impr: Capture profiles on a BG Thread

[philipphofmann]

📜 Description

Serializing and capturing a profile can run on the main thread and block it for a couple of milliseconds. Therefore, we move this to a background thread to avoid potentially blocking the main thread.

💡 Motivation and Context

This came up while investigating profiles for potentially slowing down UIViewControllers.

💚 How did you test it?

Unit test and checking if the profile is still correctly attached to UIViewController transactions with our iOS-Swift sample app.

📝 Checklist

You have to check all boxes before merging:

• I reviewed the submitted code.
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[codecov]

Codecov Report

Attention: Patch coverage is 92.45283% with 4 lines in your changes missing coverage. Please review.

Project coverage is 91.454%. Comparing base (e88fe9a) to head (8eba803).

Files with missing lines	Patch %	Lines
...y/Profiling/SentryCaptureTransactionWithProfile.mm	78.947%	4 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #4377       +/-   ##
=============================================
+ Coverage   91.450%   91.454%   +0.003%     
=============================================
  Files          629       630        +1     
  Lines        50625     50658       +33     
  Branches     18346     18358       +12     
=============================================
+ Hits         46297     46329       +32     
  Misses        4235      4235               
- Partials        93        94        +1     

Files with missing lines	Coverage Δ
...es/Sentry/Profiling/SentryProfilerSerialization.mm	91.472% <100.000%> (+0.184%)	⬆️
Sources/Sentry/SentryTracer.m	96.912% <100.000%> (+0.167%)	⬆️
...SentryProfilerTests/SentryTraceProfilerTests.swift	98.905% <100.000%> (+0.074%)	⬆️
...y/Profiling/SentryCaptureTransactionWithProfile.mm	78.947% <78.947%> (ø)

... and 3 files with indirect coverage changes

---

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1222.76 ms	1240.08 ms	17.33 ms
Size	21.58 KiB	736.65 KiB	715.06 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
1bf8571	1250.96 ms	1255.36 ms	4.40 ms
2ccbd03	1225.13 ms	1247.51 ms	22.39 ms
d011484	1220.86 ms	1237.18 ms	16.33 ms
6001822	1220.82 ms	1245.02 ms	24.20 ms
3723833	1205.22 ms	1216.94 ms	11.71 ms
ff09c7e	1240.94 ms	1262.66 ms	21.72 ms
b9d59f7	1250.71 ms	1257.78 ms	7.06 ms
94d8eb3	1234.02 ms	1249.63 ms	15.60 ms
e773cad	1219.86 ms	1238.26 ms	18.40 ms
90d17d3	1261.18 ms	1278.18 ms	17.00 ms

App size

Revision	Plain	With Sentry	Diff
1bf8571	20.76 KiB	437.12 KiB	416.36 KiB
2ccbd03	21.58 KiB	546.20 KiB	524.62 KiB
d011484	21.58 KiB	616.14 KiB	594.56 KiB
6001822	22.85 KiB	410.98 KiB	388.13 KiB
3723833	21.58 KiB	424.34 KiB	402.76 KiB
ff09c7e	20.76 KiB	427.76 KiB	407.00 KiB
b9d59f7	22.85 KiB	405.77 KiB	382.93 KiB
94d8eb3	21.58 KiB	417.86 KiB	396.28 KiB
e773cad	21.58 KiB	681.75 KiB	660.17 KiB
90d17d3	20.76 KiB	432.17 KiB	411.41 KiB

[brustolin]

Thanks for this. There is one thing to discuss.

[brustolin]

LGTM!

[philipphofmann]

I'm still waiting for @armcknight to approve this so we can avoid shooting ourselves in the foot.

[armcknight]

Could you be more specific about how you tested this? I just tried looking at a manual transaction with trace-based profiling on main vs this branch and the profiles look different. I can't get a launch profile to show up when running from this branch.

I tried comparing the differences between main and this branch with continuous profiling but I'm not even seeing what I expect on main so i need to ask the team what's going on there.

At a minimum for anything profiling right now you have to check what it looks like on main against your changes for:

• trace-based
  • regular transaction
  • launch
• continuous
  • regular transaction
  • launch

Trace-based regular txn on main (https://sentry-sdks.sentry.io/profiling/profile/sentry-cocoa/cdc0737ef92245a0b14dcdb40c5d68c3/flamegraph/?colorCoding=by%20system%20vs%20application%20frame&fov=0%2C0%2C3822472960%2C19&query=&sorting=call%20order&tid=259&view=top%20down):

and on this branch (https://sentry-sdks.sentry.io/profiling/profile/sentry-cocoa/28b4f9cc46ea4c2aa94cdfc400d11497/flamegraph/?colorCoding=by%20system%20vs%20application%20frame&fov=0%2C0%2C5127519744%2C19&query=&sorting=call%20order&tid=259&view=top%20down):

[philipphofmann]

Hmm, weird, I didn't test it in great detail as I didn't expect that when all the tests are green, such a small change can have such a big impact. @armcknight, do you have a clue about what could cause this? I will look into this tomorrow.

[armcknight]

@philipphofmann my guess is that you aren't dispatching the right call to the bg queue, and so it captures a set of profiling data much earlier than it marks the end of the transaction. You may need to look for a more granular point in that process, or create one by refactoring.

[philipphofmann]

I found the problem. This PR only impacts trace based profiles, as only the SentryTracer uses sentry_traceProfileEnvelopeItem. Continuous profiling doesn't use this method.

I tested this for manual transactions, UIViewController transactions and app start transactions.

	Main Branch	This feature Branch
Manual Transaction	Profile in Sentry	Profile in Sentry
TransactionsViewController	Profile in Sentry	Profile in Sentry
App Start Transaction	Profile in Sentry	Profile in Sentry

[philipphofmann]

@armcknight, when I move this into the async block the profiles the SDK shortens the profiles uploaded to Sentry, but I can't replicate this behavior in the testCaptureTransactionWithProfile_SetsCorrectAmountOfSamples. I would like to have a failing test for this to ensure we don't break this by mistake. Maybe the tests with the mocks behave differently than the actual profiling code, or I just don't get it.