William Yu wrote:
You can get 64-bit Xeons also but it takes hit in the I/O department due to the lack of a hardware I/O MMU which limits DMA transfers to addresses below 4GB. This has a two-fold impact:
1) transfering data to >4GB require first a transfer to <4GB and then a copy to the final destination.
2) You must allocate real memory 2X the address space of the devices to act as bounce buffers. This is especially problematic for workstations because if you put a 512MB Nvidia card in your computer for graphics work -- you've just lost 1GB of memory. (I dunno how much the typical SCSI/NIC/etc take up.)
When you say "allocate real memory 2X" are you saying that if you have 16GB of RAM only 8GB is actually usable and the other 8GB is for bounce buffers, or is it just address space being used up?
It's 2x the memory space of the devices. E.g. a Nvidia Graphics card w/ 512MB of RAM would require 1GB of memory to act as bounce buffers. And it has to be real chunks of memory in 64-bit mode since DMA transfer must drop it into real memory in order to then be copied to > 4GB.
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