samply

name	samply
description	Analyze Samply Firefox-profiler output, record focused profiles, summarize hot threads/stacks, inspect symbolication, and compare profile evidence before and after a performance change.

Samply

Overview

Use this skill when a task involves Samply recordings or Firefox-profiler JSON, especially profile.json.gz files, samply record, samply load, symbolication, thread timeline skew, or hot stack interpretation. Keep the loop evidence-driven: establish a focused baseline, record the exact target, summarize the profile before deep code reading, make one scoped change, and rerun the same target.

This skill is intentionally project-agnostic. Any project-specific benchmark harness, environment variables, metrics, or logging commands should live in a separate benchmark skill or in the current task context.

Share Evidence Early

Do not disappear into profile spelunking while useful output is already available. As soon as a timing run finishes, report the timing lines or comparison table before starting deeper analysis. As soon as a profile summary is available, report the top threads/functions/stacks before reading more code. Then continue investigating with those facts visible.

For long performance sessions, use this cadence:

1. timing command starts: say what target, input, mode, and runtime toggles are being measured;
2. timing command finishes: immediately show timing results and output path;
3. profile command finishes: immediately show profile path, a samply load command the user can run to inspect it in Firefox Profiler, and the first stack/function summary;
4. deeper analysis begins: state the concrete hotspot or hypothesis being checked.

Standard Loop

1. Check branch state and changed surface when working in a repository:

   git status --short
   git branch --show-current
   git diff --stat
   

2. Run a focused timing command before profiling. Prefer one executable, one workload/input, one mode, and enough iterations to smooth obvious noise. If the experiment has a runtime environment toggle, prefix every timing and profile command with the same env setting; this is often faster than recompiling and makes A/B comparisons clearer.

   Generic shape:

   FEATURE_TOGGLE=1 <timing-command> --iterations 5 --output /tmp/<label>.jsonl
   

   If a project has an existing benchmark harness, use that harness for the timing baseline and copy its exact target arguments into the profiled command.

3. Record a focused Samply profile. Prefer recording without --unstable-presymbolicate first, then symbolicate offline with the scripts below. This avoids chasing misleading pre-symbolicated stacks when unwinding or symbol lookup gets confused.

   FEATURE_TOGGLE=1 samply record --save-only --rate 1000 \
     --output /tmp/<label>.profile.json.gz \
     -- /absolute/path/to/<binary> <args>
   

   Put environment assignments before samply record, as shown above. Do not put them after the -- separator; everything after -- is the command Samply launches and profiles.

   On macOS, profiling through a system helper such as env, sleep, /bin/true, or system Python can be a bad sanity check because signed system executables may block Samply's task-port handoff. Prefer a locally built binary or a user-owned executable.

   In a sandboxed agent environment on macOS, Encountered an error during profiling: Unknown(1100) usually means Samply was blocked before the profiled command started. Rerun the same samply record command with the required execution permissions instead of changing the workload.

   Use a profile with debug information when stack quality matters. If the symbols or unwinding look suspect, rebuild with the project's highest-quality profiling/debug-symbol profile and record again.

   If a profile shows impossible-looking ancestry, such as hot execution frames nested under unrelated Drop::drop frames or otherwise nonsensical async stacks, do not trust the stack summary. First verify the binary UUID matches the profile, remove presymbolication from the recording command, and rebuild with better debug symbols if needed.

   After the profile is recorded, immediately show the user the command to open it themselves:

   samply load /tmp/<label>.profile.json.gz
   

   samply load starts a local Firefox Profiler server and opens the browser UI. If the user only wants the URL or the environment cannot open a browser, use:

   samply load --no-open /tmp/<label>.profile.json.gz
   

   Then report the printed local URL.

4. Summarize the profile without opening Firefox Profiler. Run a small summary first and report it immediately, then run a wider summary only if needed:

   python3 .agents/skills/samply/scripts/profile_summary.py \
     /tmp/<label>.profile.json.gz \
     --binary /absolute/path/to/<binary> \
     --symbolicate \
     --weight-mode cpu \
     --top 12 \
     --threads 2 \
     --stacks 4 \
     --stack-depth 10
   

   Wider follow-up:

   python3 .agents/skills/samply/scripts/profile_summary.py \
     /tmp/<label>.profile.json.gz \
     --binary /absolute/path/to/<binary> \
     --symbolicate \
     --weight-mode cpu \
     --top 30 \
     --threads 6 \
     --stacks 12
   

