CODEWITHSUNDEEP
c++opengl
Desperado17
Desperado17

Reputation: 1015

When is glFlush called too often?

I have an application that issues about 100 drawcalls per frame, each with an individual VBO. The VBOs are uploaded via glBufferData in a separate thread has has gl context resource sharing. The render thread tests buffer upload state via glClientWaitSync.

Now my question:

According to the documentation glClientWaitSync and GL_SYNC_FLUSH_COMMANDS_BIT cause a flush at every call, right? This would mean that for every not yet finished glBufferData in the upload thread I would have dozens of flushes in the render thread right? What impact on performance would it have if thus, in the worst case, I practically issue a flush before every drawcall?

Upvotes: 2

Views: 665

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473447

The behavior of GL_SYNC_FLUSH_COMMANDS_BIT has changed from its original specification.

In the original, the use of that bit was the equivalent of issuing a glFlush before doing the wait.

However, GL 4.5 changed the wording. Now, it is the equivalent of having performed a flush immediately after you submitted that sync object. That is, instead of doing a flush relative to the current stream, it works as if you had flushed after submitting the sync. Thus, repeated use does not mean repeatedly flushing.

You can get the equivalent behavior of course by manually issuing a flush after you submit the sync object, then not using GL_SYNC_FLUSH_COMMANDS_BIT when you wait for it.

Upvotes: 2

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