I'm really not sure about this, but I believe there could be a severe performance problem when using pci radeons (at least on x86). I haven't seen any measurements of this, and few people seem to have such hardware.
But by using the "BusType" option, I've forced pci mode on my radeon 9000pro agp, and the results are simply terrible. In theory, results should probably be pretty much the same as AGP 1x (same raw data rata, as it's operating as a pci 66mhz device afaik), but it's very different
That's not quite true. Most motherboards and cards only support 33MHz PCI. As far as I know, only a few 3dlabs and 3dfx boards ever supported 66MHz PCI fully. If a given application is texture (or vertex, or command) upload bound, that should, at worst, cut performance in half. What's beeing seen here is more like cutting it in a tenth.
It's possible that this card is really bad at PCI transfers. Maybe we should put our vertex / command buffers in on-card memory in the PCI case and avoid "AGP" texturing. That seems yucky, but I'm not even sure how we would go about tracking this down otherwise.
in practice. (Tests on other OS certainly show there is next to zero performance difference for graphic cards of this generation and fillrate-heavy games such as Q3A between AGP and PCI). However, since this isn't a "true" pci card, the chipset (or the driver?) might not like this mode of operation (not sure what could be a problem, but you never know - maybe someone with more knowledge of the drm/gart stuff could answer that).
I don't think ATI makes a "true" PCI chip. They just wire the AGP chip to the bus in such a way that it looks PCI. Remember that AGP and PCI are, both in terms of physical and signal, very, very similar.
Results (AXP 1600, 9000pro, 1GB sdram, KT133A Chipset):^^^
I didn't know Quake3 was available for the Alpha. ;) I know, I know...AXP == Athlon XP. That abreviation was also used for the Alpha architecture, though.
glxgears QuakeIII (1024x768, graphic options all set to high) AGP 4x 1910 62.5 AGP 1x 1860 61.1 PCI 200 16.8
Ouch.
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