Performance and debugging interview questions

Xcode debugging features

Once you've reproduced a bug, which Xcode debugging features do you actually reach for, beyond print statements? And when do you pull out each one?

#xcode #debugging #breakpoint #lldb

Breakpoints

Which of these can you do with Xcode breakpoints without changing any code? Select all that apply.

#xcode #breakpoint #debugging

Picking an Instruments template

Tapping a certain button freezes the screen for about a second. To see which function on the main thread is eating the time, which Instruments template do you …

#instruments #time-profiler

Data structure choice and performance

With the same 100,000 elements stored in each, which operations are clearly faster on a Set (NSSet) than on an Array? Select all that apply.

#array #set #complexity #hash

Performance and debuggingMultiple choiceIntroAppleinterview report

Performance measurement conditions

You want to measure app performance as the basis for optimization. Which are correct measurement practices? Select all that apply.

#measurement #release #device #profiling

Performance and debuggingShort answerBasicsAppleinterview report

Instruments profiling

All you've got is vague feedback that "the app is slow." How do you approach that with Instruments?

#instruments #time-profiler #allocations #profiling

Crash investigation process

You get an alert that crashes spiked in the version you shipped yesterday. What do you do, and in what order?

#crash #crashlytics #organizer #triage

Performance and debuggingShort answerBasicsAppleinterview report

Crash log symbolication

A user sends you a crash log and it only has hex addresses instead of function names. What do you need to make it readable?

#crash #dsym #symbolication #crash-log

Identifying crash types

A crash report shows EXC_BREAKPOINT (SIGTRAP). In a Swift app, which of these situations produce this type? Select all that apply.

#crash #exc_breakpoint #sigtrap #swift-runtime

os_signpost intervals

Someone reports that search results take too long to appear. Instead of printing timestamps, what would you use to measure each stage?

#os_signpost #logger #instruments #points-of-interest

Causes of scroll stutter

You inherit an app whose UICollectionView stutters while scrolling. Which of these can cause the stutter? Select all that apply.

#scroll #main-thread #offscreen-rendering #autolayout

Performance and debuggingShort answerBasicsGoogleinterview report

Navigation stack optimization

Because of deep links or state restoration you have to push dozens of view controllers onto a UINavigationController in a row. What goes wrong if you just do t…

#uinavigationcontroller #memory #view-controller #navigation

Matching a diagnostic tool

An app you're debugging in Xcode throws an intermittent EXC_BAD_ACCESS, and you don't know which object is being accessed after it was deallocated. Which diagn…

#zombies #exc_bad_access #debugging

Performance and debuggingShort answerBasicsTikTok

Approach to code optimization

You're asked to "optimize this function, it's slow." What do you do before you touch the code?

#optimization #measurement #algorithm #profiling

Performance and debuggingShort answerBasicsWalmart

Spotting memory leaks and crashes in production

A hiring manager asks: "How do you find out there's a memory leak in an app that's already shipped, not one you're developing?" Answer separately for local too…

#memory-leak #production #crash #metrickit

Performance and debuggingShort answerBasicsPayPal

Where to start on scroll stutter

On a phone screen the interviewer asks: "A user reports that scrolling stutters. Where would you start?" Walk through the diagnostic order and what you look at…

#scroll #hitch #time-profiler #main-thread

Performance and debuggingMultiple choiceBasicsKuaishou

Computing decoded image memory

You decode a 100×100 pixel RGBA PNG (3KB on disk) to draw it on screen. Which value is closest to the memory the bitmap takes?

#image #memory #bitmap #png

Performance and debuggingMultiple choiceBasicsDiDi

Crash collection for shipped apps

For an app already on the App Store, which of these are ways to detect and collect crashes? Select all that apply.

#crash #production #organizer #metrickit

Performance and debuggingShort answerBasicsMeta

Using system resources efficiently

A phone screen question: "When you design an iOS app, what do you keep in mind to use system resources (CPU, memory, network, battery, disk) efficiently?" Give…

#energy #battery #memory #network

Performance and debuggingShort answerBasicsXiaomi

Finding the cause of freezes

Reviews of your shipped iOS app say it occasionally stops responding to touch for a few seconds. When it won't reproduce in the office, how do you get your han…

#hang #main-thread #metrickit #profiling

Performance and debuggingShort answerBasicsComcastinterview report

Leak investigation with Instruments

Opening and closing a certain screen repeatedly makes the app's memory keep growing. In what order would you use Leaks, Allocations, and the Memory Graph?

#instruments #leaks #allocations #memory-graph

Performance and debuggingShort answerBasics마이리얼트립interview report

Baselines for performance improvements

You're reviewing a PR that claims the search results screen got 40% faster. It looks like it mixes the author's laptop simulator numbers with QA's real device …

#performance #baseline #p95 #regression

Performance and debuggingShort answerBasics중고나라interview report

Correlating crashes with performance

Reviews of a secondhand marketplace app keep saying "it gets slow and then closes." How do you connect crash logs with performance traces to decide which probl…

#crash #symbolication #performance #observability

Performance and debuggingShort answerBasics뤼튼

React Native performance boundaries

A chat screen built with React Native is slow on iOS. How do you separate whether the bottleneck is JavaScript rendering, calls across the native boundary, or …

#react-native #bridge #profiling #ios

Performance and debuggingShort answerAdvancedTikTok

Detecting OOM terminations

Users say "the app just closes," but Crashlytics has nothing. If you suspect out-of-memory (OOM) terminations, how do you confirm it and gather data?

