setFrameRate on lwjgl3 leads to too slow frame rate

[richardTingle]

Issue
When running JME with jme3-lwjgl3 the AppSettings#setFrameRate method call does not work correctly, causing far lower frame rates that requested. My application, when unlimited runs at around 60 fps, but when limited runs as follows:

Requested	Actual
60	30
120	37
240	45
2400	55
No limit	60

This only occurs with jme3-lwjgl3, jme3-lwjgl works fine

Investigation so far
I've had a look in the debugger at what's going on in LwjglWindow#runLoop and on line 564 I see it trying to figure out how long to sleep for:

final double sleep = frameSleepTime - (timer.getTimePerFrame() / 1000.0)

However, timer.getTimePerFrame() appears to always return zero, leading to it always sleeping for the full 1/frameRate. Meaning if the application is under any sort of load that brings it close (or worse below) to the requested frame rate it slows the application way below the requested rate.

This appears to be because Timer#update is never called (a different times update method is called, but not the one in LwjglWindow

Environment

I'm running 3.3.2-stable but I've also checked out and run against the V3.3 branch and it behaves the same (master looks like it would have the same problem but crashes for me)

[richardTingle]

There are some other problems in runLoop, timer.getTimePerFrame() is in seconds so it should be multiplied by 1000, not divided. But the bigger problem is that even if corrected it still runs at the wrong frame rate, as on frame 1 it detects and corrects the problem, then on frame 2 there is (as far as it sees) no problem, so it doesn't sleep. Meaning every other frame runs too fast.

jme3-lwjgl's version of this seems much more complicated (necessarily so) calling Sync.sync which runs a longer average thereby avoiding the problem

[riccardobl]

This is fixed by this unmerged patch:

2f62afc

from pr #1406

[richardTingle]

Awesome! I'll stop investigating in that case.

Should this issue be closed or should it stay open till the PR is merged?

[stephengold]

I'm fine leaving this open. It might help someone solve a similar issue. And it's not like we're going to clear out the backlog anytime soon...

[stephengold]

richtea reported this issue fixed in 3.4.0-beta1:

https://hub.jmonkeyengine.org/t/engine-v3-4-0-beta-testing/44507/5