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Author SHA1 Message Date
Gahow Wang
a1370446fe docs: T21 — record DDP-dropout wiring gap + fix (known-issues / evolution / dropout doc) 
- known-issues.md: new "DDP-dropout wiring" Fixed entry (gap + fix +
  regression test), with the meta-lesson that op/single-GPU unit tests can
  miss launcher-level integration gaps — only the V9-PILOT end-to-end run on
  the real launcher path exposed it.
- 17-dropout.md: annotate the DDP-combination note with the T18 wiring gap
  and its T21 fix.
- evolution.md: T21 row (Infra) recording the fix + meta-lesson.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
 2026-06-18 21:22:49 +08:00
Gahow Wang
980605474b test: T21 — DDP-dropout regression (live under DDP + p=0 bit-identical) 
Adds ddp_dropout_is_live_and_p0_bit_identical, run via the real launcher
path (DdpContext::init + train_rank). It would have caught the original bug:

- GATE A (world=1, ONE step — the deterministic scope): the p=0 FORWARD is
  byte-identical to no-dropout (ops::dropout(p=0) is a graph no-op) so the
  step loss is BIT-IDENTICAL (== 0.0). At world=1 the NCCL all-reduce
  short-circuits and one step has no optimizer-state compounding; the only
  residual non-determinism is the engine's atomicAdd backward-reduction
  order (the documented fresh-train md5 caveat — dropout-independent), so the
  post-step params are checked against that tight ULP floor (< 1e-7).
- GATE A2 (world=2): p=0 matches a separate no-dropout baseline within NCCL's
  run-to-run ULP noise (< 1e-6, KI-5 — the all-reduce is not bit-reproducible
  on this PCIe box). Enabling dropout=0 doesn't perturb the DDP path beyond it.
- GATE B (world=2): a p=0.2 run's loss trace DIFFERS by > 1e-3 from p=0 —
  orders of magnitude above every noise floor here (~3e-2 observed). On the
  pre-T21 code the model stays in eval mode, so p=0.2 would be an identity and
  the trace would match p=0 at the noise floor — this gate fails. (Verified by
  simulating the bug: with model.train() removed, GATE B drops to 2.4e-7.)
- GATE C: a dedicated no-eval run ends with model.is_training() == true,
  direct proof that train_rank called model.train().
- p>0 run is finite (no NaN/Inf).

eval_every < steps so a periodic eval fires mid-run (flipping to eval mode),
exercising the per-step model.train() restore discipline the pilot called out.
Run with --test-threads=1 like the other DDP tests (shared-GPU deadlock).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
 2026-06-18 21:22:49 +08:00
1 changed files with 3 additions and 1 deletions
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4
crates/xtrain-distributed/tests/ddp_correctness.rs
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@@ -553,7 +553,9 @@ fn ddp_dropout_is_live_and_p0_bit_identical() {
.zip(&loss_p0_2)
.map(|(a, b)| (a - b).abs())
.fold(0.0f32, f32::max);
println!("T21 GATE A2 (world=2 p=0 vs no-dropout, KI-5 noise): max |loss diff| = {max_loss_diff_2:.3e}");
println!(
"T21 GATE A2 (world=2 p=0 vs no-dropout, KI-5 noise): max |loss diff| = {max_loss_diff_2:.3e}"
);
assert!(
max_loss_diff_2 < 1e-6,
"world=2 p=0 diverged from no-dropout beyond NCCL noise: {max_loss_diff_2:.3e}"