coorl-lost-cities/docs/performance.md

Deep CFR Performance Notes

This document tracks current runtime bottlenecks for the active Deep CFR training path. The numbers below are observational, not a benchmark contract.

Current Default Runtime

Source run:

runs/tmp/2026-05-07_171535_deep-cfr-default/metrics.jsonl

The run used configs/deep_cfr/default.yaml with CUDA enabled. At the time of inspection, completed metrics covered iterations 70 through 95. The training process was still running, so later rows may differ.

Non-evaluation iterations averaged:

Metric	Mean	Share
iteration_seconds	17.85s	100%
traversal_seconds	10.71s	60%
advantage_train_seconds	4.72s	26%
strategy_train_seconds	2.39s	13%
memory_add_seconds	1.25s	7%
checkpoint_seconds	0.02s	<1%

batch_tensor_seconds averaged 4.60s. This is not an additional phase; it is included inside the advantage and strategy training timers. It measures the cost of building per-step tensors from sampled replay entries, including np.stack and transfer to the trainer device.

One evaluation iteration was present in the inspected window:

Iteration	iteration_seconds	evaluation_seconds
75	40.30s	18.29s

With the default evaluation.eval_every: 25, an 18.29s evaluation amortizes to about 0.73s per training iteration. This makes evaluation noticeable in logs but not the primary long-run wall-clock bottleneck.

If the same 100-game evaluation is run more often, the amortized cost becomes:

evaluation.eval_every	Amortized eval cost
25	0.73s/iteration
10	1.83s/iteration
5	3.66s/iteration

If evaluation.games increases from 100 to 1000, evaluation cost should be expected to grow roughly linearly unless fixed overhead or batching effects dominate. Using the observed 18.29s eval as a rough base, a 1000-game eval would be about 183s:

evaluation.games	evaluation.eval_every	Rough amortized eval cost
1000	25	7.3s/iteration
1000	10	18.3s/iteration
1000	5	36.6s/iteration

At that point evaluation becomes a first-order wall-clock concern.

Current Bottleneck Shape

Default training is split between CPU-heavy traversal and CUDA-backed network training:

• Traversal is the largest measured phase at roughly 60% of non-eval iteration time.
• Advantage and strategy training together are roughly 40% of non-eval iteration time.
• Tensor materialization is a major part of training time, so reducing pure GPU compute alone cannot remove all of the training cost.

The default traversal settings are:

traversal:
  traversals_per_player: 280
  num_workers: 8
  worker_chunk_size: 8

This gives 560 traversals per iteration, split into 70 worker batches.

Device Use

The trainer constructs the advantage and strategy networks on run.device. configs/deep_cfr/default.yaml sets:

run:
  device: cuda
  use_amp: false

Traversal workers currently reconstruct networks on CPU:

device = torch.device("cpu")

So, in the default multiprocessing traversal path, setting run.device: cuda accelerates the trainer's optimization steps and trainer-device evaluation, but does not move traversal worker inference to GPU.

On ROCm systems, AMD GPUs may still appear through PyTorch's cuda device API if a compatible ROCm build is installed. The project does not have a separate AMD-specific device path.

AMP Status

run.use_amp exists in configuration, but automatic mixed precision is not currently wired into the training loop. There are no active autocast or GradScaler calls in the Deep CFR trainer.

Practical implication: setting

--set run.use_amp=true

should be treated as a no-op until the trainer implements AMP explicitly.

If implemented, AMP would mainly target the network optimization phases:

• advantage_train_seconds
• strategy_train_seconds

It would not directly reduce traversal CPU time or replay tensor materialization overhead.

Batching Status

There are two separate meanings of batching in the current codebase.

Implemented:

• Worker batching via traversal.worker_chunk_size.
• Evaluation policy batching via evaluation.batch_size.
• Optimization batching via optimization.advantage_batch_size and optimization.strategy_batch_size.

Not implemented:

• Batched network inference inside traversal.

Traversal policy evaluation currently encodes one state and runs a single-row network call, effectively batch_size == 1, then converts the result back to CPU/Numpy for Cython-side policy logic. This means traversal is not structured to feed many states to the GPU in a single inference call.

Evaluation batching is already implemented. During evaluation, active games that need a policy-network action are grouped into chunks of evaluation.batch_size, then evaluated together on the evaluation device. Default config uses:

evaluation:
  batch_size: 64

This makes evaluation much more suitable for GPU inference optimization than traversal is today. The remaining question is whether policy-network forward time is actually the dominant part of evaluation.

