iOS Profiling

Prerequisites

iOS profiling requires macOS with Xcode Command Line Tools installed. Lanterna uses xcrun xctrace under the hood, which is only available on macOS.

Install the required tools:

xcode-select --install

Verify the installation:

xcrun xctrace version

You should see the xctrace version number corresponding to your installed Xcode version.

How Metrics Are Collected

Lanterna uses a 7-step pipeline to collect and process iOS performance data:

Step 1 — Find App PID

Lanterna locates the target app’s process ID using platform-appropriate discovery:

Simulator: xcrun simctl spawn <deviceId> launchctl list to enumerate running processes
Physical device: pgrep -f <bundleId> to find the process by bundle identifier

Step 2 — Create Temp Directory

A temporary directory is created to store the trace file and exported XML. This directory is cleaned up automatically after processing.

Step 3 — Record Trace

xcrun xctrace record \
  --template "Time Profiler" \
  --attach <pid> \
  --time-limit <duration>s \
  --output trace.trace

The Time Profiler template captures CPU sampling data at regular intervals. The --attach flag connects to an already-running process, and --time-limit controls the measurement duration.

Step 4 — Export XML

xcrun xctrace export \
  --input trace.trace \
  --xpath '/trace-toc/run[@number="1"]/data/table[@schema="time-profile"]'

The trace binary is exported to XML format using an XPath query that targets the time profile data table.

Step 5 — Parse CPU Metrics

The exported XML contains weight values in nanoseconds representing CPU time spent in each function. Lanterna aggregates these weights and converts them to a CPU utilization percentage relative to the total measurement duration.

Step 6 — Collect Memory

top -l 1 -pid <pid> -stats pid,rsize

A single snapshot of the process’s resident set size (rsize) is captured to measure memory consumption in megabytes.

Step 7 — Cleanup

Temporary trace files and exported XML are deleted to avoid accumulating large files on disk.

Limitations

There are several important limitations to be aware of when profiling iOS apps:

No real-time streaming. The xctrace tool follows a record-then-export model. Data is only available after the recording completes. For live monitoring, use the Tier 2 in-app module with lanterna monitor.
macOS only. iOS profiling cannot be performed from Linux or Windows. This affects CI environments — you must use macOS runners for iOS performance checks.
Recording overhead. The Time Profiler template adds a small amount of overhead to the profiled process. Results are representative but not perfectly identical to uninstrumented performance.

Xcode Version Compatibility

Lanterna detects the installed Xcode version and adjusts its XML parsing accordingly. The xctrace export format is underdocumented by Apple and can vary between Xcode releases, particularly in:

XML element naming and attribute structure
Table schema identifiers
Weight value encoding

Lanterna maintains parsers for Xcode 14.x, 15.x, and 16.x. If you encounter parsing errors after an Xcode update, check for a Lanterna update or file an issue.

Simulator vs Physical Device

Both iOS Simulator and physical devices are supported.

Aspect	Simulator	Physical Device
Process discovery	`xcrun simctl spawn`	`pgrep -f`
Performance fidelity	Runs on Mac hardware (not representative)	Real device performance
CI compatibility	Works on macOS runners	Requires connected device
Setup	No additional setup	Developer Mode required

For accurate performance measurements, always validate on physical devices. The simulator runs on desktop-class hardware and does not reflect real-world mobile performance.

Troubleshooting

”xctrace not found”

The Xcode Command Line Tools are not installed or not properly linked.

xcode-select --install

If already installed, ensure the correct Xcode is selected:

sudo xcode-select --switch /Applications/Xcode.app

Permissions on Physical Devices

Profiling a physical device may require:

Developer Mode enabled in Settings > Privacy & Security > Developer Mode (iOS 16+)
The device must be trusted by the Mac (connect via USB and approve the trust dialog)
For wireless debugging, the device must be on the same network and paired

”Process not found”

Verify the app is running on the target device
Check that the bundle identifier is correct
On Simulator, ensure you are targeting the correct simulator instance (use xcrun simctl list devices booted to see running simulators)

Trace Recording Fails

Ensure no other Instruments or xctrace session is active (only one trace can record at a time)
Check available disk space — trace files can be several hundred megabytes
On physical devices, ensure the device is not locked during recording