Hangs with non-busy main thread using Instruments

Question

Created Feb ’24

Replies 3

Boosts 0

Participants 3

Hi! I watched the 'Analyze hangs with Instruments' video from WWDC2023. It's such a great video! Unfortunately, I found a case that the video doesn't cover when profiling my browser app using Instruments:

As shown in the image, the hangs still occur even though my main thread is not busy, and I've also used the 'Thread State Trace instrument' to confirm that the main thread is not blocked either.

I'm not sure what the next step is to resolve the hangs. Any insight or guidance would be much appreciated. Thank you!"

Boost

Answer 1

Developer Tools Engineer OP

Apple

Feb ’24

Hi there,

happy to hear you found the WWDC session helpful!

Based on your screenshot it looks very much like a (mostly) blocked main thread. You mentioned that you also tried running with the "Thread State Trace" instrument. Would you have a screenshot of the thread states of the Main Thread during one of the hangs as well? If possible, also include what the "Narrative" detail view for the Main Thread shows for the time of the hang.

1

Answer 2

startover205 OP

Feb ’24

Hi!

Thank you so much for confirming it's a blocked main thread!

It turns out I was silly enough not to realize I need to toggle the Thread track button to see the Thread graph. So it IS a blocked main thread.

blocked Large.jpeg

However, the Narrative view only shows one of my function causing the issue but doesn't specify which line is the cause. I've scratched my head for hours, but couldn't understand why. The function is basically reusing the WKWebView for using it in SwiftUI UIViewRepresentative, which should only use the main thread, isn't that it...? Is there a way to let the instrument tell us which line is causing the blocking?

narrative Large.jpeg

1

Answer 3

3saul OP

Jun ’24

Did you ever resolve this? I have a similar issue where my app has an intermittently 'frozen' UI on start but when I investigate I see a similar situation to you. The issue is it doesn't show where in the code is causing the issue. Frustratingly, I see a blocked main thread in instruments whether the UI is blocked or not! I'm curious if you see something similar.

0