Evaluation Breakdown

The current source emits evaluation metrics as:

eval/<opponent>/<metric>

The inspected run uses an older flattened scheme:

eval_<opponent>_<metric>

For that run, iteration 75 had evaluation_seconds = 18.29s. Evaluation was parallelized by opponent, so per-opponent elapsed_seconds values overlap and must not be summed as wall-clock time. The slow safe-heuristic opponents dominated the eval wall-clock.

Representative per-opponent breakdown:

Opponent	Elapsed	Network	Postprocess	Opponent act
random	0.57s	0.18s	0.25s	0.06s
passive_discard	0.36s	0.13s	0.17s	0.00s
safe_heuristic	14.54s	2.65s	3.34s	7.57s
safe_heuristic_loose	11.20s	2.55s	3.30s	4.63s
safe_heuristic_strict	15.94s	2.51s	3.14s	9.22s
noisy_safe	1.68s	0.40s	0.58s	0.57s

The important read is that safe-heuristic evaluation is not primarily GPU network forward time. opponent_act_seconds and policy post-processing are larger than policy_network_seconds for the slowest opponents.

Useful eval runtime keys to inspect:

eval_<opponent>_elapsed_seconds
eval_<opponent>_policy_network_seconds
eval_<opponent>_policy_encoding_seconds
eval_<opponent>_policy_postprocess_seconds
eval_<opponent>_policy_legal_mask_seconds
eval_<opponent>_opponent_act_seconds
eval_<opponent>_apply_action_seconds
eval_<opponent>_diagnostics_seconds
eval_<opponent>_final_scoring_seconds

For newer runs, replace eval_<opponent>_<metric> with eval/<opponent>/<metric>.

Evaluation Optimization Options

The practical eval tuning levers are:

1. Tune evaluation.batch_size. Try 128 or 256 if GPU memory allows. This helps most when policy_network_seconds is a large fraction of opponent elapsed time.

2. Tune evaluation.num_workers. Multiple workers parallelize opponents, but they can also split GPU work across processes and duplicate model copies. Compare 1, 2, and 4 workers for CUDA eval instead of assuming the largest value is fastest.

3. Split light and full evaluation. A useful schedule would run a small opponent/games set frequently and the full opponent suite less often. The current config has one eval schedule, so this would require a feature change.

4. Reduce frequent opponents. The safe-heuristic opponents dominate wall-clock in the inspected run. For frequent checks, evaluate against one or two representative opponents and run the full suite less often.

5. Add a diagnostics-light mode. Current eval records many action-quality and expedition diagnostics. The measured diagnostics_seconds is small in the inspected run, but a basic win-rate/score-only mode would still make frequent large evals simpler and cheaper.

6. Consider asynchronous evaluation. A separate process can evaluate checkpoints while training continues. This does not reduce total compute, and it can contend for GPU if run on the same device, but it removes eval pauses from the trainer wall-clock.

7. Consider eval-only AMP or compiled inference. This is simpler than training AMP because evaluation has no backward pass. It should be measured against policy_network_seconds; it will not reduce opponent policy time or game-state transition time.

TensorRT Assessment

TensorRT is not an obvious high-priority optimization for the current default training loop.

Reasons:

• The largest phase is traversal, and default multiprocessing traversal runs on CPU workers.
• Traversal network inference is single-state, control-flow-heavy, and crosses between encoded state arrays, PyTorch tensors, and CPU/Numpy outputs.
• TensorRT mainly helps inference, while the CUDA-backed trainer phases are training steps with backward passes and optimizer updates.
• Evaluation can benefit from inference optimization in principle, but default evaluation is only every 25 iterations. Even making evaluation much faster has limited effect on long-run average iteration time.

For evaluation specifically, TensorRT is more plausible because GPU batching is already implemented. It would replace or wrap the strategy-network forward pass with a precompiled inference engine. Its maximum impact is bounded by policy_network_seconds, not by total eval time.

In the inspected eval row, the slow safe-heuristic opponents spent about 2.5-2.6s in policy-network forward but 4.6-9.2s in opponent action selection and about 3.1-3.3s in policy post-processing. That means TensorRT could help eval, especially for larger evaluation.games, but it is not expected to collapse the 18.29s eval to a tiny number by itself.

