Async Workgroup Update. Barthold Lichtenbelt

Transcription

1 Async Workgroup Update Barthold Lichtenbelt 1

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 2

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 3

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 4

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. 5

6 Example RTT with two contexts Context A Sync_objectA = glcreatesync(attrib); <render to texture that context B needs> glfence(sync_objecta); glflush(); // prevent deadlock Context B glclientwaitsync(sync_objecta,0,gl_forever); glbindtexture(.); // Just rendered <render using texture> 6

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 <sync> are reflected in OS event and vice-versa Both <sync> 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 7

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 8

9 Example Streaming video processing Loop Draw frame 1 glfence(sync_object1); // To a FBO, for example // inserts a fence in the command stream Draw frame 2 glfence(sync_object2); Read back data in frame 1 Read back data in frame 2 9

10 Variation with asynchronous read back Loop Draw frame 1 Read back frame 1 into PBO 1 glfence(sync_object1); // To a FBO, for example // Asynchronous readback // Inserts a fence in the command stream Draw frame 2 Read back frame 2 into PBO 2 glfence(sync_object2); glmapbuffer( ); // Access the data of frame 1 in PBO 1 glmapbuffer( ); // Access the data of frame 2 in PBO 2 10

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 11

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); 12

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 13

14 Example Streaming video processing Loop Draw frame 1 glfence(sync_object1); // To a FBO, for example // inserts a fence in the command stream Draw frame 2 glfence(sync_object2); Read back data in frame 1 Read back data in frame 2 14

15 Variation with asynchronous read back Loop Draw frame 1 Read back frame 1 into PBO 1 glfence(sync_object1); // To a FBO, for example // Asynchronous readback // Inserts a fence in the command stream Draw frame 2 Read back frame 2 into PBO 2 glfence(sync_object2); glmapbuffer( ); // Access the data of frame 1 in PBO 1 glmapbuffer( ); // Access the data of frame 2 in PBO 2 15