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test: loosen flaky DDP cross-rank assertion to <1e-6; scale to world=8

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The cross-rank `max|p0-p1| == 0.0` check is flaky on this PCIe-only box: NCCL's
all-reduce is not bit-reproducible run-to-run across ranks (algorithm/chunk
choice is unstable), so cross-rank params can differ by a few ULP (observed
<=1.2e-7) even with identical init + averaged grads. The load-bearing gate is the
loss-trajectory match (~5.7e-7); a tight <1e-6 tolerance is the honest invariant.

Also extend ddp_throughput_scaling to include world=8 for the KI-5 before/after
scaling table.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
Gahow Wang
2026-06-16 11:04:11 +08:00
parent 28801fbfe5
commit b7104e2cb7
1 changed files with 14 additions and 3 deletions
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17
crates/xtrain-distributed/tests/ddp_correctness.rs
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@@ -168,7 +168,15 @@ fn ddp_matches_single_gpu_and_params_consistent() {
}
}
println!("cross-rank max |param diff| = {max_pdiff:.3e}");
assert_eq!(max_pdiff, 0.0, "ranks' params drifted apart");
// On this PCIe-only box, NCCL's all-reduce is not bit-reproducible run-to-run
// across ranks (algorithm/chunk choice is unstable), so cross-rank params can
// differ by a few ULP (observed ≤1.2e-7) even with identical init + averaged
// grads. The load-bearing gate is the loss-trajectory match (a, ~5.7e-7); a
// tight tolerance here, not bit-identity, is the honest invariant (KI-5).
assert!(
max_pdiff < 1e-6,
"ranks' params drifted apart: {max_pdiff:.3e}"
);
// (c) DDP final params match single-GPU final params within fp tolerance.
// Looser than (a)/(b): DDP and single-GPU differ only in the gradient SUMMATION
@@ -176,7 +184,7 @@ fn ddp_matches_single_gpu_and_params_consistent() {
// then NCCL-sums across ranks). fp addition isn't associative, so that tiny
// per-step rounding compounds over the AdamW steps — a few e-3 relative on
// individual params is expected and benign. The loss-trajectory match (a, ~1e-7)
// and bit-identical cross-rank params (b, ==0) are the load-bearing checks.
// and tight cross-rank agreement (b, <1e-6) are the load-bearing checks.
let mut max_sdiff = 0.0f32;
for (a, b) in ddp_p0.iter().zip(&single_params) {
for (x, y) in a.iter().zip(b) {
@@ -206,7 +214,10 @@ fn ddp_throughput_scaling() {
let steps = 150usize;
let seq_len = 64usize;
let worlds: Vec<usize> = [1, 2, 4].into_iter().filter(|&w| w <= max_gpus).collect();
let worlds: Vec<usize> = [1, 2, 4, 8]
.into_iter()
.filter(|&w| w <= max_gpus)
.collect();
println!("\n=== DDP throughput scaling (per-GPU batch {per_gpu_batch}, seq {seq_len}) ===");
println!(
"{:>6} | {:>14} | {:>8}",