TensorRT becomes more attractive if:

• evaluation.games is raised substantially, such as 1000 games.
• evaluation.eval_every is reduced to 5 or 10.
• policy_network_seconds / elapsed_seconds rises after batch-size and worker tuning.

TensorRT may also become relevant for traversal after a larger traversal redesign that batches many policy-needed states into GPU inference requests.

Optimization Priorities

Based on the current metrics, the more plausible performance work is:

1. Improve traversal throughput. Tune worker count and chunk size, then profile the Cython traversal hot path.

2. Reduce training tensor materialization cost. batch_tensor_seconds is a large part of train time. More contiguous replay storage or tensor-ready sampled batches may help more than model-kernel tuning alone.

3. Implement and test AMP. This should be gated by run.use_amp and measured against loss stability and wall-clock, since it only targets the optimization phases.

4. Consider torch.compile for the trainer networks. This should be measured separately from traversal because the default training loop has substantial non-kernel overhead.

5. Consider batched traversal inference only as a structural project. This is the path that could make GPU inference accelerators more meaningful, but it requires changing traversal scheduling, not just swapping the network backend.

6. For eval-heavy runs, optimize the safe-heuristic opponents and policy post-processing before assuming TensorRT is the main lever. The inspected eval row shows those costs dominate the slowest opponents.

Experiments

torch.compile on trainer networks (2026-05-07, regression)

Wrapped both advantage networks and the strategy network with torch.compile() at trainer construction time. Implementation also required a _clean_state_dict() helper to strip the _orig_mod. prefix that compiled modules add to state_dict(), plus a _orig_mod-routed path for load_state_dict() so multiprocessing traversal workers and checkpoint restoration could keep using the uncompiled DeepCFRMLP class.

Measurement (8 iterations on default.yaml, eval and checkpoint disabled, iteration 1 dropped as compile warm-up):

	iter mean	1000-iter projection
Baseline (no compile)	17.93s	4.98h
torch.compile on trainer nets	18.79s	5.22h
Effect	+0.86s (+4.8%)	+14 min

Net result: regression. Two reasons:

• Traversal is ~60% of iteration time and runs in CPU multiprocessing workers that reconstruct networks from cleaned state_dicts, so they bypass the compiled wrapper entirely.
• DeepCFRMLP (512-hidden, 3-layer) is small enough that the compiled call dispatch overhead exceeds the kernel-fusion benefit.

Implementation preserved on branch experiments/torch-compile for revisiting if the trainer model grows substantially or after the batched-traversal-inference work in Optimization Priorities #5 lands — that is the change that would put compile on the dominant phase, not just on the trainer's optimization steps. Not enabled on main.

AMP on trainer networks (2026-05-07, regression)

Wrapped the trainer optimization phases with torch.autocast(fp16) and torch.amp.GradScaler: _train_advantage and _train_strategy now run their network forward/backward/optimizer step through the AMP path when run.use_amp=true and the trainer device is CUDA.

Safety mitigations included in the implementation:

• GradScaler.unscale_(optimizer) is called before clip_grad_norm_.
• Non-finite loss guard increments amp/nonfinite_loss_count and skips the bad step instead of applying it.
• Advantage squared loss computes diff.float().square() so the loss reduction is fp32 even when the forward path is autocast to fp16.
• Strategy logits are cast back to fp32 before masked_fill and log_softmax.
• Metrics now expose amp/grad_scale and amp/nonfinite_loss_count.

Measurement used the small smoke.yaml config with synthetic replay-memory samples via:

uv run python scripts/bench_amp_trainer.py \
  --config configs/deep_cfr/smoke.yaml \
  --runs 3 \
  --warmup 1 \
  --device cuda

	mean ms/call	speedup vs fp32
fp32	3.22	1.00×
AMP (fp16)	3.92	0.82×

Net result: regression. This matches the same dispatch-overhead-vs-kernel benefit dynamic as the torch.compile regression above: the current trainer model and smoke workload are too small for AMP's lower-precision kernels to pay back autocast and scaler bookkeeping overhead.

The full default.yaml 100-iteration A/B was intentionally skipped. Given the small-model regression and the matching torch.compile precedent on the same model family, there is no current evidence that spending GPU time on the longer A/B would produce a different decision. The infrastructure is kept merged but default-off: run.use_amp=false remains the default, and re-enabling is a one-field config flip.

