iPhone 13 Pro 120hz, something is wrong with display synchronisation inside Unity

[PavelDoGreat]

Hey! I have an app Fluid Simulation with +10m install base ‎Fluid Simulation on the App Store

I have a problem though. Sometimes the app runs perfectly at 120hz/fps then suddenly and randomly it gets locked to 60fps. Sometimes it gets unlocked to 120fps again.

In profiler it shows that time got spent on Gfx.WaitForPresentOnGfxThread, on display synchronisation or waiting for GPU. One can assume that it is a GPU performance issue but Xcode Profiler shows a different picture:

Here are stats when it runs at 120fps. Perfect!

And now when it locks to 60fps. Notice very low GPU and CPU load.

I’m using Unity 2020.3.33f1, it also does this on newer 2021.* versions

It happens totally randomly. Neither battery level of the phone, neither quality settings inside the app affects this. ProMotion is enabled in player’s settings.

[PavelDoGreat]

Here is more info from Unity’s profiler attached to iPhone 13 Pro.

This is when it runs perfectly at 120fps. Only WaitForTargetFPS

This is when at 60fps. Notice Gfx.WaitForPresentOnGfxThread is taking all the time

And the last one is more interesting. It is also 60fps but have both WaitForTargetFPS and Gfx.WaitForPresentOnGfxThread evenly distributed

[Neonlyte]

This could be the power management of iOS rather than Unity’s synchronization issue. This person used cocos2d-x and sees similar behavior. FPS drops when tapping the screen … | Apple Developer Forums

I’d recommend submitting a bug report to Unity so that they can get the attention and indirectly get Apple to look at it.

Edit: Here is an interesting quote on the Apple Developer Site:

iOS 15 provides special priority to 30Hz and 60Hz refresh rates (if your app provides hints that request either of these rates) to help with early adoption for game developers. This prioritization is specific to iOS 15.

[PavelDoGreat]

@Neonlyte

This could be the power management of iOS rather than Unity’s synchronization issue. This person used cocos2d-x and sees similar behavior. https://developer.apple.com/forums/thread/695097

I’d recommend submitting a bug report to Unity so that they can get the attention and indirectly get Apple to look at it.

Edit: Here is an interesting quote on the Apple Developer Site:

Thanks, Neonlyte! Don’t know if it is an Apple’s issue. The game Thumper on iOS for example doesn’t have this problem and can run at 120hz steadily and it wasn’t made in Unity engine. Maybe they used some hack with UIView animations or something to force iOS to run at 120hz.

I’ve submitted a bug report, case 1425276

[eliphat]

I’m also meeting this bug here with iPad Pro 2021. Exactly the same behavior. Normally my stuff is done within 1ms, and the profiler shows 1ms stuff and 6ms WaitForTargetFPS. But suddenly it gets through 60fps, or even spikes to 30fps occasionally. It is really weird.

Would you please share a link to the issue tracker so we can follow the progress? @PavelDoGreat

[PavelDoGreat]

@eliphat

I’m also meeting this bug here with iPad Pro 2021. Exactly the same behavior. Normally my stuff is done within 1ms, and the profiler shows 1ms stuff and 6ms WaitForTargetFPS. But suddenly it gets through 60fps, or even spikes to 30fps occasionally. It is really weird.

Would you please share a link to the issue tracker so we can follow the progress? @PavelDoGreat

I can’t give direct link as it would also share my email address. I can copy paste the conversation in images

You can send bug reports too. It will increase the chance of them finally reproducing the bug

[Neonlyte]

I’ve just made my attempt. Report number is IN-7012. I have made the project so that it can be replicated with just a build and no extra inputs.

I also attached a video. Should be enough.

iPhone 13 Pro Frame Rate Yo-yo

[eliphat]

After a bit of digging I found that the code for synchronization is visible in the XCode project, search for preferredFrameRateRange – you can find that in a #if ios 15 block.
I currently didn’t find any information about how the dirty iOS internals govern the frame rates. I checked info.plist and the .mm files and unity seems to be doing nothing wrong.

[PavelDoGreat]

@Neonlyte

I’ve just made my attempt. Report number is IN-7012. I have made the project so that it can be replicated with just a build and no extra inputs.

