How does a Mac Pro handle two gpu's?

FranzyPants

macrumors newbie
Original poster
Nov 1, 2009
12
0
I have a Mac Pro 3,1 with an 8800gt in it. I've been thinking of upgrading to one of the newer ati/amd cards available, the 5770 and 5870. But the 5770 doesn't seem to have enough power over my 8800 to justify the $250 and the 5870 is a bit to pricey for me.

If I run my 8800gt and a 5770 in my Mac, how will osx handle them? Will the gpu only handle applications running on the monitor plugged into them? Meaning if display 1 is running portal 2 and is plugged into my 8800, will only the 8800 process portal? Or does osx do a good job of dividing up the work between the gpu's?

I realize that the Mac Pro doesn't support sli/xfire under osx, and it might not be a great idea to mix an nvidia and ati in the same machine. I'm just trying to get the most bang for my buck.

I'd appreciate any help/links with more info about this. And apologies if this is already posted somewhere, kinda difficult topic to search for.
 

Neodym

macrumors 68000
Jul 5, 2002
1,927
510
It is no problem to mix nVidia and ATI (i have a 7300 nVidia running together with an AMD 6870 without a single problem from that combination). However, OSX won't distribute load between two cards. As you already expected - if Portal 2 is running on the monitor plugged into your 8800 then only the 8800 will calculate the graphic.

An inexpensive alternative for you could be the 6870, which is very close to the 5870 in terms of performance (though consuming less power). Its standard PC versions are supported by OSX 10.6.8 (and onwards) out of the box. The only drawback is that you have to manually mitigate three issues:

1. No DVD player (the OSX one).
2. No Steam games.

Those two can be cured by installing ATY_Init.kext from Netkas.

3. No boot screen.

As you are on a MP 3,1, you can cure that one as well by flashing the card with another firmware (search netkas.org for details). If you don't want to flash, you can alternatively keep the 8800 as helper card to show the boot screen (e.g. for chosing a boot device) and the 6870 will show a picture when either the finder or the login screen will eventually be displayed.
 

FranzyPants

macrumors newbie
Original poster
Nov 1, 2009
12
0
It is no problem to mix nVidia and ATI (i have a 7300 nVidia running together with an AMD 6870 without a single problem from that combination). However, OSX won't distribute load between two cards. As you already expected - if Portal 2 is running on the monitor plugged into your 8800 then only the 8800 will calculate the graphic.

An inexpensive alternative for you could be the 6870, which is very close to the 5870 in terms of performance (though consuming less power). Its standard PC versions are supported by OSX 10.6.8 (and onwards) out of the box. The only drawback is that you have to manually mitigate three issues:

1. No DVD player (the OSX one).
2. No Steam games.

Those two can be cured by installing ATY_Init.kext from Netkas.

3. No boot screen.

As you are on a MP 3,1, you can cure that one as well by flashing the card with another firmware (search netkas.org for details). If you don't want to flash, you can alternatively keep the 8800 as helper card to show the boot screen (e.g. for chosing a boot device) and the 6870 will show a picture when either the finder or the login screen will eventually be displayed.
Great, thanks for the help. I'll look into ATY_Init.kext. Couple questions though, I'm running my Mac Pro with the 64 bit EFI enabled. Will I still be able to keep my EFI in 64 bit mode with Netkas's .kext? And do I just add .kext files to /System/Library/Extensions ?
 
Last edited:

minifridge1138

macrumors 65816
Jun 26, 2010
1,025
52
The Mac Pro can not spread one job across multiple cards.

If a monitor is plugged into card A, then everything on that monitor is processed by card A.
Regardless of the presence of additional video cards.
 

Neodym

macrumors 68000
Jul 5, 2002
1,927
510
Great, thanks for the help. I'll look into ATY_Init.kext. Couple questions though, I'm running my Mac Pro with the 64 bit EFI enabled. Will I still be able to keep my EFI in 64 bit mode with Netkas's .kext?
I don't know why you shouldn't. Though i'm no expert in that area. If you want to know more you could browse through the forum over at netkas.org.

And do I just add .kext files to /System/Library/Extensions ?
Get the 6870 package from Cindori's site. It contains a little tool for .kext installation, so you can't go wrong...
 

FranzyPants

macrumors newbie
Original poster
Nov 1, 2009
12
0
Thanks Neodym, great resources. I'm gonna do some searching for a nicely price 6870. Maybe I'll have it up and running and a week or so.

Thanks again for the help!