Accessing empty table causes 100% lag spike?!

Corecii · June 1, 2018, 3:56am

I think the dump you provided was too small to capture what you’re talking about in this instance.

I’m not sure what WaitUntilCompleted or Present do, but I know it’s related to rendering. I think it might be described in Roblox’s microprofiler tutorial videos, or some staff might have commented about it before.

This reply seems to describe what’s going on here well.

More detailed Microprofiler sublabels under Present

There are two cases why this can happen:

There is a lot of GPU workload, and Present is waiting for it. There’s a separate GPU timeline available on NVidia/AMD on PC (we disable it on Intel because of some driver bugs that interfere with the client closing correctly…)

The system activates VSync behind our back. We generally currently try to disable it, but there are cases where we can’t; particularly, on Mac with Metal VSync is currently enabled because there’s no way to disable it up until macOS 10.13 which isn’t live.

There’s no way for us to bisect Present further in general because all of the above happens in the OS + the graphics driver and we ourselves unfortunately have zero visibility into that.

However, note that in the case 2 above, this isn’t really doing “work” - it’s just waiting. All other processing (like scripts etc.) is executing during this, so you can think of this as throttling to 60 Hz as opposed to “it takes 15 ms to do smth”.

TL;DR: It’s probably taking a whole frame to wait for the start of the next frame.

Results of course won’t be the same every frame. You have to render and compute different things on each frame. It looks like maybe your GPU usage went up due to the explosion particles during those frames so you saw a longer Present than in other frames. Not 100% sure though.

What do you mean by “It’s from my main script”? The microprofiler does not show the WaitUntilCompleted, Present, or Perform as directly related to any script as far as I can see – it’s just part of the general workload. (Unless WaitUntilCompleted is a custom label)