OpenCL: number of platforms

how do I choose the number of platforms in my OpenCL programs?
Does it affect the program-speed?


The number of platforms depends on what hardware your system has, and what drivers are installed. Then your program sees all the platforms, and all of the devices in each platform, and can select which one it uses (or, if you software has lots of work to do you could use them all). Some software lets the user select which device is used.