Fun With Menubars

My Mac Pro has just gotten weirder - I put a Radeon HD 3870 into the second PCIe x16 slot. (The machine comes with  a GeForce 8800.)  I now have one monitor in each.

So here's where things get fun:

• Start X-Plane.  60 fps.
• Drag the window to the second monitor.  30 fps.
• Quit, move the menu bar to the second monitor, restart.  (X-Plane is now on the right.)  160 fps.
• Drag the window back to the primary monitor on the left.  100 fps.

What's going on?  Two things:

• On OS X, X-Plane's graphics are rendered by one video card, and that video card (in 921) is the card that has the menu on one of its monitors.
• When an OpenGL window is displayed on a monitor that is not attached to the video card that is doing the rendering, OS X will copy the image from one video card to another, at a cost of some framerate.

So what's going on above?  Well, the 60 fps is my 8800.  When I drag the window, the OS starts copying the graphics, slowing fps.  When I move the menu bar, the 3870 does the rendering, and we get much higher fps.  Once again, put the window on the monitor that is not attached to the video card, and fps hit.

Final note: fps tests of the 8800 vs 3870 with X-Plane 921:

Fps test 2, 8800: 46,49,51

Fps test 2, 3870: 70,75,80

Fps test 3, 8800: 24,25,25

Fps test 3, 3870: 40,41,43

In other words, the 3870 is significantly faster.  I believe that this is due to the OS X drivers, not the cards themselves.  Note that the 3870 is in a PCIe 1.0 slot and the 8800 is in a PCIe 2.0 slot.