Re-measure AMP only after the model grows to at least hidden_size >= 1024 or num_layers >= 6. At that point run both the fast scripts/bench_amp_trainer.py micro-bench and the formal 100-iteration fp32-vs-AMP A/B. If AMP still provides less than 5% speedup at that larger model size, keep it default-off and raise the next re-measure trigger to an even larger model.

GPU forward profiling for batched traversal (2026-05-07, decision support)

To decide whether Optimization Priorities #5 (batched traversal inference) is worth implementing, profiled DeepCFRMLP from default.yaml (input_dim=365, output_dim=22, hidden=512, 3 layers, ReLU) on an RTX 3090 in eval() + inference_mode, with 10-iter warm-up and 1000-iter measurement per batch size. Script: scripts/profile_gpu_forward.py.

Batch size	μs/call	μs/state	Speedup vs bs=1
1	80.07	80.074	1.00×
4	81.20	20.299	3.94×
16	91.30	5.706	14.03×
64	93.61	1.463	54.75×
256	88.34	0.345	232.03×
1024	161.95	0.158	506.30×

Policy-call supply from runs/tmp/2026-05-07_181155_deep-cfr-default/metrics.jsonl: mean traversal/nodes ≈ 205,810 over 280 traversals/player → ~368 policy calls per traversal (rough upper bound on batchable states), ~200k per iteration across 560 traversals.

Verdict: Priority #5 is worth pursuing. Per-state cost drops from 80 μs at bs=1 to 0.34 μs at bs=256 (>230×). The available supply of ~368 states per traversal sits comfortably in the bs=64–256 range where μs/call plateaus near 90 μs. End-to-end gain will be bounded by encoding and worker-GPU coordination overhead, but the GPU forward is not the limiter once batching is in place.

Batched Traversal Inference: Design Decision (2026-05-07)

Three structural options were considered for Priority #5:

• A. Central inference server. Workers stay in multiprocessing and reconstruct nothing on GPU. A separate server process owns the model, batches incoming policy requests across workers, runs GPU forward, returns logits. Worker traversal logic and the Cython recursion are untouched.
• B. Per-worker batching. Each worker interleaves multiple traversals internally to form its own batches. GPU process count = worker count, so model copies and GPU contention scale with workers. Batching efficiency is bounded by per-worker in-flight count.
• C. Single-process vectorized traversal. Drop multiprocessing entirely. Main process runs N traversals lockstep with explicit recursion stacks, forming a natural batch dimension across traversal instances. Existing recursive traversal can be kept and a new traversal/batched.{py,pyx} added as a parallel backend gated by config; existing code is not modified.

Decision: A

Reasons:

• Hardware fit dominates. A central inference server keeps multiprocessing, so all available CPU cores stay productive on game logic. C is single-process, so on a 32-core remote machine with a weak GPU it wastes 31 cores while the weak GPU caps batching gains; A is strictly better there. On a 6-core / RTX 3090 box A and C are competitive but uncertain — C only wins when GPU forward is the dominant share of traversal, and game logic in CFR traversal is not negligible.
• C is not the "ultimate" answer on multi-core machines. A truly maximal design would combine C's batched GPU forward with nogil threaded game logic, which is strictly more complex than C alone. Plain C, by being single-process, gives up CPU parallelism that the existing multiprocessing path already exploits.
• A is mostly additive. New modules: inference_server.py, inference_client.py, shared-memory tensor pool, weight-sync hook. Existing touches are small: worker policy call site (one line), worker spawn/teardown (server start/stop), trainer (periodic weight push). Cython traversal recursion, game engine, replay/training paths are unchanged.
• The hard part is IPC tuning, not code volume. Latency budget vs GPU forward, weight-staleness window, backpressure, and shared-memory tensor layout. Code is small; the design surface is concentrated in one place.

IPC: what crosses the process boundary

Only the encoded policy input and its response cross IPC:

• Forward request: encoded state vector, ~365 floats ≈ 1.5KB.
• Forward response: action logits, ~22 floats ≈ 88 bytes.

Game state, traversal recursion stack, event log, CFR regret/strategy accumulators, and chance-node sampling history all stay inside the worker process. The policy network consumes a flat encoded state (input_dim=365), so the server needs no game-tree context to answer a request.

The replay-buffer write path (workers shipping collected regret/strategy samples to the trainer) is separate, already exists today, and is reflected in memory_add_seconds ≈ 1.25s/iter; A does not add to it.

