ln-653-runtime-performance-auditor

name	ln-653-runtime-performance-auditor
description	Runtime performance audit worker (L3). Checks blocking IO in async, unnecessary allocations, sync sleep in async, string concat in loops, missing to_thread for CPU-bound, redundant data copies. Returns findings with severity, location, effort, recommendations.
allowed-tools	Read, Grep, Glob, Bash

Paths: File paths (shared/, references/, ../ln-*) are relative to skills repo root. If not found at CWD, locate this SKILL.md directory and go up one level for repo root.

Runtime Performance Auditor (L3 Worker)

Specialized worker auditing runtime performance anti-patterns in async and general code.

Purpose & Scope

• Worker in ln-650 coordinator pipeline - invoked by ln-650-persistence-performance-auditor
• Audit runtime performance (Priority: MEDIUM)
• Check async anti-patterns, unnecessary allocations, blocking operations
• Write structured findings to file with severity, location, effort, recommendations
• Calculate compliance score (X/10) for Runtime Performance category

Inputs (from Coordinator)

MANDATORY READ: Load shared/references/task_delegation_pattern.md#audit-coordinator--worker-contract for contextStore structure.

Receives contextStore with: tech_stack, best_practices, codebase_root, output_dir.

Domain-aware: Supports domain_mode + current_domain.

Workflow

1. Parse context from contextStore

   • Extract tech_stack, best_practices, output_dir
   • Determine scan_path
   • Detect async framework: asyncio (Python), Node.js async, Tokio (Rust)

2. Scan codebase for violations

   • Grep patterns scoped to scan_path
   • For Rules 1, 3, 5: detect async def blocks first, then check for violations inside them

3. Collect findings with severity, location, effort, recommendation

4. Calculate score using penalty algorithm

5. Write Report: Build full markdown report in memory per shared/templates/audit_worker_report_template.md, write to {output_dir}/653-runtime-performance.md in single Write call

6. Return Summary: Return minimal summary to coordinator (see Output Format)

Audit Rules (Priority: MEDIUM)

1. Blocking IO in Async

What: Synchronous file/network operations inside async functions, blocking event loop

Detection (Python):

• Find async def functions
• Inside them, grep for blocking calls:
  • File: open(, .read_bytes(), .read_text(), .write_bytes(), .write_text(), Path(...).(read|write)
  • Network: requests.get, requests.post, urllib.request
  • Subprocess: subprocess.run(, subprocess.call(
• Exclude: calls wrapped in await asyncio.to_thread(...) or await loop.run_in_executor(...)

Detection (Node.js):

• Inside async function or arrow async, grep for fs.readFileSync, fs.writeFileSync, child_process.execSync

Severity:

• HIGH: Blocking IO in API request handler (blocks entire event loop)
• MEDIUM: Blocking IO in background task/worker

Recommendation: Use aiofiles, asyncio.to_thread(), or loop.run_in_executor() for file operations; use httpx.AsyncClient instead of requests

Effort: S (wrap in to_thread or switch to async library)

2. Unnecessary List Allocation

What: List comprehension where generator expression suffices

Detection:

• len([x for x in ...]) - allocates list just to count; use sum(1 for ...)
• any([x for x in ...]) - allocates list for short-circuit check; use any(x for ...)
• all([x for x in ...]) - same pattern; use all(x for ...)
• set([x for x in ...]) - use set comprehension {x for x in ...}
• "".join([x for x in ...]) - use generator directly "".join(x for x in ...)

Severity:

• MEDIUM: Unnecessary allocation in hot path (API handler, loop)
• LOW: Unnecessary allocation in infrequent code

Recommendation: Replace [...] with generator (...) or set comprehension {...}

Effort: S (syntax change only)

3. Sync Sleep in Async

What: time.sleep() inside async function blocks event loop

Detection:

• Grep for time\.sleep inside async def blocks
• Pattern: await some_async_call() ... time.sleep(N) ... await another_call()

Severity:

• HIGH: time.sleep() in async API handler (freezes all concurrent requests)
• MEDIUM: time.sleep() in async background task

Recommendation: Replace with await asyncio.sleep(N)

Effort: S (one-line change)

4. String Concatenation in Loop

What: Building string via += inside loop (O(n^2) for large strings)

Detection:

• Pattern: variable result, output, html, text with += inside for/while loop
• Grep for: variable followed by += containing string operand inside loop body

Severity:

• MEDIUM: String concat in loop processing large data (>100 iterations)
• LOW: String concat in loop with small iterations (<100)

Recommendation: Use list.append() + "".join(), or io.StringIO, or f-string with "".join(generator)

Effort: S (refactor to list + join)

5. Missing to_thread for CPU-Bound

What: CPU-intensive synchronous code in async handler without offloading to thread

Detection:

• Inside async def, find CPU-intensive operations:
  • JSON parsing large files: json.loads(large_data), json.load(file)
  • Image processing: PIL.Image.open, cv2.imread
  • Crypto: hashlib, bcrypt.hashpw
  • XML/HTML parsing: lxml.etree.parse, BeautifulSoup(
  • Large data transformation without await points
• Exclude: operations already wrapped in asyncio.to_thread() or executor

Severity:

• MEDIUM: CPU-bound operation in async handler (blocks event loop proportionally to data size)

Recommendation: Wrap in await asyncio.to_thread(func, *args) (Python 3.9+) or loop.run_in_executor(None, func, *args)

Effort: S (wrap in to_thread)

6. Redundant Data Copies

What: Unnecessary .copy(), list(), dict() when data is only read, not mutated

Detection:

• data = list(items) where data is only iterated (never modified)
• config = config_dict.copy() where config is only read
• result = dict(original) where result is returned without modification

Severity:

• LOW: Redundant copy in most contexts (minor memory overhead)
• MEDIUM: Redundant copy of large data in hot path

Recommendation: Remove unnecessary copy; pass original if not mutated

Effort: S (remove copy call)

Scoring Algorithm

MANDATORY READ: Load shared/references/audit_scoring.md for unified scoring formula.

Output Format

MANDATORY READ: Load shared/templates/audit_worker_report_template.md for file format.

Write report to {output_dir}/653-runtime-performance.md with category: "Runtime Performance" and checks: blocking_io_in_async, unnecessary_list_allocation, sync_sleep_in_async, string_concat_in_loop, missing_to_thread, redundant_data_copies.

Return summary to coordinator:

Report written: docs/project/.audit/ln-650/{YYYY-MM-DD}/653-runtime-performance.md
Score: X.X/10 | Issues: N (C:N H:N M:N L:N)

Critical Rules

• Do not auto-fix: Report only
• Async context required: Rules 1, 3, 5 apply ONLY inside async functions
• Exclude wrappers: Do not flag calls already wrapped in to_thread/run_in_executor
• Context-aware: Small files (<1KB) read synchronously may be acceptable
• Exclude tests: Do not flag test utilities or test fixtures

Definition of Done

• contextStore parsed successfully (including output_dir)
• scan_path determined
• Async framework detected (asyncio/Node.js async/Tokio)
• All 6 checks completed:
  • blocking IO, unnecessary allocations, sync sleep, string concat, CPU-bound, redundant copies
• Findings collected with severity, location, effort, recommendation
• Score calculated using penalty algorithm
• Report written to {output_dir}/653-runtime-performance.md (atomic single Write call)
• Summary returned to coordinator

Reference Files

• Worker report template: shared/templates/audit_worker_report_template.md
• Audit scoring formula: shared/references/audit_scoring.md
• Audit output schema: shared/references/audit_output_schema.md

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Version: 1.0.0 Last Updated: 2026-02-04