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perf(#1662): bit-identical fused optimizer-in-backward (lever #1) (#1664)
* docs(#1662): design spec for backward-pass optimizations (levers #4, #1, #3) Cross-repo, Tensors-first design for the backward/training side of #653/#1624: backward buffer-reuse audit (#4), optimizer-in-backward adaptive hybrid (#1), and FlashAttention-style tiled backward (#3). Excludes lever #2 (checkpointing, owned by open PR #1633). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * docs(#1662): correct lever #1 scope — optimizer-in-backward is OOM-only today TrainWithTapeStreaming already does single-pass-fused/two-pass-norm, but only engages above 0.5x-RAM; the common (fits-in-memory) case is collect-then-step, which merely ties PyTorch. Lever #1's real deliverable: promote single-pass fused optimizer-in-backward to the common-case default for unclipped training, add opt-in fast-clip for single-pass clipped, and prove a handy PyTorch CPU win on per-step time / peak RSS / allocation via --trainbench (default-on flip gated on that proof). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * docs(#1662): implementation plan — levers #4, #3, #1 (Tensors-first, PyTorch proof) Phased TDD plan: Tensors #4 (trainbench probe + alloc audit + arena guard + fused-norm overload) -> Tensors #3 (tiled FlashAttention backward, parity-gated) -> v0.102.0 release -> AiDotNet #1 (full-precision streaming optimizer, engage single-pass fused-in-backward as the unclipped common-case default, opt-in fast-clip, PyTorch CPU comparison proving a handy win on time/alloc/RSS). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * feat(#1662): full-precision streaming Adam (bit-identical fused-in-backward foundation) Adds FullPrecisionStreamingOptimizer<T> (per-tensor double[] moments, one global step counter, no quantization, no update clamping) + FullPrecisionStreamingAdam<T> matching the classic AdamOptimizer formula exactly. This is the bit-identical streaming optimizer that lets single-pass fused optimizer-in-backward be the common-case DEFAULT for models that fit in memory (vs the 8-bit OOM-survival variants whose ClampUpdate + block quantization diverge from classic Adam). Compiles clean; not yet wired into GetOrCreateStreamingOptimizer (next commit) — the bit-identical gate test lands with the wiring. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * feat(#1662): wire full-precision fused optimizer-in-backward (unclipped) + gate test Resolver gains SupportsFullPrecision (Adam-family today) + a full-precision branch; TrainWithTapeStreaming uses the bit-identical FullPrecisionStreamingAdam when the user opts in (ForceOn) for an unclipped Adam model. Auto default unchanged (the unclipped-fitting default-on flip is gated on the §5d PyTorch proof). Gate test FusedInBackward_Unclipped_MatchesClassicAdam_ToFloatPrecision: fused single-pass optimizer-in-backward tracks classic eager collect-then-step Adam to float precision (1e-3 over 20 steps, identical init). PASSES. Surfaced (pre-existing, separate): MaxGradNorm defaults to 1.0, so the COMMON case is clipped -> the two-pass clipped streaming path, which currently throws "persistent tape activations released" (sets Persistent=true but not ReleaseStreamingActivations=false). Fix tracked next. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * fix(#1662): clipped two-pass streaming path + bit-identical full-precision for clipped Fixes the pre-existing clipped-streaming crash: the two-pass path builds a Persistent tape but ComputeGradientsStreaming releases activations by default (process-global GradientTape<T>.ReleaseStreamingActivations), so pass 2 threw "activations released". Now save/restore that flag (false during the clipped passes) so both passes share the recorded graph; the setting never leaks. Also drops the !clip restriction on full-precision selection: clipping only chooses single-pass vs two-pass, not precision, so the clipped (common-case, MaxGradNorm=1.0 default) path now uses the bit-identical FullPrecisionStreamingAdam too. PyTorch's apply_optimizer_in_backward does not support clipping at all. Gate: FusedInBackward_Clipped_MatchesClassicAdam_ToFloatPrecision (+ the unclipped one) both PASS — fused optimizer-in-backward tracks classic clip-then-step Adam to float precision over 20 steps. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * test(#1662): pytorch cpu comparison - honest result (we do not beat torch on speed) Adds benchmarks/trainbench_torch.py (mirrors the --trainbench residual-FFN shape) and the head-to-head results. Finding on S=128/D=384/10-layer/8-thread: torch: median 69.6 ms/step, peak RSS 312 MB aidotnet: median 252.9 ms/step (3.6x SLOWER), peak WS 489 MB, alloc 0.127 MB/step Identical loss. The optimizer-in-backward + arena wins ONLY on allocation/GC churn (near-zero), not throughput or peak RSS. The 3.6x per-step gap is GEMM + autodiff overhead (#653 core CPU-parity), orthogonal to optimizer-in-backward. Consequence: the §5a default-on flip is NOT justified (no speed win) — fused optimizer-in-backward stays opt-in (ForceOn), valued for bounded peak-grad memory + zero GC churn (bit-identical) + clipped support PyTorch lacks. "Beat PyTorch on all metrics" is not currently true and must not be claimed. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * docs(#1662): gap investigation — per-step deficit is small-M GEMM parallel scaling Decomposed the 3.6x per-step gap by batch: S=128 -> 3.6x, S=1024 -> 1.43x. The gap collapses with larger M, matching the known #475 finding (managed microkernel is MKL-parity, but small-M GEMM parallel scaling plateaus ~2x). So the speed deficit is the #653 core CPU-parity problem (small-M GEMM dispatch/scaling), orthogonal to optimizer-in-backward, which can't change matmul cost. Lever #1's real win is the bit-identical allocation/peak-grad-memory reduction, which holds at all batch sizes. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> * feat(#1662): opt-in single-pass fast approximate grad-clip (§5c) Adds FastApproxGradClip (default OFF). When on + clipping active, the streaming clipped path runs as a SINGLE backward pass: the clip scale comes from an EMA of the previous step's global grad-norm (NFNet-style adaptive clipping), and this step's exact norm is accumulated in the same pass to update the EMA. First step seeds the EMA without clipping. NOT bit-identical (documented approximation) — this is the clipped path that beats PyTorch on backward-pass count (torch's apply_optimizer_in_backward cannot clip at all). Branch restructured into unclipped single-pass / fast-clip single-pass / exact two-pass; persistent tape + ReleaseStreamingActivations toggle now gated on the genuine two-pass only. Convergence test (loss decreases, stays finite) PASSES; 14/14 fused+streaming tests green. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com> --------- Co-authored-by: franklinic <franklin@ivorycloud.com> Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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benchmarks/trainbench_torch.py

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#!/usr/bin/env python3
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"""
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#1662 lever #1 (§5d) PyTorch CPU baseline for the fused optimizer-in-backward proof.
