In my relatively short time learning OpenCL I frequently see my application cause the operating system UI to become significantly less responsive (several seconds for a window to respond to a drag for example). I have encountered this problem on Windows Vista and Mac OS X both with NVidia GPUs.
What can I do when using OpenCL on the same GPU as the display to ensure that my application does not significantly degrade the UI responsiveness like this? Also, can this be done without taking needless performance losses within my application? (Ie, if the user is not doing some UI intensive task then I would not expect my application to run any slower than it does now.)
I understand that any answers will be very platform specific (where platform includes OS/GPU/driver combo).
As described in Dr. David Gohara’s OpenCL Tutorial Episode 6 (beginning at 43:49), graphics cards cannot be preemptively scheduled at this time. As a result, using the same graphics card both for an intensive OpenCL kernel and the UI (or other GPU-using operations) will result in clunkiness or the visual appearance of freezing. Until graphics cards get preemptively scheduled multitasking (if ever), there’s no way to do exactly what you want with just a single graphics card. I don’t believe this is a platform-specific issue at all.
However, this problem might be solvable by dividing the problem up. Given the relative speed of whatever single GPU is available (you’ll have to do testing to find the right setup), divide up your OpenCL problem to run the kernel multiple times with different parts of the input data, and later combine the output data when all sets of kernels are complete. I would recommend creating kernel sets that take less than 100 milliseconds to run (on a given GPU) so that lag would be, if not unnoticeable, not significantly annoying (the 100 milliseconds figure is a good “rule of thumb” according to this paper).
Based on your comment about your program being a command-line application, I assume your application will only run once at any given time, versus being a continuously running application with real-time output, as a lot of OpenCL demos are. My above answer is only satisfactory for non-continuous applications, since real-time performance isn’t inherently expected. However, if your application is supposed to be continuous, the only solution currently available is to add a second, simpler graphics card that will only be used for UI.