career-progression-2026/week-01-python-internals/day-001-gil/week-01-python-internals_day-01-gil_notes.md

Day 1 — GIL + CPU-bound vs I/O-bound threading

Goals (what "done" means)

• I can explain the GIL in 2–3 sentences and why it impacts CPU-bound threading more than I/O-bound threading.
• I can choose between threads, processes, and asyncio for a given workload and justify it.
• I have benchmark results (numbers) that demonstrate the above.

Reading (mostly)

• Read: What Is the Python Global Interpreter Lock (GIL)? (Real Python)
  • Write 5 bullets: what the GIL is, what it is not, and why it exists.
• Read: Understanding the Python GIL (PDF)
  • Write 5 bullets: where threads do help, where they don’t, and what "switch interval" means (high level).
• Skim (reference): concurrent.futures — Python docs
  • Note the difference between ThreadPoolExecutor vs ProcessPoolExecutor.

Hands-on (benchmark + notes)

• Create a benchmark script that compares these variants for a CPU-bound function:
  • Serial (single process, no threads)
  • ThreadPoolExecutor
  • ProcessPoolExecutor
• Use a repeatable timing method (pick one):
  • timeit (preferred for small benchmarks): timeit — Python docs
  • or wall-clock timing with time.perf_counter() (fine for longer tasks)
• Record results in a small table (example format):
  • workload size parameters (so you can reproduce)
  • runtime for each approach
  • notes on CPU utilization / behavior
• Add an I/O-bound comparison:
  • Serial loop calling time.sleep(x)
  • Thread pool version (expect improvement)
• Write "Conclusions" (minimum 6 sentences):
  • What happened for CPU-bound?
  • What happened for I/O-bound?
  • What I would choose in real systems and why

Stretch (optional)

• Skim: PEP 703 – Making the Global Interpreter Lock Optional in CPython
  • Write 3 bullets: what changes, tradeoffs, and why it matters.

Deliverables (to commit)

• notes.md updated with:
  • explanation bullets
  • benchmark table
  • conclusions
• benchmark.py (or similar) added with instructions to run

Suggested commits

• day01: add GIL notes + benchmark scaffolding
• day01: record CPU vs IO benchmark results and conclusions