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Nvidia + AMD food for thought...

post #1 of 3
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Just a rant / thought process for the odd behavior seen when using Sli on a AMD system.



I think Nvidia has a 8bit switch in their drivers from the old nvidia chipset days, lol. Some of the older AMD nvidia chipsets were built with 8 bit HT buses to link to the north bridge (all modern day AMD HT implementations use 16bit). This would effectively limit ht 2.0 at the time to 2000mb/s. if using a 8 bit switch on a modern day ht 3.0 interface, that would limit the bandwidth to 4000mb/s. That is enough to saturate a PciE x16 1.x lane, however is only half as much is needed to saturate a PciE x16 2.x lane (8000mb/s).

When using a AMD chipset that does not utilize the NF200 repeater chip (which are seen primarily in conjunction with Intel chipsets), both cards require full bandwidth capability when in Sli. Not to mention the latency discrepancy between PciE and the HT bus. Even if the HT bus was being utilized to its full potential, Im sure there is still little optimization within the drivers themselves for the HT bus communication to the chipset's north-bridge.

So as it stands there is enough bandwidth on a AMD chipset to saturate ONE PciE Graphics cards needs, but when Sli is introduced the "Bottleneck" (as so many are anxious to refer) is in all reality in the lack of optimization to utilize the chipset's full potential. This is also why, I believe, AMD users see low GPU usage while utilizing so little CPU resources. Its the only logical explanation for the symptoms seen with AMD + Sli, and furthermore why these symptoms are not seen with the use of AMD's CrossFire.


*Constructive* Thoughts? Input?
Edited by MadGoat - 11/4/11 at 5:41pm
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post #2 of 3
Interesting analysis, but I wasn't aware of any SLI impedance as compared to a set of AMD GPUs when using an AMD CPU. Is this something that's very common or is it just from your own experience?
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post #3 of 3
Yea, I kinda thought something was up with AMD Chipset SLI, since results always seemed kinda strange to me.

Didn't really say anything about it because I didn't have any proof to back it up since i don't own 2 of any video card
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