IPC mechanism: multiprocessing + shared memory

• Big payload (state, logits): shared-memory tensors. Either torch.multiprocessing with tensor.share_memory_() and a pre-allocated buffer pool indexed by slot id, or multiprocessing.shared_memory.SharedMemory with manual slot management. Pickle is bypassed for the data itself.
• Control messages (slot index, request id): small Queue. Pickle still happens here but only for ints/tuples, which is sub-microsecond and negligible against ~90 μs GPU forward.
• The naive path (multiprocessing.Queue(tensor) with default pickle) is the one that is slow and is what causes the "Python IPC is slow" reputation. With shared memory, multiprocessing IPC is effectively on par with thread shared-memory access for tensor traffic.

Why not Cython nogil + threading instead of multiprocessing

Threading would avoid IPC entirely, but it requires the game-engine hot path to be genuinely nogil-clean — no Python objects touched anywhere on the path. Whether the existing Cython traversal qualifies is unknown and likely no: auditing and migrating it to be fully nogil-clean is a substantial, high-risk change to existing code, contradicting A's "mostly additive" property. Additional drawbacks: a single segfault kills all threads; multi-threaded CUDA usage has subtle context-sharing pitfalls; tooling and prior art are weaker than for the multiprocessing pattern. Revisit only after free-threaded Python (PEP 703) stabilizes or if a future profile shows the shared-memory IPC is itself the limiter.

Implementation plan

1. Prototype A on the 6-core / 3090 host with a single worker: validate end-to-end correctness and measure IPC round-trip latency vs GPU forward.
2. Scale to multiple workers; tune batch_window_us, max_batch, and sync_every (weight push frequency).
3. Deploy to the 32-core / weak-GPU remote and confirm CPU-side scaling holds and the weak GPU is still the right place to keep the model.
4. Defer C. Re-evaluate only if A's measurements show GPU forward is no longer on the critical path and game-logic CPU cost dominates — in that case the right next step is C with nogil threading, not plain C.

Post-A Optimization Calculus (forward-looking, 2026-05-07)

Once Option A lands, the bottleneck shape changes. This section records the expected sequencing for follow-up work. It is forward-looking and has not been measured yet — verify against bench numbers after A is benchmarked.

Why compile / TensorRT are negligible today but become meaningful later

Today (small model: 3-layer, 512 hidden):

• torch.compile on the trainer's networks already regressed (see the 2026-05-07 experiment above). The model is too small for kernel fusion to beat compile dispatch overhead.
• torch.compile / TensorRT on the inference-server forward (post-A) would shave ~30–50% off ~90μs/call → ~50–70μs/call. With forward share of an iter reduced to <1% by A's batching, the iter-level multiplier is ~1.00–1.01×. Negligible.

Two compounding shifts can flip this:

1. Larger model. Going from 512 hidden / 3 layers to ~1024 hidden / ~6 layers pushes the forward call out of dispatch-bound territory into kernel-bound territory. Compile fusion and TensorRT both deliver real 1.5–2× on the forward call itself once the kernel is large enough to amortize launch overhead. Forward share of iter time also rebalances upward because per-call time scales with FLOPs while batching gain is fixed.
2. Denser, larger evaluation. Moving toward eval_every: 5 and evaluation.games: 1000 makes evaluation about half of iteration wall-clock (see the amortized eval table earlier in this doc). Eval is pure inference, so TensorRT on the inference-server's forward path applies directly.

When both shifts happen together, an illustrative future iter (rough order of magnitude only):

Configuration	Iter time (rough)
Today (small model, eval_every=25)	17.85s
+ A (batched traversal inference)	~14s
+ larger model (≈4× FLOPs), no compile/TRT	~50s
+ dense eval (eval_every=5, games=1000)	~70s
+ compile (trainer) + TensorRT (inference)	~45s

That last row is where compile/TensorRT contributes ~1.5× iter — the same tooling that is iter-neutral today. The numbers above are illustrative; real ratios depend on model size, kernel autotune outcomes, and the eval-vs-train balance.

Tooling split

• TensorRT: applies only to inference (no backward). Targets:
  • inference-server forward in traversal,
  • inference-server forward in evaluation. Both are served by the same A-era server, so a single TensorRT integration covers both.
• torch.compile: applies to trainer's advantage/strategy training (forward+backward+optimizer). The 2026-05-07 regression on a small model does not generalize — it must be re-measured on whatever larger model config we settle on. Do not conclude "compile is bad" from the small-model data point.

