Workflow Perf

Workflow: Performance Evaluation

Use this skill when tasked with improving the performance or memory usage of a workload in V8. This workflow focuses on identifying bottlenecks and applying V8-side optimizations.

Activation Criteria

• User requests to optimize a specific benchmark or script.
• Goal is to reduce execution time, CPU cycles, or memory footprint.

Core Principles

1. Data-Driven: Always base optimization decisions on profiling data, not intuition.
2. V8-Centric: For V8 engineers, performance work usually means changing V8 to better handle the JS pattern, rather than changing the JS itself (though both are valid for general users).
3. Holistic View: Look for general efficiency improvements, not just the single hottest function.
4. Contextual Interpretation: Assume unknown terms in performance tasks are likely benchmark names or domain-specific concepts, not environmental terms (e.g., "WSL" is likely a JetStream story, not the OS). Verify before assuming. If in doubt, ask the user.
5. Mandatory Orchestration: The agent executing this workflow MUST act as an Orchestrator. Delegate execution of benchmarks, profiles, or searches to subagents to maximize parallelism.
6. Local Experiment Baseline: Whenever asked to perform a local experiment, always compile the baseline first and store it in a separate directory like x64.release-baseline for better gm.py integration. This avoids recompiling at later stages.

Workflow

Planning

For performance analysis, you do NOT need to create a full implementation_plan.md until you are actually fixing the performance issue you've detected. Instead, maintain an Analysis Plan (e.g., in task.md or as a list of questions to answer) to guide the investigation.

1. Parallel Track Initialization

Initialize the following tracks concurrently:

• Track A: Profiling & Tracing:
  • Run the workload with the v8-profiling skill (e.g. linux-perf and/or pprof).
  • Use V8 tracing flags to gather specific runtime telemetry.
• Track B: V8 Log Analysis
  • Use the v8-log skill to extract v8.log and analyze internal state of V8
• Track C: JS Source Analysis:
  • Study the JavaScript benchmark to understand the core operations and potential hotspots.
  • Benchmark Source Access: If benchmark sources (such as JetStream 3) are not locally available under test/benchmarks/, enable "checkout_benchmarks": True in your ../.gclient configuration (custom_vars) and run gclient sync.
• Track D: Static V8 Research:
  • Search for known optimization patterns or issues related to the observed JS patterns in the V8 codebase.

3. Running Benchmarks / Pages in Chrome with with Crossbench

• Generate Performance Logs and Profiles: use the crossbench skill to gather v8.log, detailed perfetto traces and sampling profiles from pages or benchmarks.
• Probes:
  • Use --probe=profiling for full-browser or d8 profiles
  • Use --probe=perfetto for detailed perfetto traces
  • Use --probe=v8.log for extracting internal v8 logs from chrome

4. Alternative: Running Benchmarks with jsb_run_bench

In the jetski environment, you can also use the jsb_run_bench tool from v8-utils as an alternative for quick runs.

• Run for Scores: Call jsb_run_bench with paths to d8 binaries to compare performance.
• Profile: Supports record: "perf" and record: "v8log".

5. Profile Analysis & Tick Processor

• Generate and Analyze Profile: Use the v8-profiling skill to generate and analyze linux-perf and tickprocessor profiles. Note that you can use crossbench for generating chromium-level profiles.
• Cross Reference:
  • Correlate JS sources with v8.log to understand what V8 is doing when executing the JS sources.
  • Use the v8.log to further drill down on internal v8 state to understand bottlenecks.
• Interpretation:
  • Look at the C++ entry points and JS functions taking the most ticks.
  • Check if time is spent in runtime functions vs. generated code.
  • Identify if specific builtins are taking significant time.
  • Identify inefficient JS code patterns in the workloads
  • Suggest builtins, C++ code that can be optimized

6. Tracing Compiler Graphs (Turbolizer)

For peak performance, it is often necessary to inspect the intermediate representations (IR) of the optimizing compiler (TurboFan or Turboshaft).

• Generate Graph Data: Run d8 --trace-turbo script.js or pass flags in Crossbench/jsb_run_bench. This generates JSON files containing the graph state at various optimization phases (e.g., turbo-*.json).
• Visualize with Turbolizer: Use the Turbolizer tool (available internally at go/turbolizer or in the V8 repository under tools/turbolizer).
• Analysis:
  • Inspect the graph at different phases to see how nodes are simplified, combined, or eliminated.
  • Look for missed optimizations, such as redundant checks that were not hoisted or allocations that failed to be eliminated by escape analysis.
  • Identify unexpected deoptimization points.

7. Identifying General Efficiency Improvements

Beyond hotspots, look for areas where V8 can be improved to handle patterns better:

• Reducing Allocations: High GC overhead implies frequent allocations. Investigate if V8 can optimize allocation folding, escape analysis, or if the allocations are unavoidable.
• Optimizing Hot Loops: Ensure loops are not deoptimizing in V8. Check if checks can be hoisted by the compiler or if loop peeling is effective in the VM.
• Hidden Class (Map) Stability: Understand how object shapes evolve and cause polymorphic or megamorphic IC states. Investigate if V8 can be optimized to handle these transitions better.

8. Analysis & Reprioritization

• Analyze profile results (e.g., flamegraphs, top functions).
• Dynamic Reprioritization:
  • High GC Time: If profile shows significant time in GC, pivot to allocation analysis and reducing memory churn. Also explain which JS code is causing frequent allocations.
  • High IC Misses: If --log-ic shows frequent misses, pivot to investigating object layout and stabilizing hidden classes.
  • Dominant Hotspot: If a single function dominates execution time, focus all efforts on that component.
  • Pattern Identification: If a V8 change is identified that could improve the pattern generally, prioritize implementing and testing it over further analysis.

9. Optimization & Verification

• Propose a V8 change to improve performance (e.g., specialized builtin, improved optimization pass).
• Verification on Pinpoint (Preferred):
  • Commit changes to a local branch.
  • Use the automation script to upload a CL and start a Pinpoint job:

    scripts/upload_and_pinpoint.py \
      --benchmark=<benchmark_name> \
      --bot=<bot_name> \
      --message="Experiment: My performance optimization"
    

  • Use ./cb.py pinpoint help to understand the available options.
• Local Verification:
  • Re-run the benchmark locally if Pinpoint is unavailable.
  • Compare perf stats (cycles, instructions).
  • Ensure no regressions in correctness or other benchmarks.