   For timeline skew, quantify worker occupancy instead of relying only on the visual timeline:

   python3 .agents/skills/samply/scripts/profile_activity.py \
     /tmp/<label>.profile.json.gz \
     --thread-regex '<worker-thread-regex>' \
     --bin-ms 10
   

   For a focused inverted call tree over a thread class or time range, use:

   python3 .agents/skills/samply/scripts/profile_inverted_tree.py \
     /tmp/<label>.profile.json.gz \
     --binary /absolute/path/to/<binary> \
     --symbolicate \
     --thread-regex '<worker-thread-regex>' \
     --start-ms <start> \
     --end-ms <end> \
     --contains '<frame-regex>'
   

5. Inspect code near the actual hot path. Load the workload definition when it matters; do not rely on memory for the workload shape.

6. Make one narrow change, rerun the focused timing/profile command, and record the before/after command lines and results. Do not broaden the workload until the narrow target explains the change.

Samply JSON Schema

Samply writes Firefox-profiler JSON, often compressed as profile.json.gz.

• Top-level keys commonly include meta, libs, threads, pages, counters, and profilerOverhead.
• meta.product names the process, meta.interval is the sampling interval in milliseconds, and meta.startTime is an epoch timestamp in milliseconds.
• libs[] records loaded binaries with name, path, debugPath, codeId, breakpadId, and arch. Use this to verify symbol files still match the profile.
• Each threads[] entry has name, tid, samples, stackTable, frameTable, funcTable, resourceTable, and stringArray.
• samples.length is the number of stored sample rows. samples.stack[] points into stackTable. samples.weight[] is the number of collapsed samples represented by that row; use weight instead of row count when present. samples.threadCPUDelta[] is per-thread CPU delta in microseconds when present.
• stackTable is a linked list: stackTable.frame[i] is the current frame and stackTable.prefix[i] points to the caller stack. Follow prefixes to null and reverse to get root-to-leaf order.
• frameTable.func[frame] points into funcTable; funcTable.name[func] points into stringArray. funcTable.resource[func] points into resourceTable, whose name or lib fields point back into stringArray.

Symbolication

If function names are raw addresses such as 0x3db28a0, the profile is not symbolicated. Before using atos, verify the binary UUID/code ID matches the profile:

gzip -cd /tmp/<label>.profile.json.gz | jq '.libs[] | {name, path, codeId, breakpadId}'
dwarfdump --uuid /absolute/path/to/<binary>

If the UUID/code ID does not match, do not trust symbol names from the current binary. Re-profile, or keep the exact binary plus any symbol sidecar emitted by the recorder.

On macOS, Samply stores app addresses as offsets. Add the Mach-O text load address when using atos manually:

atos -o /absolute/path/to/<binary> -l 0x100000000 0x103db28a0

For a raw offset 0x3db28a0, the address passed to atos is 0x100000000 + 0x3db28a0.

When using the bundled scripts, pass --symbol-lib <library-name> if the binary name in profile.libs[] differs from the basename of --binary.

Reading Profiles

• Treat the main thread as orchestration unless its CPU delta is high. Useful CPU time is often on worker threads, but thread naming is runtime-specific.
• Sort threads by total sample weight and CPU delta. Many idle worker threads can have high wall time but near-zero CPU.
• Use profile_activity.py when the Firefox Profiler timeline shows empty space. Good parallel traces keep worker occupancy high through the timed region; a low-occupancy tail points to scheduling skew, stragglers, dependency ordering, partition imbalance, blocking, or insufficient work admission.
• Use profile_inverted_tree.py with --contains for allocation frames, blocking frames, or a hot leaf function to see the caller contexts that produce the samples.
• A stack with many samples may mean the operation is slow, or it may mean it is called many times. Samply alone usually cannot distinguish those. Pair hot stacks with counters, metrics, or logs: operation count, byte count, rows/items processed, cache hits/misses, per-operation max/median duration, and lock wait/hold time.
• Prefer inclusive stacks to understand which subsystem owns time, then self frames to find tight loops.
• Look for repeated work: allocation, parsing, cloning, serialization/deserialization, expression evaluation, decompression, canonicalization, redundant I/O, and synchronization overhead.
• If profile output only shows addresses and symbolication is blocked by a UUID mismatch, you can still use thread CPU, stack repetition, and library ownership, but re-profile before making a code-level claim.

Reporting

Summaries should include:

• timing command and profile command;
• command for the user to open the profile, usually samply load <profile.json.gz>;
• branch, binary, and profile used;
• before/after timings or run IDs;
• top hot threads/functions/stacks;
• confirmed facts versus inferences;
• checks run and checks skipped.