Async Workgroup Update Barthold Lichtenbelt

OpenGL Siggraph BOF page 2 Goals Provide synchronization framework for OpenGL - Provide base functionality as defined in NV_fence and GL2_async_core - Build a framework for future, more complex, functionality, some of which discussed in GL2_async_core - Initially support CPU GPU synchronization - Support synchronization across multiple OpenGL contexts Resulted in GL_ARB_sync spec - Finished April Posted draft to opengl.org for feedback - Not quite official ARB extension yet

OpenGL Siggraph BOF page 3 Functionality overview ARB_sync provides synchronization primitives - Can be tested, set and waited upon Specifically, a “Fence Synchronization Object” and corresponding Fence command Fence completion allows for partial glFinish - All commands prior to the fence are forced to complete before control is returned to caller Fence Sync Objects can be shared across contexts - Allows for synchronization of OpenGL command streams across contexts New data type: GLtime represents intervals in nanoseconds - 64 bit integer, same encoding as UST counter in OpenML - Accuracy implementation dependent, precision in nanoseconds If you have used the Windows Event model, this will feel familiar

OpenGL Siggraph BOF page 4 Synchronization model in ARB_sync 1/2 A “sync object” is a primitive used for synchronization between CPU and GPU, CPU, or ‘something else’. - Sync object has state: type, condition, status A sync object’s status can be signaled or non-signaled - when created status is signaled unless a flag is set in which case it is non-signaled A “fence sync object” is a specific type of sync object - Provides partial finish semantics - Only type of sync object currently defined A “fence” is a token inserted in the GL command stream - A sync object is not inserted into the command stream - Fence has no state A fence is associated with a fence sync object. - Multiple fences can be associated with the same sync object When a fence is inserted in the command stream, the status of its sync object is set to non-signaled A fence, once completed, will set the status of its sync object to signaled

OpenGL Siggraph BOF page 5 Synchronization model in ARB_sync 2/2 A wait function waits on a sync object, not on a fence A poll function polls a sync object, not a fence A wait function called on a sync object in the non-signaled state will block. It unblocks when the sync object transitions to the signaled state.

OpenGL Siggraph BOF page 6 Example – RTT with two contexts Context A Sync_objectA = glCreateSync(attrib); glFence(sync_objectA); glFlush(); // prevent deadlock Context B glClientWaitSync(sync_objectA,0,GL_FOREVER); glBindTexture(….); // Just rendered

OpenGL Siggraph BOF page 7 OS specific functionality Convert sync object to the window system native event primitive - Allows applications to synchronize all events in a system using one API All operations on are reflected in OS event and vice-versa Both and the OS event are valid to use in your code On windows, convert to an Event HANDLE wglConvertSyncToEvent(object sync); - Need to specify, when sync object is created, that it can be converted to OS event - Separate extension: WGL_ARB_sync_event On Unix, convert to a file-descriptor, x-event or semaphore? - Still TBD

OpenGL Siggraph BOF page 8 Possible future functionality Add a WaitForMultipleSync(uint *sync_objects, ….) command - Synchronize with multiple sync objects at once Add a “payload” to a fence - For example, the time it completed Allow one GPU stream to wait for another GPU stream - WaitSync(sync_object); A sync object whose status will pulse with every vblank A sync object that can signal when data binding has completed - As opposed to when rendering has completed using the data

OpenGL Siggraph BOF page 9 Example – Streaming video processing Loop Draw frame 1 // To a FBO, for example glFence(sync_object1); // inserts a fence in the command stream Draw frame 2 glFence(sync_object2); while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking Read back data in frame 1 while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking Read back data in frame 2

OpenGL Siggraph BOF page 10 Variation with asynchronous read back Loop Draw frame 1 // To a FBO, for example Read back frame 1 into PBO 1// Asynchronous readback glFence(sync_object1); // Inserts a fence in the command stream Draw frame 2 Read back frame 2 into PBO 2 glFence(sync_object2); while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking glMapBuffer(…); // Access the data of frame 1 in PBO 1 while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking glMapBuffer(…); // Access the data of frame 2 in PBO 2

OpenGL Siggraph BOF page 11 Differences with GL_NV_Fence No separation of sync objects and fences in NV_Fence - NV version only has fence objects - Fence object has state Creation of sync object and inserting a fence in one command - SetFenceNV creates and inserts a fence (old object model) NV Fence objects not shared across contexts

OpenGL Siggraph BOF page 12 API Overview 1/2 Create a sync attribute object object CreateSyncAttrib(); - SYNC_TYPE has to be FENCE - SYNC_CONDITION has to be SYNC_PRIOR_COMMANDS_COMPLETE - SYNC_STATUS SIGNALED or UNSIGNALED Create the sync object object CreateSync(object attrib); Insert a fence, associated with a sync object, into command stream void Fence(object sync);

OpenGL Siggraph BOF page 13 API Overview 2/2 Wait or test the status of a fence sync object enum ClientWaitSync(object sync, uint flags, time timeout); - Blocks until sync is signalled or timeout expired - If timeout == 0, does not block, returns the status of sync - If timeout == FOREVER, call does not timeout - Optionally will flush before blocking - Returns 3 values: ALREADY_SIGNALED, TIMEOUT_EXPIRED, CONDITION_SATISFIED Signal or unsignal a sync object void SignalSync(object sync, enum mode); - If status transitions from unsignaled to signaled, ClientWaitSync will unblock

OpenGL Siggraph BOF page 14 Example – Streaming video processing Loop Draw frame 1 // To a FBO, for example glFence(sync_object1); // inserts a fence in the command stream Draw frame 2 glFence(sync_object2); while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking Read back data in frame 1 while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking Read back data in frame 2

OpenGL Siggraph BOF page 15 Variation with asynchronous read back Loop Draw frame 1 // To a FBO, for example Read back frame 1 into PBO 1// Asynchronous readback glFence(sync_object1); // Inserts a fence in the command stream Draw frame 2 Read back frame 2 into PBO 2 glFence(sync_object2); while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking glMapBuffer(…); // Access the data of frame 1 in PBO 1 while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) // App uses CPU cycles instead of blocking glMapBuffer(…); // Access the data of frame 2 in PBO 2