Recommended sequencing

Do this in order. Skipping ahead is the failure mode that creates misleading "compile/TRT didn't help" data.

1. Now: benchmark A (scripts/bench_inference_backend.py) and confirm the local vs server multipliers on home and remote. Validate the iter 1.2–1.3× / traversal 1.5–2× working estimate.
2. Next: experiment with a larger network config. Measure compute vs learning-curve trade-off with the existing toolchain (no compile/TRT yet). This step decides the model size that future optimizations target. It is also the prerequisite for revisiting AMP, torch.compile, and TensorRT: all three are dispatch-overhead-bound on the current small model.
3. Then: re-measure torch.compile on the trainer at the chosen model size. The earlier regression was size-bound; expect a different result.
4. Then: integrate TensorRT into the inference server (covers traversal and eval forward simultaneously). Bound the gain by the post-step-2 policy_network_seconds share, not the headline TensorRT speedup.
5. In parallel with 2–4: if denser eval is operationally useful, raise evaluation.games and lower evaluation.eval_every. This step does not require code changes but sharply increases the value of step 4.

Out of scope until A bench numbers are in: Option C, nogil threading, async inference client, compiled encoding.

Option A Bench Result and Structural Ceiling (2026-05-07)

Option A (traversal.inference_backend: server) was implemented and benchmarked. Result: regression. A is deferred. default.yaml stays on local. The implementation is preserved behind the flag for future revisit.

Bench numbers

scripts/bench_inference_backend.py --device cuda --iterations 5 --warmup 1, RTX 3090, after a per-call IPC fix (replaced multiprocessing.Manager() queues/events with spawn-context primitives, slot reuse per worker batch, shared memory confirmed in use for state/response payloads).

Backend	iter	traversal	adv_train	strat_train	mem_add	batch_tensor
local	16.75s	10.81s	3.91s	2.02s	0.95s	3.56s
server	57.61s	51.61s	3.96s	2.02s	0.50s	3.57s
Speedup	0.29×	0.21×	0.99×	1.00×	1.89×	1.00×

Raw: runs/bench/2026-05-07_193335_inference_backend/results.json.

Training and eval phases are unchanged (as expected — A only touches the traversal forward path). The regression is contained in traversal_seconds, which is ~5× worse.

Diagnosis

The server emits per-flush batch stats. Mean batch size: ~7.2–7.9, max 8. This is the structural ceiling, not a tunable misconfiguration:

• Traversal recursion is sync-blocking at the policy call site. Each worker has at most one in-flight policy request at a time.
• In-flight requests at the server ≤ num_workers = 8.
• The server further splits each batch by (network_kind, network_index), so the actual GPU forward group size is roughly half of that — about 4 rows per group.

Per-state cost at this realized batch size, from the GPU profile table:

Realized batch	μs/state
1	80.07
4	20.30
8 (extrapolated)	~12
64	1.46
256	0.34

So the GPU is doing ~12μs per state instead of the projected ~1.5μs at bs=64. The IPC round-trip per call (queue post + server scheduler + event wakeup, even with shared-memory payload) is on the order of hundreds of μs per call, which exceeds both the local CPU forward (~80–200μs at bs=1 on this small MLP) and the marginal GPU gain. Net: per-call cost roughly doubles or triples, compounded across ~205k calls/iter, gives the observed 5× traversal regression.

batch_window_us and max_batch tuning cannot escape this ceiling — there are simply not 64 concurrent in-flight requests to coalesce when only 8 workers are blocking-sync.

What this means for the headline GPU profile (scripts/profile_gpu_forward.py)

The earlier "230× speedup at bs=256" is a per-state GPU forward microbenchmark, not an end-to-end traversal speedup. Realizing that gain requires actually feeding the GPU with bs=64+ batches. Sync-blocking multi-worker traversal cannot do that. Reaching the bs=64 regime needs either:

• Per-worker traversal interleaving (worker advances worker_chunk_size traversals concurrently, suspending at each policy call — Option B shape), which requires turning Cython traversal recursion into a resumable state machine. Same scope as a partial Option C, localized to worker scope.
• Option C proper (single-process vectorized traversal).

Both require restructuring traversal. Option A's "additive, no traversal changes" property turned out to also mean "cannot drive the batch size up."

