How to combine CPU and GPU in JavaCL? - opencl

Question

My laptop had
- one CPU core i5: Intel(R) Core(TM) i5-3210M CPU # 2.50GHz
- one Graphic card: Intel(R) HD Graphics 4000
- one Nvidia card ( external card ): GeForce GT 630M
But When I tried to use JavaCL.createBestContext(), it looks like just use one card Intel HD Graphics. So I tried to combine 3 : CPU and 2 GPUs by using:
List<CLDevice> devices = new ArrayList<CLDevice>();
// try to list all platform and devices
for(CLPlatform platform : JavaCL.listPlatforms()) {
//System.out.println(platform.getName());
for (CLDevice device : platform.listAllDevices(true)) {
System.out.println(device.getName().trim());
devices.add(device);
}
}
CLDevice device1 = (CLDevice)devices.get(0);
CLDevice device2 = (CLDevice)devices.get(1);
CLDevice device3 = (CLDevice)devices.get(2);
CLContext context = JavaCL.createContext(null, device1, device2, device3);
But I got error when try to use 3 at the same. So How can compile CPU and GPUs in JavaCL ? Because I read that OpenCL is standard to support parallel programming by using CPU and GPU. So If I miss something, please let me know. Any idea or answers will be appreciated.
Thanks,
Duy.

Answer 1

Sadly, its not that easy. When creating a single context across multiple devices, the devices all have to come from the same platform. Creating a context containing the Intel CPU and GPU should work, but the Nvidia GPU has to be in its own context (different platform, Nvidia not Intel).
Here's how I handle this scenario: I create a context for each device and a thread for each context. Each thread takes a portion of the data I'm working on and dispatches it to its assigned OpenCL device. This way, you can mix, CPUs, GPUs from both AMD and Nvidia, and any other hardware that comes along.
Its important to do load balancing across the threads so that you don't have faster devices sitting idle waiting for a slower device to catch up.