#oom #jetsam #metrickit #memory

Responding to memory warnings

A photo editing app gets frequent memory warnings and sometimes gets killed. In what order would you track down the cause?

#memory-warning #image #nscache #downsampling

App launch time

You take over an app whose cold start takes more than 3 seconds. How would you split up and measure the launch time?

#launch-time #dyld #pre-main #app-launch

Pre-main optimization

Which of these actually reduce the pre-main part of app launch (before main() is called)? Select all that apply.

#launch-time #dyld #pre-main #mergeable-libraries

Performance and debuggingShort answerAdvancedTikTok

Memory bug diagnosis tools

Besides retain cycle leaks, what other kinds of memory bugs do you run into on iOS? And if one took you a long time to track down, which kind was it?

#memory #leaks #zombies #memory-graph

Diagnosing hangs

Users report that a certain screen occasionally freezes for about a second, but you can't reproduce it in development. How do you narrow down the cause?

#hang #main-thread #metrickit #thread-performance-checker

App size optimization

Your app's download size is over 200MB and users get the cellular download warning. How do you analyze where the size comes from?

#app-size #app-thinning #asset-catalog #on-demand-resources

Reading Animation Hitches

You recorded scrolling with the Instruments Animation Hitches template. Which statements about reading the result are correct? Select all that apply.

#hitch #animation-hitches #instruments #commit

Performance and debuggingMultiple choiceAdvancedMicrosoft

Post-main launch optimization

You want to cut app startup time. Which of these actually shorten the post-main stage, from main() to the first frame? Select all that apply.

#launch-time #post-main #first-frame #didfinishlaunching

Performance and debuggingShort answerAdvancedTrip.com

Memory pressure from large images

You need to show a single 50MB photo on screen. If you just load it with UIImage(contentsOfFile:), how much memory does it take?

#image #downsampling #imageio #memory

Performance and debuggingShort answerAdvancedTencent

Building an in-app hang monitor

You want to add a feature to your in-house APM that captures a stack whenever the main thread stalls past a threshold. How would you detect jank from inside th…

#hang #runloop #watchdog #main-thread

Performance and debuggingShort answerAdvancedMeituan

CPU vs GPU bottlenecks in dropped frames

Can you explain the causes of dropped frames, split into the CPU side and the GPU side?

#frame-drop #gpu #cpu #offscreen-rendering

Performance and debuggingMultiple choiceAdvancedXiaohongshu

Finding where the main thread is blocked

After a certain action the app freezes completely (no input response). Which of these are valid ways to find where the main thread is blocked? Select all that …

#hang #main-thread #deadlock #debugging

Performance and debuggingShort answerAdvanced티맵모빌리티interview report

Battery optimization for navigation apps

Map and navigation apps use a lot of battery. How would you design one to minimize battery drain while keeping location tracking accurate?

#battery #energy #location #core-location

Performance and debuggingShort answerAdvanced밀리의서재interview report

Adopting and running app monitoring (APM)

If you were introducing monitoring like Sentry or Firebase Crashlytics to a team for the first time, which metrics would you define?

#monitoring #sentry #crashlytics #release-health

Performance and debuggingShort answerAdvancedHSBCinterview report

Instrumenting a 5 second screen load

The account detail screen in a banking app takes 5 seconds to first display. To avoid fixing on gut feeling, into which stages would you split and measure thos…

#performance #signpost #time-profiler #loading

Performance and debuggingShort answerAdvanced네이버interview report

Large image pipeline

An iOS feature that edits and saves photos of tens of MB gets killed after a memory warning. From decoding to saving, how would you lower the memory peak?

#images #downsampling #memory #imageio

Performance and debuggingShort answerAdvanced더스윙interview report

Analyzing scroll hitches

A photo feed collection view stutters while scrolling. How do you use Instruments to tell whether the cause is image decoding, layout computation, or cell setu…

#scrolling #core-animation #prefetch #performance

Reading the Time Profiler call tree

You run Time Profiler and 80% of main thread time lands in CA::Transaction::commit or UIKit internals. Your own code doesn't show up. How do you read this resu…

#time-profiler #call-tree #instruments #sampling

ARC traffic optimization

In Time Profiler, the top entries on the scrolling hot path are swift_retain, swift_release, and swift_bridgeObjectRetain. Why does this happen?

#arc #swift_retain #time-profiler #borrowing

MetricKit

Which of these can an app actually receive through MetricKit (MXMetricManager)? Select all that apply.

#metrickit #diagnostics #hang #crash

Performance and debuggingShort answerExpertAppleinterview report

Investigating unreproducible production bugs

You have a production bug that affects 0.1% of users and never reproduces in development. How do you approach it?

#debugging #oslog #observability #feature-flag

Performance and debuggingShort answerExpertKuaishou

Designing a crash reporting system

You can't use third-party SDKs, so you have to design your own crash log collection. How do you catch and record a crash inside the app?

#crash-reporting #signal #mach-exception #dsym