Launch time, memory and hangs

Every run on a simulator takes four numbers while it checks your flow. They land in perf in results.json, in the HTML report, in the pull request comment and in JUnit (properties perf.launchMs, perf.memoryMb and perf.hangs).

Set a budget

Set limits with budget in a check file (the keys):

budget:
  launch_ms: 2500
  memory_mb: 400
  hangs: 0

A run over a limit fails with class budget_exceeded and says which number and by how much. A limit on a number the run couldn’t take, such as memory_mb on an iPhone, ends the run needs_review with class budget_unmeasured. Mobster never passes a budget it can’t check.

Read what a check measures

Number	How Mobster takes it
Cold launch	The time from the `simctl launch` call to the first accessibility read of the app’s own screen that the next read agrees with: `perf.launch.interactiveMs`. It reads every 0.1 s, so it has that resolution plus the time of one read. `foregroundMs` is the time until WebDriverAgent names your app as the app in front
Memory	The app’s resident memory, from `ps` on its process when it is in front, every 500 ms after that, and once more at the verdict read. `peakMb` is the largest sample and `p95Mb` the nearest-rank 95th percentile. Resident size is larger than the footprint Xcode shows, and it includes shared frameworks
Hangs	The lines `Hang detected: 0.52s` in the app’s own log of 250 ms or longer. `count` and `longestMs`
CPU	The largest `ps` CPU share of the process, in `memory.cpuPeakPct`. One busy core is about 100

What Mobster doesn’t measure is in perf.notMeasured, so a missing number says why:

First frame. A simulator writes no log line that marks the app’s first frame, so launch has no firstFrameMs. Cold launch ends at the first stable screen, which comes after the first frame.
Dropped frames. A simulator exposes no Core Animation commit times, and MetricKit isn’t available in one.
Memory on an iPhone. A phone run takes the two launch times and the hang count, and memory is null.
Hangs on a simulator. A simulator may not log hangs at all. A count of 0 means no hang line appeared.

Watch the numbers over time

mobster test --history keeps the numbers per run and marks the last run against the median of the last 10 passes before it. ↑ is 20% more launch time or memory, or more hangs than the median. ↓ is 20% less, and → is in between. It marks nothing until 3 earlier passes have the number.

stable  Feed loads  on iPhone 17 Pro  ✓✓✓✓✓✓✓  100% of 7 runs, flake score 0.00
    Last run: launch 1,329 ms →, memory 278 MB →, hangs 0 →.

The marker flags a trend and never fails a check. Only budget fails one. --history --json adds perfLatest (the last run’s numbers) and perf (each number’s marker, latest value, baseline median and run count) to every row.

Run checks

Devices

Suites and results

Pull requests and CI

Your stack

App Store

Launch time, memory and hangs

Set a budget

Read what a check measures

Watch the numbers over time