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Mirrors ConvParallelProbe's `--trainbench` shape EXACTLY (residual-FFN MLP stack:
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per layer h = h + W2( gelu( W1 h ) ), scalar loss = sum(h*h), SGD) so the two
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runtimes can be diffed apples-to-apples on the metrics that are comparable across a
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managed (.NET) and a native (libtorch) runtime: per-step wall time and peak process
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RSS. (Per-step *managed* allocation is not comparable — torch allocates in C++ — so
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it is not reported here; the AiDotNet side reports it separately as the GC-churn win.)
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PyTorch does the classic collect-then-step: loss.backward() materializes the full
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gradient set, then a separate SGD sweep updates every parameter. AiDotNet's streaming
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path applies the optimizer to each gradient the moment it is produced and frees it
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(optimizer-in-backward), so the comparison is exactly the architecture difference.
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Usage:
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python trainbench_torch.py --s 128 --d 384 --layers 10 --reps 20 --threads 8
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"""
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import argparse
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import statistics
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import threading
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import time
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import psutil
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import torch
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import torch.nn.functional as F
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def main():
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ap = argparse.ArgumentParser()
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ap.add_argument("--s", type=int, default=128) # sequence length (rows)
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ap.add_argument("--d", type=int, default=384) # model dim
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ap.add_argument("--layers", type=int, default=10)
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ap.add_argument("--reps", type=int, default=20)
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ap.add_argument("--warmup", type=int, default=5)
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ap.add_argument("--threads", type=int, default=psutil.cpu_count(logical=True))
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ap.add_argument("--lr", type=float, default=1e-3)
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args = ap.parse_args()
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torch.set_num_threads(args.threads)
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torch.manual_seed(0)
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S, D, L = args.s, args.d, args.layers
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x = (torch.rand(S, D) - 0.5) # fixed input, no grad
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w1 = [((torch.rand(D, 4 * D) - 0.5) * 0.02).requires_grad_(True) for _ in range(L)]
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w2 = [((torch.rand(4 * D, D) - 0.5) * 0.02).requires_grad_(True) for _ in range(L)]
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params = w1 + w2
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def step():
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for p in params:
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p.grad = None
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h = x
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for l in range(L):
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f = h @ w1[l]
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f = F.gelu(f)
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f = f @ w2[l]
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h = h + f
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loss = (h * h).sum()
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loss.backward()
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with torch.no_grad():
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for p in params: # classic collect-then-step SGD sweep
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p -= args.lr * p.grad
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return float(loss.detach())
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for _ in range(args.warmup):
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step()
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proc = psutil.Process()
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peak_rss = proc.memory_info().rss
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stop = False
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def sampler():
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nonlocal peak_rss
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while not stop:
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rss = proc.memory_info().rss
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if rss > peak_rss:
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peak_rss = rss
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time.sleep(0.002)
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t = threading.Thread(target=sampler, daemon=True)
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t.start()
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times = []
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last_loss = 0.0
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for _ in range(args.reps):
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t0 = time.perf_counter()
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last_loss = step()
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times.append((time.perf_counter() - t0) * 1000.0)
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stop = True
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t.join()
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times.sort()
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print(
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f"TRAINBENCH engine=torch block=mlp S={S} D={D} layers={L} threads={args.threads} "
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f"median_ms={times[len(times)//2]:.3f} min_ms={times[0]:.3f} "
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f"peak_rss_mb={peak_rss/(1024*1024):.1f} last_loss={last_loss:.3e}"
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)
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if __name__ == "__main__":
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main()

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