4x only?
no ... not at all ... what happens is that the bandwidth of the *2nd* is restricted - 10-25% in extreme cases; You get the "horsepower" of the first card [clocked to it's highest OC] with the 2nd card contributing whatever it can - also on its own separate OC.
we are still unsure of what a 256-bit coupled with a 512-bit card does to the wider bandwidth card .... but i am surmising it is *moot* in my case since my resolutions are only up to 16x12.
Originally posted by: apoppin
i remember the discussions here that the 512-bit for the 2900xt was OverKill ... evidently
Originally posted by: betasub
Originally posted by: apoppin
i remember the discussions here that the 512-bit for the 2900xt was OverKill ... evidently
Thinking about this, I realised all I needed to do was to simulate the 256bit card was run my 512bit card at half memory clock. Using 3dmark06 as a convenient bench, the results were surprising to me, having accepted conventional wisdom, but agree with some of your links. (CPU scores just to show unchanged basic platform).
2900 Clocks......850/850 ... 850/425
Bandwidth Gb/s....108.8 ... 54.4
3DMark Score......11127 ... 9791
Sm2.0 Score........4526 ... 3801
GT1.....................34.568 ... 28.001
GT2.....................40.868 ... 35.347
HDR/SM3.0...........5314 ... 4542
HDR1...................48.507 ... 42.028
HDR2...................57.777 ... 48.819
CPU Score............2889 ... 2907
CPU1....................0.916 ... 0.923
CPU2....................1.458 ... 1.466
OK - no more than 10fps difference in any one game test, but enough of a difference to show bandwidth does matter at 1280x1024. And for gaming at higher resolutions, while feeding two GPUs in crossfire, the effect is only going to get larger.
Now to try out that PCIe 4x slot