Significant Device.Present with VSync turned off

[CHPedersen]

I’m having some trouble understanding how to effectively turn off VSync in Unity and/or my machine. I did this, of course:

Then, realizing that VSync could also be forced upon applications by the graphics card drivers, I went to the config software for my video card, too. It’s one of those integrated Intel Graphics processors built into the Core i7, an HD Graphics 4000, to be precise. Going to its Control Panel, I found this under the 3D settings and changed it from “On” to “Use Application Settings”:

With these settings, I’m expecting it to obey Unity’s request for not syncing, and I would expect it to just race ahead and blast frames at me, with no concern for screen tearing. Yet, when I run an external build against the profiler, I’m seeing this:

Notice the huge amount of time spent on Device.Present in the GPU profiler. This is usually the “wait for vsync”-task. Why is that still on? Are there other places to disable it I just haven’t looked?

It’s worth noting that it’s only on under certain conditions. If I start up an empty scene, it works as expected, and the task is < 1ms. I just don’t understand why, under some workloads, it suddenly kicks in, even when disabled in both Unity and driver software? And when it does kick in, why 33ms? Along with the ~8ms from the other, actual GPU tasks, that lands the GPU at ~41 ms. What does that accomplish? That doesn’t correspond to any syncing frequency. It’s certainly not waiting for the CPU, either. The sum of all CPU tasks is giving me <5ms.

I’m on Windows 7, and I even considered that it could be forcing VSync at the OS level to support its 3D accelerated Aero interface. But disabling that and running the Windows Basic profile does nothing for this issue. I’m out of ideas. O_O Anybody know what exactly this is, and if it’s VSync, why does it seem to aim for something that isn’t an integer division of 60 FPS?

[CHPedersen]

Interestingly, and a little surprisingly, this turned out to be the profiler itself. O_O

After speaking to ToreTank in Unity3D’s IRC channel, I did some alternative performance testing using a simple GUI.Label with the FPS (1 / Time.deltaTime) instead of using the profiler to measure performance. Then I ran the external build many more times in multiple configurations, in Windowed mode and Full Screen, using both standard builds and Developement Builds, using both VSync Count = Every VBlank and Don’t Sync, and the app’s behavior started completely corresponding to expectations. With VSync Count = Don’t Sync, the FPS was approximately 125 in the same scene, completely as expected from a GPU load of approximately 8ms.

It appears measuring and sending data to the profiler is such a burden that it cripples the application and causes a much heavier Device.Present than you’d expect from disabling VSync. This appears especially true for scenes with a high number of objects and many scripts, since such a setup will generate a complex object hierarchy for the profiler to display and update. That’s why it kicked in when rendering the more complex scenes.

Lesson learned: Use profiler to pinpoint troublesome objects and code, but never for measuring the raw FPS.