5 Replies Latest reply on Jun 12, 2012 3:25 PM by Meteorhead

    OpenCL-OpenGL Interop and multi-device

    etiienne

      Hi !

       

      I'm working on OpenCL-OpenGL interop possibility. More precisely, I use GL for graphical and renderring stuff, and CL for calculation. I don't use NVIDIA SDK (neither AMD).

       

      I'm trying to make it multidevice, but I'm not a GL expert and I don't know how to deal with context creation in a such situation. To initiate the interoperability, all I do is get the current gl context, using specificic method (wglGetCurrentContext for instance), and create my cl context with it. (it works fine)

       

      I'm trying to have multiple OpenGL context at the same time (to work on multiple GPU), and get all these gl context in order to create cl contexts.

      Knowing this, I have two questions :

      - is it possible to have multiple gl context running a the same time in the same program ?

      - if yes, how can i get there id ?

       

      Thanks in advance,

       

      Etienne

        • Re: OpenCL-OpenGL Interop and multi-device
          Meteorhead

          Hi!

           

          You are in the same shoes as myself. I'm just ahead of putting together some classes that ease my job of multi-GPU render and calculations.

           

          Practically OpenGL contexts are a pain. One thread can only have one OpenGL context active at a time, and one context can only be active in one thread at a time. (Mutual exclusion in both directions) wglGetCurrentContext will only acquire the context used for desktop render, more precisely the default adapter that belongs to desktop no.1.

           

          If you would like to have multiple OpenGL contexts used at the same time with OpenCL contexts that contain them, you need at least one thread for each OpenGL-OpenCL context pair. You can use any threading class you like to achieve this. (OpenMP, MPI, Boost::Thread, SFML, etc.)

           

          AMD OpenCL Programming Guide at the end shows sample codes in the appendix that shows how to query non-default contexts under both linux and windows. Yes, it's a pain that you have to interact with the windowing API for that. Coming to interop programming from OpenCL side, it's very strange that OpenGL has not defined standard context, device, etc. types over the past 15 years.

           

          So to put it short, you will need at least as many threads as devices you want to use, but if you use more, and some OpenGL contexts are accessed from multiple threads, you have to mutex around setActive() functions of OpenGL. (make sure to unset contexts after finishing with them and unlocking the mutex.

           

          Basically, multi-device interop is a real pain and is far from being straight forward to create a generic solution to many application setups.

           

          I was told, that there will some webinar, tutorial or something about windowing system tricks for these kind of cases. (I have no idea what happend, if I pull the window over to a desktop being rendered by a different card. How should I handle this change? Must I follow with my OpenCL context the window itself?)

           

          Anyhow, good luck!

           

          Máté

          1 of 1 people found this helpful
            • Re: OpenCL-OpenGL Interop and multi-device
              etiienne

              Thank you for such a quick answer.

               

              I hadn't seen this appendix in the AMP APP Guide before.

              I just gave it the once-over, and the mechanism seems a bit awkward. I'll dig into it.

               

              I think we are in the same situation (library to ease cl development, generic really matter, etc.), so I'll let you know if find any other help, example or information.

               

              I'm sorry i can't help you with your interrogation.

               

              Etienne.

              • Re: OpenCL-OpenGL Interop and multi-device
                etiienne

                Hi !

                 

                I've look into in the appendix at the end of AMD APP Guide and found this line :

                "G.1.3 Limitations

                • It is recommended not to use GLUT in a multi-GPU environment.

                AMD currently supports  CL-GL interoperability only in a single-GPU

                environment."

                 

                So I decided to focus on the single CL-GL interop. I will come back on it later..

                 

                 

                I have a tiny further question. Because its concerning the interop, I assumed you would know the answer.

                 

                I noticed the "cl_khr_gl_sharing" extension on my CPU device. In the case of a CPU, i dont understand the meaning of it.

                 

                1) Do I have to check the host sharing ability (it would surprise me that not every CPU are able to host a cl-gl interop program) ?

                2) Is OpenGL able to emulate a GPU on a CPU (without the use of a MESA-like API..) ?

                 

                The presence of this extension bother me..

                 

                Thanks again