argent-native-profiler

1. Tools

• native-profiler-start: start profiling on a booted device. iOS: xctrace recording for CPU, hangs, and leaks.
• native-profiler-stop: stop the profiler and export trace data to timestamped XML files.
• native-profiler-analyze: parse exported trace data and return a structured bottleneck payload.
• profiler-stack-query: drill into parsed data: hang stacks, function callers, thread breakdown, leak details.
• profiler-load: list and reload previous trace sessions from disk for re-investigation.
• Physical iPhone: not supported; use a simulator.

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2. Platform Support

• iOS: Backend: Xcode Instruments via xctrace on a booted simulator or connected device. Requires Xcode command-line tools on PATH. Surfaces CPU hotspots, UI hangs, and memory leaks (instruments Leaks table).
• Android: Backend: Perfetto via adb shell perfetto + an in-process WASM trace-processor engine. Surfaces CPU hotspots and UI hangs, with per-hang jank reason codes, a main-thread state breakdown with blocked_function attribution, and a GC overlap annotation. Also reports an RSS-growth signal for memory pressure; treat it as a hint to confirm manually, not a confirmed leak. The target app must be debuggable or include <profileable android:shell="true"/> in its manifest for perf_sample callstacks to be captured.

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3. Investigation Patterns

After native-profiler-analyze surfaces findings, use profiler-stack-query to drill into root causes:

• Hang detected → profiler-stack-query mode=hang_stacks for full native call chains → mode=function_callers for the suspected function → read native source.
• CPU hotspot → profiler-stack-query mode=thread_breakdown for per-thread distribution → mode=function_callers for the dominant function.
• Memory leak → profiler-stack-query mode=leak_stacks filtered by object_type for responsible frames and libraries.
  • iOS: if leaks come back unattributed (responsible frame <Call stack limit reached>), re-run native-profiler-start with malloc_stack_logging: true. This cold-launches the app with Malloc Stack Logging so leaks carry a real allocation backtrace (responsible frame + library). It restarts the app and adds overhead, so use it only when you need leak attribution, not for CPU/hang passes.

After presenting findings, ask the user whether to investigate further, implement fixes, or stop. After applying fixes, always re-profile the same scenario and compare with profiler-load. Report honestly whether the target metric improved, regressed, or stayed flat. If the fix showed no net benefit or introduced regressions elsewhere, say so and reconsider.

Tip: For reproducible before/after comparisons, record the interaction sequence as an e2e test using the argent-create-e2e skill before the first profiling run. Replay with e2e-execute on subsequent runs to eliminate interaction variance.

Note: The argent-react-native-profiler instructs to start native profiling automatically alongside React profiling. This skill's workflow and investigation patterns apply in both cases.

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4. Workflow

Complete all steps in order; do not break mid-flow.

Step 0: Ensure the target app is running

The native-profiler-start tool auto-detects the running app on the device. You do not need to derive app_process manually; just make sure the app is launched.

1. If the app is already running on the device, skip to Step 1 (do not pass app_process).
2. If the app is not running, use launch-app with the correct bundle ID first.
3. Only pass app_process explicitly if the tool reports multiple running user apps and you need to disambiguate.

Note: If multiple build flavors are installed (dev, staging, prod), the tool will detect whichever one is currently running. If both are running, it will ask you to specify.

Step 1: Start recording

Call native-profiler-start with device_id (iOS UDID or Android serial). The tool auto-detects the running app and saves the trace to /tmp/argent-profiler-cwd/ with a timestamped filename. Let the user interact with the app or drive interaction via simulator tools (see argent-device-interact skill).

Step 2: Stop and export

Call native-profiler-stop with device_id. iOS sends SIGINT to xctrace, waits for trace packaging, and exports CPU, hangs, and leaks data to XML; check exportDiagnostics for any export warnings. Android sends SIGTERM to the on-device perfetto daemon, polls /proc/<pid> until it exits, then adb pulls the .pftrace to the host.

Step 3: Analyze

Call native-profiler-analyze with device_id. Returns a markdown report with bottlenecks categorized as CPU hotspots, UI hangs, or memory leaks, sorted by severity.

Step 4: Present findings and ask about next steps

Present a concise summary of the key findings. Then follow the "After analysis" guideline: ask whether to investigate further with query tools, implement fixes, or stop.

Step 5: Drill-down investigation

Use profiler-stack-query to investigate specific findings. See §3 Investigation Patterns for chaining guidance.

Step 6: Reload previous sessions

To revisit a previous trace:

1. Call profiler-load mode=list to see available sessions.
2. Call profiler-load mode=load_native session_id=<timestamp> device_id=<UDID> to re-parse the XML files.
3. Use profiler-stack-query to investigate the reloaded data.

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5. Understanding Results

Bottlenecks are categorized by severity:

• RED: CPU functions taking >15% of total time, all UI hangs, and attributed memory leaks (those with a resolved responsible frame). These require immediate attention.
• YELLOW: CPU functions taking 3-15% of total time, and unattributed memory leaks (<Call stack limit reached>, no library; see the memory-leaks caveat below). Worth investigating but may be acceptable.

Each bottleneck type indicates a different class of problem:

• CPU hotspots: Native functions consuming excessive CPU time. Look for tight loops, expensive computations, or redundant work.
• UI hangs: Main thread blocked long enough to cause visible jank or unresponsiveness. Often caused by synchronous I/O, heavy layout passes, or lock contention.
• Memory leaks: Objects allocated but never freed. Common causes include retain cycles, unclosed resources, or forgotten observers. Argent records via xctrace --attach, which has no malloc-stack history, so on the simulator most leaks come back unattributed (<Call stack limit reached>, no library) and are dominated by benign system allocations; these are reported as a low-confidence YELLOW summary, not confirmed RED leaks. For attributed stacks, capture with malloc stack logging enabled at launch.

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6. Important Caveats

• Simulator vs device: Simulator profiling reflects host Mac performance, not real device hardware. Use device profiling for accurate CPU timings and memory behavior.
• xctrace availability (iOS): Requires Xcode command-line tools installed. Verify with xcrun xctrace version.
• Profiler overhead: xctrace instrumentation adds CPU load. If JSLexer, JSONEmitter, or Hermes runtime internals dominate the JS thread in CPU hotspot results, those reflect profiler overhead, not app work. Discount those entries when evaluating findings.
• Run-to-run variance: Small fluctuations in CPU percentages between runs are normal. Treat only consistent directional changes (across 2+ runs or >15% delta) as actionable signal.
• Live data variability: If the app fetches live API data, different responses between runs change rendering workload independently of code changes. Note when data-dependent screens show variance.