Clarifying the traversal bottleneck: sync policy boundary, not SIMD

The tempting shorthand is "Python/GIL prevents traversal from using SIMD or threads." The more precise diagnosis is narrower:

• Lost Cities game mechanics are already mostly Cython C-level operations. legal_actions, action push/pop, and cached scoring are not Python list walks on the hot path.
• The traversal recursion is Cython, but it synchronously crosses back into Python/PyTorch at every policy-needed state: encode a single info state, run one-row PyTorch forward, copy logits back to CPU/Numpy, then continue recursion.
• This boundary makes every traversal worker sync-blocking. With num_workers=8, the inference server can see at most eight in-flight requests before per-network splitting, no matter how large max_batch is.
• GIL-free threading would help only after the same path is made nogil-clean or after traversal is restructured so policy calls can be batched. Simply "using SIMD" does not address the one-row policy boundary.

So the actionable bottleneck is policy-call scheduling shape, not scalar game-rule arithmetic. The highest-leverage experiment is Option B: per-worker interleaved traversal, where one worker advances many traversals, suspends each at a policy request, batches those requests, and resumes the corresponding continuations.

Microbench evidence (2026-05-07, configs/deep_cfr/default.yaml, experiments/traversal_policy_boundary/bench_policy_boundary.py):

Device	Component	median μs/call	p95 μs/call
CPU	encode + legal	3.10	3.81
CPU	push + pop	0.15	0.22
CPU	policy boundary bs=1	111.50	125.46
CPU	torch forward bs=64	12.84	13.14
CUDA	policy boundary bs=1	181.30	194.77
CUDA	torch forward bs=64	2.55	2.75

This confirms the bottleneck is not Cython game-rule scalar work. The single-request policy boundary is ~36× larger than encode+legal on CPU, while CUDA bs=64 forward is ~71× cheaper than the current CUDA bs=1 boundary.

Expected upside is bounded by the fraction of traversal currently spent at policy calls. Moving realized GPU forward from the current ~4-8 row regime (~12-20μs/state) to bs=64 (~1.46μs/state) is an ~8-14× improvement on the forward component, but not on game recursion, sample creation, or replay writes. For the observed local traversal around 10-13s/iter, a realistic first target is roughly 1.5-3× traversal speedup if Option B reaches the bs=64 regime without adding comparable scheduler overhead. Larger claims need a prototype because traversal has substantial non-forward work.

Why deferring A (not deleting) is the right call

• The plumbing (server process, shared-memory client, weight sync, config flag) is complete and tested. Re-enabling is a config flip.
• The fundamental issue at this model size is that GPU forward time is too small to amortize IPC overhead at any realistic batch size we can drive without restructuring traversal. Bigger model changes that arithmetic; the same plumbing then becomes useful.
• The mem_add_seconds row showed a real 1.89× win, suggesting the shared-memory replay-write path adopted along the way is worth keeping even with local backend. (Confirm separately; this is a side effect.)

Re-enable A when one of these holds

1. Model grows to ~1024 hidden / ~6 layers (compile/TRT discussion above). Forward time scales with FLOPs while IPC overhead is fixed; at some point IPC becomes a small fraction.
2. Per-worker interleaved traversal ships (Option B-shape refactor). Drives realized batch toward 64 and reclaims the profile table's gains.
3. Eval becomes the dominant phase (eval_every: 5, evaluation.games: 1000). Eval is not sync-blocking traversal; it is already batch_size=64 in eval code. The same inference server can serve eval directly without the worker-side ceiling.

Free-threaded Python (3.13t) note

Free-threaded Python + Cython nogil would let many threads (well above core count) run game logic concurrently in one process, with shared memory and no IPC. With ~64 threads sync-blocking on policy calls, the server would naturally see bs=64. In principle this is the cleanest endpoint.

In practice as of early 2026:

• Free-threaded Python is an opt-in build (python3.13t), still experimental, with measurable single-thread overhead.
• PyTorch's free-threaded compatibility is partial.
• Cython nogil-cleanliness audit on the existing game engine is still required and was the original reason nogil threading was deferred in the design decision above.
• No project-level adoption pressure on python3.13t today.

So free-threaded Python does change the architectural answer, but it does not unblock A now. Track the ecosystem; revisit when (a) python3.13t becomes mainstream or (b) the Cython engine is nogil-cleaned for other reasons.