Name MESA_multithread_makecurrent Name Strings GLX_MESA_multithread_makecurrent Contact Eric Anholt (eric@anholt.net) Status Not shipping. Version Last Modified Date: 21 February 2011 Number TBD Dependencies OpenGL 1.0 or later is required. GLX 1.3 or later is required. Overview The GLX context setup encourages multithreaded applications to create a context per thread which each operate on their own objects in parallel, and leaves synchronization for write access to shared objects up to the application. For some applications, maintaining per-thread contexts and ensuring that the glFlush happens in one thread before another thread starts working on that object is difficult. For them, using the same context across multiple threads and protecting its usage with a mutex is both higher performance and easier to implement. This extension gives those applications that option by relaxing the context binding requirements. This new behavior matches the requirements of AGL, while providing a feature not specified in WGL. IP Status Open-source; freely implementable. Issues None. New Procedures and Functions None. New Tokens None. Changes to Chapter 2 of the GLX 1.3 Specification (Functions and Errors) Replace the following sentence from section 2.2 Rendering Contexts: In addition, a rendering context can be current for only one thread at a time. with: In addition, an indirect rendering context can be current for only one thread at a time. A direct rendering context may be current to multiple threads, with synchronization of access to the context thruogh the GL managed by the application through mutexes. Changes to Chapter 3 of the GLX 1.3 Specification (Functions and Errors) Replace the following sentence from section 3.3.7 Rendering Contexts: If ctx is current to some other thread, then glXMakeContextCurrent will generate a BadAccess error. with: If ctx is an indirect context current to some other thread, then glXMakeContextCurrent will generate a BadAccess error. Replace the following sentence from section 3.5 Rendering Contexts: If ctx is current to some other thread, then glXMakeCurrent will generate a BadAccess error. with: If ctx is an indirect context current to some other thread, then glXMakeCurrent will generate a BadAccess error. GLX Protocol None. The GLX extension only extends to direct rendering contexts. Errors None. New State None. Issues (1) What happens if the app binds a context/drawable in multiple threads, then binds a different context/thread in one of them? As with binding a new context from the current thread, the old context's refcount is reduced and the new context's refcount is increased. (2) What happens if the app binds a context/drawable in multiple threads, then binds None/None in one of them? The GLX context is unreferenced from that thread, and the other threads retain their GLX context binding. (3) What happens if the app binds a context/drawable in 7 threads, then destroys the context in one of them? As with GLX context destruction previously, the XID is destroyed but the context remains usable by threads that have the context current. (4) What happens if the app binds a new drawable/readable with glXMakeCurrent() when it is already bound to another thread? The context becomes bound to the new drawable/readable, and further rendering in either thread will use the new drawable/readable. (5) What requirements should be placed on the user managing contexts from multiple threads? The intention is to allow multithreaded access to the GL at the minimal performance cost, so requiring that the GL do general synchronization (beyond that already required by context sharing) is not an option, and synchronizing of GL's access to the GL context between multiple threads is left to the application to do across GL calls. However, it would be unfortunate for a library doing multithread_makecurrent to require that other libraries share in synchronization for binding of their own contexts, so the refcounting of the contexts is required to be threadsafe. (6) Does this apply to indirect contexts? This was ignored in the initial revision of the spec. Behavior for indirect contexts is left as-is. Revision History 20 November 2009 Eric Anholt - initial specification 22 November 2009 Eric Anholt - added issues from Ian Romanick. 3 February 2011 Eric Anholt - updated with resolution to issues 1-3 3 February 2011 Eric Anholt - added issue 4, 5 21 February 2011 Eric Anholt - Include glXMakeCurrent() sentence along with glXMakeContextCurrent() for removal.