Run-01 was killed at step 19 by operator instruction to root-cause before
spending a ~13.9h window. Two independent methods now agree on where the step
time went, and neither was the hypothesis the consult panel converged on.
Scaling fit (3 points, 2 params, residuals <3ms over an 8x range):
A = 6.87e-4 s/token, B = 8.85e-8 s/token^2
quadratic share 20.9% @ w=2048 -> 67.8% @ w=16384
No fixed term was needed, which refutes launch-bound outright.
Profiler kernel table (device rows only):
attention 22,835.8 ms 65.2% fmha_cutlass*_sm80
dense GEMM 2,774.0 ms 7.9%
other 5,739.0 ms 16.4%
The attention kernels are sm80 — Ampere-generation CUTLASS running on an
sm_120 Blackwell card, with the forward on the gmem fallback tier. That is the
mechanism behind 100% SM utilisation at 27 of 304 available TFLOPS.
Correctness cleared separately: the sliding mask asserts at max 1024
allowed/row, so the 25 windowed layers were genuinely windowed. The same probe
found that right-padding is what pins the 5 global layers to an explicit 4D
mask and off the is_causal fast path — measured at 9.4% slower for 24% less
loss work at fixed width.
The largest available win is not the attention kernel. The corpus is 29.9%
padding, and bucket-to-pair + shuffle-to-mix takes it to 0.0% for >=35.5% wall
clock, no new dependency, unchanged peak memory. Bucket size turned out not to
be a diversity knob — roots per accumulation window are flat across a 256x
range, so the global micro-batch shuffle does that work alone and the bucket
should be tight.
Adds docs/pfi/training-throughput-playbook.md as the durable model-agnostic
home (sibling to the quantization playbook), the four probes under
scripts/training-probes/ with raw output kept for re-derivation, and a §6 to
the sizing doc carrying the Gemma-4-specific numbers and round-2 restart
parameters.
Measured negatives recorded so they are not re-chased: grouped_mm (0.9%
slower, and MoE is only 7.9% of the step), CUDA graphs / torch.compile over
the expert loop (no fixed cost to amortise), liger fused CE (~1-3% lever),
FA4 on sm_120.
Round-1 state preserved: 609MB encode cache, order manifest, truncation
report, resume script. No checkpoints — it died at step 19 and the first was
due at 100, so the lora_B inert-adapter gate never ran and moves to the
restart.
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