I also attached a video. Should be enough.

https://www.youtube.com/watch?v=-pM6Eiz6r6Q

Cool!

@eliphat

After a bit of digging I found that the code for synchronization is visible in the XCode project, search for preferredFrameRateRange – you can find that in a #if ios 15 block.
I currently didn’t find any information about how the dirty iOS internals govern the frame rates. I checked info.plist and the .mm files and unity seems to be doing nothing wrong.

I’ve also found this and played with values. There everything seems to be right. It’s something internal in the engine.

[Neonlyte]

Update: My submission was accepted as reproducible. I’ll update once I hear back.

[amamaenko]

Not sure if related, but in my game I have nasty frame drops specifically on iPhone 13 pro.

I have Application.targetFrameRate set to 60, and it works fine on older devices: iPhone11, iPhone XR, and even iPhone SE (2016). But on iPhone 13 pro there is a periodic (~ once per second) frame drop from 60 to 40s, and a noticeable hiccup. I haven’t profiled it all yet, and not sure if 100% related to this thread, but at least it’s the closest one I found so far.

[amamaenko]

Ok, just digged a bit deeper. The issue in my case is also with Gfx.WaitForPresentOnGfxThread. Although, the manifestation is different, instead of randomly dropping to some other frame-rate, I see crippling spike every 60th frame. Below are three screenshots of the profiler with some comments

1. First a “normal” frame. It’s the same picture that I can see on iPhone 11 and iPhone XR: solid 16.5 ms (60 FPS) with most of the time spend in the WaitForTargetFPS.
   
   
2. At some point, there starts a “build-up” of WaitForPresentOnGfxThread

3. Finally, every 60th frame, there is a massive 30 ms peak of WaitForPresentOnGfxThread
   
   

[amamaenko]

An update. One thing that I must have mentioned is that in my case I was using URP pipeline for iOS. At the beginning I wasn’t relating the problem with the render, however, then I found this thread Gfx.WaitForPresentOnGfxThread takes long time and decided to just give it a try, and migrate to “legacy” built-in render pipeline. Miraculously, it worked, there are no spikes in URP on iPhone 13Pro or other devices.

[eliphat]

@amamaenko

An update. One thing that I must have mentioned is that in my case I was using URP pipeline for iOS. At the beginning I wasn’t relating the problem with the render, however, then I found this thread https://discussions.unity.com/t/808114 and decided to just give it a try, and migrate to “legacy” built-in render pipeline. Miraculously, it worked, there are no spikes in URP on iPhone 13Pro or other devices.

Thanks but unfortunately, I’ve been using the built-in pipeline from the beginning.

[Neonlyte]

Update: My report is now visible on the Issue Tracker. Feel free to follow the progress using this link:

Unity IssueTracker - [iOS] Framerate drops below 120fps when tapping the...

[EthanRushbrook2_1]

+1, I’m having the exact same issue. I’ve got a 13 Pro Max with precicely what you’re describing here. I’m using Unity 2021.3.5f1 with the latest version of the URP. My heart sank when I saw how recent all of this was… hopefully they get it sorted soon. I voted on the issue on issuetracker.

[PavelDoGreat]

Unity, why have you marked this issue as “By Design”? Unity Issue Tracker - [iOS] Framerate drops below 120fps when tapping the screen in a near-empty scene on iPhone 13 Pro

I would like to have answers on why this problem is not fixable. Your explanation that it is performance issue doesn’t count because this problem happens even in empty projects. Are there plans to fix it in a future?

[Neonlyte]

@PavelDoGreat

Are there plans to fix it in a future?

I just messed around with Unity’s Metal support code in the generated Xcode project. Try comment out the following code below and see if the 120FPS is stable for you. My reproduction project could maintain it easily (I did switch to use iPhone 14 Pro, so I am not sure what the behavior for 13 Pro would be):

Edit: I may have spoken too early. I did just see the reproduction project dip to 60, although not as often as before. I still do think this is because of the device difference, unless people here can test it for me.

[Neonlyte]

Well seems like they reversed the decision and the issue is active again. Whoever made them do it (or themselves did it), I thank you.

[polatefekayaNBT]

Any updates? Same issue, it’s special for 120hz I guess. I tried with 90 (but it limited with 80) and its working much better than 120. Still dipping 60 but not as often as before.