OpenCL creates new threads upon first function call, why?

Question

When I call an OpenCL function I wouldn't imagine would create new threads, in this case to simply get platform IDs, my program creates 8 new threads.

    cl_platform_id platforms[10] = {0};
    cl_uint numberofplatforms = 0;
    clGetPlatformIDs(10, platforms, &numberofplatforms);//this creates 8 threads

Due to me not creating a context, but simply asking for platform IDs to see what is available, why does this function create all these threads? I'm using Windows 7 64 bit, i7 920 with HT (I am suspicious it is creating 8 threads because I have 8 cores), both the Intel and Nvidia SDK( I have a GTS 250 and GTX 560), while I'm linking with the Nvidia OpenCL libraries and using its headers.

This isn't a big concern, but what if I decide to not use OpenCL after analyzing the devices, only to have 8 useless threads "lying around". Anyone know why this happens?

I guess that communicating with the GPU could be simpler with separate threads monitoring it. — Basile Starynkevitch

Erik Smistad Erik Smistad · Accepted Answer · 2012-10-21T09:21:32

A lot of the OpenCL functions are non-blocking, meaning that they issue commands to the device in the form of a queue and I'm pretty sure the threads are used to control the device while the host program continues to run the rest of the code.

To illustrate: When you call clEnqueueNDRangeKernel, the kernel is not necessarily run at once, but the program continues to run code after the clEnqueueNDRangeKernel call. So I guess this functions passes some information to separate threads that controls the compute device and makes sure that the kernel is run eventually.

OpenCL creates new threads upon first function call, why?

1 Answers