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openglopencl
Nghia Bui
Nghia Bui

Reputation: 3784

What is the purpose of clEnqueueAcquireGLObjects?

I did google for the question, and got from this link

clEnqueueAcquireGLObjects

Acquire OpenCL memory objects that have been created from OpenGL objects.

These objects need to be acquired before they can be used by any OpenCL commands queued to a command-queue.

I really don't understand why these objects need to be acquired. In my opinion, the reason of the acquiring is NOT OpenGL/OpenCL synchronization because the synchronization can be achieved by glFinish and clFinish.

I mean, if clEnqueueAcquireGLObjects/clEnqueueReleaseGLObjects are used, then glFinish/clFinish are redundant, and vice-versa.

Upvotes: 0

Views: 972

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474366

I mean, if clEnqueueAcquireGLObjects/clEnqueueReleaseGLObjects are used, then glFinish/clFinish are redundant, and vice-versa.

You're thinking about this in entirely the wrong way.

glFinish causes OpenGL to perform a full CPU synchronization, such that the implementation will have completed all commands afterwards. clFinish does something similar for OpenCL.

The fact that you called one or the other has absolutely no effect on what a different system does. OpenGL has no idea that OpenCL exists, and vice-versa. glFinish has nothing to do with clFinish and vice-versa. So while OpenGL may have finished making some modification to an object, OpenCL has no idea that these modifications took place.

The purpose of acquiring and releasing OpenGL objects is for OpenCL and OpenGL to talk to one another. When objects are acquired, OpenCL tells OpenGL, "Hey, see these objects? They're mine now, so give them to me." This means that the OpenGL/OpenCL driver will do whatever mechanics are necessary to transfer access control over those objects to OpenCL.

For example, if an object has been paged out of GPU memory, OpenCL acquiring it may need to make it resident again. OpenCL and OpenGL have two separate sets of records that refer to this memory; by acquiring the object, you synchronize the OpenCL data with changes made by OpenGL. And so forth.

Notice that these mechanics have nothing at all to do with synchronizing GPU operations. They are about making the objects accessible to OpenCL.

If your OpenCL implementation doesn't have cl_khr_gl_event, then you must use OpenGL's synchronization mechanism to ensure that those objects are no longer in use before you acquire them. The two functions aren't redundant; they're doing different things to ensure the integrity of the system.

Upvotes: 3

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