{
  "lesson": "45-gradient-clipping-amp",
  "title": "Gradient Clipping and Mixed Precision",
  "questions": [
    {
      "stage": "pre",
      "question": "What problem does global L2 gradient clipping solve?",
      "options": [
        "A single bad batch can spike the gradient norm by orders of magnitude; clipping bounds the worst-case optimizer step so one batch cannot reset hours of learning",
        "Vocabulary drift",
        "Slow training",
        "Random restart"
      ],
      "correct": 0,
      "explanation": "Clipping is the safety belt that keeps a noisy batch from wrecking the loss curve."
    },
    {
      "stage": "check",
      "question": "What is the correct order of GradScaler operations in a step?",
      "options": [
        "scaler.scale(loss).backward(); scaler.unscale_(optimizer); clip_grad_norm_; scaler.step(optimizer); scaler.update()",
        "backward, step, scale, update",
        "Any order; the scaler reorders internally",
        "step, scale, unscale, backward, update"
      ],
      "correct": 0,
      "explanation": "Clip on unscaled gradients between unscale_ and step; update always runs."
    },
    {
      "stage": "check",
      "question": "What does scaler.update() do on a skipped step?",
      "options": [
        "Closes the run",
        "Nothing",
        "Halves the scaling factor and resets the no-inf counter; forgetting to call it on the skip path is the bug that produces 'the scaling factor never changed'",
        "Doubles the scaling factor"
      ],
      "correct": 2,
      "explanation": "The scaler tunes its factor based on the skip / clean ratio across recent steps."
    },
    {
      "stage": "check",
      "question": "Why is the loss finiteness check necessary even with a GradScaler?",
      "options": [
        "Because GradScaler is slow",
        "It is decorative",
        "PyTorch requires it",
        "GradScaler handles backward-pass overflow but a non-finite loss does not produce useful gradients; skipping before backward saves compute and avoids polluting the scaler's state"
      ],
      "correct": 3,
      "explanation": "Loss check covers forward-pass failures; scaler covers backward-pass failures."
    },
    {
      "stage": "post",
      "question": "What is the production signal in the rolling skip rate?",
      "options": [
        "Always page on any skip",
        "A handful of skips per run is healthy; hundreds of skips per epoch (above roughly 5 percent rolling rate) is a hard alert that the model is in a regime FP16 cannot hold",
        "Skip rate is irrelevant",
        "Page only on consecutive skips"
      ],
      "correct": 1,
      "explanation": "Rate-based alerting catches silent failure modes that occasional-skip pages would not."
    },
    {
      "stage": "post",
      "question": "Why switch from FP16 to BF16 autocast when skips are frequent?",
      "options": [
        "BF16 has a wider exponent range than FP16 and rarely needs loss scaling; the skip rate typically drops to zero on the same model",
        "BF16 is slower so it skips less",
        "BF16 is newer",
        "Random tradition"
      ],
      "correct": 0,
      "explanation": "FP16 favors mantissa precision over range; BF16 trades the other way, removing the overflow class entirely."
    }
  ]
}