Saagar Jha (@saagar@saagarjha.com)

Saagar Jha

04/07/2025 (replying to Saagar Jha)

It’s beautiful…

The analyzed trace, showing all tracks filled (but not expanded). The cursor is hovering over one track, actually one stack frame, showing that it is “specialized Array.init(_unsafeUninitializedCapacity:initializingWith:) (8.00ms)”

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2 replies

James Atkinson

04/07/2025 (replying to Saagar Jha)

@saagar maybe @KacperHarasim can advise?

Pierre H.

04/07/2025 (replying to Saagar Jha)

@saagar isn’t it?!

Saagar Jha

04/07/2025 (replying to Saagar Jha)

Ever wondered system libraries have initializers? How about which ones are the most expensive? Which one is the first to touch the Objective-C runtime? Well here’s your answer in a single image

Screenshot of early dyld initialization, specifically libSystem_initializer calling _libc_initializer, _sanitizers_init, __malloc_init, _dyld_initializer, dyld4::APIs::_libdyld_initialize(), libdispatch_init, _libxpc_initializer in that order

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4 replies

Adam Bell :catPOWER:

04/07/2025 (replying to Saagar Jha)

@saagar this is actually so sick

Ben Scheirman

04/07/2025 (replying to Saagar Jha)

@saagar do you mean "static initializers” ?

Saagar Jha

04/07/2025 (replying to Ben Scheirman)

@bens They’re morally kind of similar but I would shy away from calling them that because they are invoked differently

Pierre H.

04/07/2025 (replying to Saagar Jha)

@saagar you have no idea how familiar I am with this particular image.

Joachim

04/07/2025 (replying to Saagar Jha)

@saagar also you can now see how much time objc_msgSend actually takes in your app. And how often it was called.

Saagar Jha

04/08/2025 (replying to Joachim)

@cocoafrog Of course there’s always DTrace for that ;)

Joachim

04/08/2025 (replying to Saagar Jha)

@saagar not in iOS, iirc ;)

Saagar Jha

04/08/2025 (replying to Joachim)

@cocoafrog Yeah the iOS people are mean

Saagar Jha

04/07/2025 (replying to Saagar Jha)

Anyway I have actual work to do so I’ll play with this later. It’s very rough but also very cool and I expect it to be invaluable for any serious performance or debugging work. I hope to see it continue to improve :)

Saagar Jha

04/07/2025 (replying to Saagar Jha)

Also, fair warning: the entire process to generate the screenshots above created about 4 TB of disk traffic. Hopefully you have AppleCare

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4 replies

Jevin Sweval

04/07/2025 (replying to Saagar Jha)

@saagar fffffffuuuuuuuuuuuu well that’s disappointing. Guess we need to figure out how to plug one of the 4U units in.

https://www.exostivlabs.com/exostiv-blade/

Kacper Harasim

04/07/2025 (replying to Jevin Sweval)

@jevinskie @saagar Developers can control how much data gets recorded using the recording options ("Limit to:"). We recommend starting with smaller traces, which should not consume as much RAM and load faster as well. We're devoted to continuing to optimize this experience and offering reasonable performance for larger traces over time.

Saagar Jha

04/08/2025 (replying to Kacper Harasim)

@KacperHarasim @jevinskie Curious what this number corresponds to

Jevin Sweval

04/07/2025 (replying to Saagar Jha)

@saagar is it tracev3 format? I wonder if some other compressor could be hooked in…
Any idea how the trace system avoids self-tracing/minimizes perturbation?

Pierre H.

04/07/2025 (replying to Jevin Sweval)

@jevinskie @saagar that’s not how tracing works. The CPU traces everything and then the trace is post processed. You know which asid is current and prune it