async workgroup update barthold lichtenbelt. opengl siggraph bof 2006 - page 2 goals provide...

Async Workgroup UpdateAsync Workgroup Update

Barthold Lichtenbelt

OpenGL Siggraph BOF 2006 - page 2

GoalsGoals

• 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 2006- Posted draft to opengl.org for feedback- Not quite official ARB extension yet


Functionality overviewFunctionality 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


Synchronization model in ARB_sync 1/2Synchronization 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


Synchronization model in ARB_sync 2/2Synchronization 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.


Example – RTT with two contextsExample – 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>


OS specific functionalityOS 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 EventHANDLE 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


Possible future functionalityPossible 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


Example – Streaming video processingExample – Streaming video processing

• LoopDraw frame 1 // To a FBO, for exampleglFence(sync_object1); // inserts a fence in the command stream

Draw frame 2glFence(sync_object2);

while (glClientWaitSync(sync_object1,0,0)!=GL_ALREADY_SIGNALED) <Do some useful work> // App uses CPU cycles instead of blocking

Read back data in frame 1




Variation with asynchronous read backVariation with asynchronous read back

• LoopDraw frame 1 // To a FBO, for exampleRead back frame 1 into PBO 1 // Asynchronous readbackglFence(sync_object1); // Inserts a fence in the command stream

Draw frame 2Read back frame 2 into PBO 2glFence(sync_object2);


glMapBuffer(…); // Access the data of frame 1 in PBO 1




Differences with GL_NV_FenceDifferences 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


API Overview 1/2API 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 objectobject CreateSync(object attrib);

• Insert a fence, associated with a sync object, into command streamvoid Fence(object sync);


API Overview 2/2API Overview 2/2

• Wait or test the status of a fence sync objectenum 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 objectvoid SignalSync(object sync, enum mode);- If status transitions from unsignaled to signaled, ClientWaitSync will unblock


Example – Streaming video processingExample – Streaming video processing

• LoopDraw frame 1 // To a FBO, for exampleglFence(sync_object1); // inserts a fence in the command stream

Draw frame 2glFence(sync_object2);






Variation with asynchronous read backVariation with asynchronous read back

• LoopDraw frame 1 // To a FBO, for exampleRead back frame 1 into PBO 1 // Asynchronous readbackglFence(sync_object1); // Inserts a fence in the command stream

Draw frame 2Read back frame 2 into PBO 2glFence(sync_object2);





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