I own a MacBook Pro 17″ with Core2 Duo 2.8Ghz, IGP MCP79 (GeForce 9400M w/16SP), and GeForce 9600MGT (32SP), to develop for CUDA and OpenCL. I like the ability to use GeForce 9400M and 9600M GT in parallel, the GeForce 9400M to try PINNED MAPPED MEMORY exchange with the CPU while kernels are running, 9600M GT for 2.5X more performance and dedicated 512MB, and both to improve dynamic load-balancing between GPUs or CPU and GPUs.
This computer could be set to use only the GeForce 9400M is active, 9600M GT shutdown, it’s called “Better battery life” on System Preferences. I dont’t use it since I want best performance for OpenCL and CUDA, and the ability to use any of the GPU at any time for computing, and even both sometimes.
The “Better Performance” setting
In this default mode, that I use each and everyday, each GPU is active and visible on both OpenCL and CUDA. They also appears together on the “About this Mac” page as graphic displays.
So you could run your CUDA or OpenCL code in any of these GPU, albeit GeForce 9600M GT is running at 1.25Ghz and GeForce 9400M only around 400Mhz. But they are both usable.
In this mode, Galaxy OpenCL Benchmark will give you 20Gflops CPU, 7 Gigaflop 9400M (400Mhz) and 43 GFlops 9600M GT.
The “Better battery life” setting
The GeForce 9600M GT disappear from the “About this Mac” video-card list, so it seems to be deactivated completely… but… taddammmmmm
-edit- on last system version 10.6.2 they both appear again in “better battery life” setting!
On OpenCL Galaxy benchmark, and OpenCL list of devices, it re-appears, fully useable. Moreover, the GeForce 9600M GT run at full speed, 1.25Ghz, and the GeForce 9400M too, at 1.1Ghz (instead 400Mhz in “Better Performance” setting!).
And so there goes the Galaxy OpenCL Benchmark results: CPU 22 Gflops (+10%), 9400M 19Gflops (2.7X faster) and 9600M GT 45 Gflops (+5%). Yes, it’s faster whatever the metric you consider than using “Best Performance” mode, at elast in case of OpenCL development, with a total gain of 16Gflops (+23% overall).
GeForce 9600M GT is faster because it don’t have to handle graphic anymore, CPU is faster because IGP is running at 1.1Ghz instead 400Mhz and it improves memory IO, 9400M is far faster running at 1100Mhz instead 400Mhz even while it needs to drive the video output and OpenGL display!
And unplugged on battery?
Performances are totally identical, albeit GPU took more time to go to their maximum frequencies, due to energey-saving policy. So battery or AC-plugged doesn’t matter from a performance point-of-view, either in “Better battery life” or “Better performance” mode.
So which mode to choose
It’s clear if you have OpenCL-enabled software, go for the “Better battery life” setting, because CPU is faster anyway (10% more, FREE upgrade of your Mac! lol!) and OpenCL is faster too, whichever GPU is used by the application!
Notice that a laptop may provide 86 Gflops of processing power on Galaxy benchmark, that is a real-world astro-physic application, not a simple MAD benchmark that only favorise number of core on a GPU. These 86 Gflops are largely over an actual Mac Pro 8-core 2.66Ghz with 16threads (2 quad-core Xeon processors).
I want to see more and more OpenCL-enabled application!
