btw texdown showed that texture transfers to card memory are faster than to AGP memory, but not by very much (something like 100MB/s vs. 140MB/s in the best case, though the numbers I got fluctuated quite a bit).
How are AGP texture uploads being done?
The card memory uploads are actually done via agp buffers - ie the data is written by the driver to agp memory, the card then copies that to card memory. If the AGP case is the same, the data probably travels up to the card and then back down again to AGP memory, accounting for the relative slowdown.
One benefit of using the card to do the up/downloads is synchronization with the graphics engine - if you were to write the texture data directly you'd have to have some extra mechanisms to ensure that the memory wasn't being used by commands still unprocessed by the GPU. This actually wouldn't be that hard to organize.
Also, note that there is quite a bit of copying going on:
- Application calls glTexImage
- Mesa allocates system memory and copies image
- Driver allocates agp buffers and copies image into them
- Card receives blit command and copies image to final destination.Currently Mesa needs to keep the system memory copy because texture images in card or agp memory can be clobbered by other apps at any time - Ian's texture manager will address this.
In the via and sis drivers, texture allocations are permanent, so I've been able to try a different strategy:
- Application calls glTexImage
- Mesa allocates AGP/card memory and copies texture directly to final destination (using memcpy().)
This resulted in an approximate 2x speedup in texture downloads against a strategy similar to the first one outlined (but implemented with cpu copies, not a hostdata blit).
Keith
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