docs/MESA_multithread_makecurrent.spec

   1 Name
   2
   3     MESA_multithread_makecurrent
   4
   5 Name Strings
   6
   7     GLX_MESA_multithread_makecurrent
   8
   9 Contact
  10
  11     Eric Anholt (eric@anholt.net)
  12
  13 Status
  14
  15     Not shipping.
  16
  17 Version
  18
  19     Last Modified Date:  21 February 2011
  20
  21 Number
  22
  23     TBD
  24
  25 Dependencies
  26
  27     OpenGL 1.0 or later is required.
  28     GLX 1.3 or later is required.
  29
  30 Overview
  31
  32     The GLX context setup encourages multithreaded applications to
  33     create a context per thread which each operate on their own
  34     objects in parallel, and leaves synchronization for write access
  35     to shared objects up to the application.
  36
  37     For some applications, maintaining per-thread contexts and
  38     ensuring that the glFlush happens in one thread before another
  39     thread starts working on that object is difficult.  For them,
  40     using the same context across multiple threads and protecting its
  41     usage with a mutex is both higher performance and easier to
  42     implement.  This extension gives those applications that option by
  43     relaxing the context binding requirements.
  44
  45     This new behavior matches the requirements of AGL, while providing
  46     a feature not specified in WGL.
  47
  48 IP Status
  49
  50     Open-source; freely implementable.
  51
  52 Issues
  53
  54     None.
  55
  56 New Procedures and Functions
  57
  58     None.
  59
  60 New Tokens
  61
  62     None.
  63
  64 Changes to Chapter 3 of the GLX 1.3 Specification (Functions and Errors)
  65
  66     Remove the following sentence from section 3.3.7 Rendering Contexts:
  67         If ctx is current to some other thread, then
  68         glXMakeContextCurrent will generate a BadAccess error.
  69
  70     Remove the following sentence from section 3.5 Rendering Contexts:
  71         If ctx is current to some other thread, then
  72         glXMakeCurrent will generate a BadAccess error.
  73
  74 GLX Protocol
  75
  76     None.  The GLX extension is client-side.
  77
  78 Errors
  79
  80     None.
  81
  82 New State
  83
  84     None.
  85
  86 Issues
  87
  88     (1) What happens if the app binds a context/drawable in multiple
  89         threads, then binds a different context/thread in one of them?
  90
  91     As with binding a new context from the current thread, the old
  92     context's refcount is reduced and the new context's refcount is
  93     increased.
  94
  95     (2) What happens if the app binds a context/drawable in multiple
  96         threads, then binds None/None in one of them?
  97
  98     The GLX context is unreferenced from that thread, and the other
  99     threads retain their GLX context binding.
 100
 101     (3) What happens if the app binds a context/drawable in 7 threads,
 102         then destroys the context in one of them?
 103
 104     As with GLX context destruction previously, the XID is destroyed
 105     but the context remains usable by threads that have the context
 106     current.
 107
 108     (4) What happens if the app binds a new drawable/readable with
 109         glXMakeCurrent() when it is already bound to another thread?
 110
 111     The context becomes bound to the new drawable/readable, and
 112     further rendering in either thread will use the new
 113     drawable/readable.
 114
 115     (5) What requirements should be placed on the user managing contexts
 116         from multiple threads?
 117
 118     The intention is to allow multithreaded access to the GL at the
 119     minimal performance cost, so requiring that the GL do general
 120     synchronization (beyond that already required by context sharing)
 121     is not an option, and synchronizing of GL's access to the GL
 122     context between multiple threads is left to the application to do
 123     across GL calls.  However, it would be unfortunate for a library
 124     doing multithread_makecurrent to require that other libraries
 125     share in synchronization for binding of their own contexts, so the
 126     refcounting of the contexts is required to be threadsafe.
 127
 128 Revision History
 129
 130     20 November 2009 Eric Anholt - initial specification
 131     22 November 2009 Eric Anholt - added issues from Ian Romanick.
 132     3 February 2011 Eric Anholt - updated with resolution to issues 1-3
 133     3 February 2011 Eric Anholt - added issue 4, 5
 134     21 February 2011 Eric Anholt - Include glXMakeCurrent() sentence
 135     along with glXMakeContextCurrent() for removal.