Debugging

Split a misbehaving run into rollout and training halves, then debug the side that is actually wrong.

When a Miles run misbehaves, the first question is always: rollout or training? The flags on this page exist to answer it by cutting the loop into pieces you can run one at a time. Once you know which side is wrong, it becomes an ordinary debugging session.

Cut the loop in half

Flag	What it does
--debug-rollout-only	Run generation only. The training backend (Megatron or FSDP) is never initialized.
--debug-train-only	Run training only. No SGLang engines are started.
--save-debug-rollout-data <path>	Pickle every rollout to path.format(rollout_id).
--load-debug-rollout-data <path>	Train from those recordings instead of generating. Implies --debug-train-only, since it does not start engines.

The two --debug-*-only flags are mutually exclusive and argument validation rejects setting both.

The pattern worth internalizing:

# 1. capture a few well-formed rollouts, no trainer involved
--debug-rollout-only --save-debug-rollout-data /tmp/rollout_{rollout_id}.pt

# 2. iterate on training with those exact inputs, no engines involved
--load-debug-rollout-data /tmp/rollout_{rollout_id}.pt

Step 2 removes sampling randomness from the loop, which is what makes a bisect or an A/B of a loss change trustworthy.

Replaying without killing the engines

--load-debug-rollout-data gives up the engines, so it cannot tell you whether the weights that reached the engines are right. --ci-inject-rollout-data-path is the variant that keeps them:

--ci-inject-rollout-data-path /tmp/rollout_{rollout_id}.pt \
--ci-inject-rollout-data-start-rollout-id 2 \
--ci-inject-rollout-data-min-match-ratio 0.9

From the start rollout id onwards, generation still runs and its output is still compared, then discarded, and training consumes the recording instead. The comparison is the point: if the mean response-token match ratio between the fresh generation and the recording falls below --ci-inject-rollout-data-min-match-ratio (default 0.9), the engines are holding the wrong weights. Legitimate ulp-level drift only flips the occasional sampled token, so the ratio stays high when the sync is correct.

Cut a single step into pieces

Flag	What it removes
--debug-skip-weight-update	The actor-to-rollout weight update itself, while keeping the offload and onload schedule around it. Separates “the sync is wrong” from “the schedule around the sync is wrong”.
--debug-disable-optimizer	Optimizer and LR-scheduler construction, and the optimizer step. Rollout, log-prob forward and actor forward/backward still run, so this isolates optimizer-state memory and update behavior from the rest.
--debug-exit-after-rollout <n>	Everything after rollout n. Built for exercising checkpoint resume with consistent scheduler state.

Make two runs comparable

--debug-deterministic-collective runs the training world on the det_nccl backend from miles.utils.test_utils.det_process_group, which folds order-sensitive SUM and AVG reductions in a fixed tree order. That is what makes two different reduction topologies bitwise comparable, and it is how the Megatron-versus-FSDP alignment test can assert equality at all. It is slow; never enable it in production.

For end-to-end reproducibility of a whole run, including the sampling path, see the Reproducibility recipe.

The assertion harness CI uses

An e2e test in Miles is mostly a normal training run with --ci-test added. That flag turns on a set of in-process assertions, so a violated invariant fails the run where it happens instead of showing up as a wrong number hours later. You can use the same flags on your own runs, and each checker has an escape hatch for the case where your change makes it legitimately not hold.

Checker	What it asserts	Turn off with
KL	At step 0, abs(train/ppo_kl) < 1e-9 and train/pg_clipfrac < 1e-10, and train/kl_loss is zero. MLA relaxes to 1e-8; LoRA relaxes because the Megatron-to-HF adapter conversion is not bit-exact	--ci-disable-kl-checker
Log-probs	At rollout 0, trainer rollout/log_probs matches rollout/ref_log_probs within 1e-8 (5e-3 under R3, whose reference does not replay routing); trainer versus engine log-probs within 0.03; rollout entropy in (0, 0.7). Under --true-on-policy-mode the two must be exactly equal	--ci-disable-logprobs-checker
Weight update	Sets check_weight_update_equal, comparing trainer and engine weights after a sync. Skipped automatically under either --debug-*-only	--ci-disable-weight-update-checker

Three more take values rather than switching off:

Accuracy gate. --ci-metric-checker-key <key> --ci-metric-checker-threshold <x> checks eval metrics when the checker is disposed at the end of the run. By default, at least one eval must report key >= x. Set --ci-metric-checker-expect-num <n> to require exactly n eval checks and require all of them to succeed. A run with no eval always fails.

Gradient-norm comparison. --ci-save-grad-norm <path> writes the grad norm, and --ci-load-grad-norm <path> asserts a later run matches within rel_tol=abs_tol=0.03. Both accept {role}, {rollout_id} and {step_id} in the path, and only rank 0 participates. This is how the Megatron-versus-FSDP alignment test compares two backends that cannot share a process.

Per-layer weight hashes. --ci-save-model-hash and --ci-check-model-hash compute SHA256 over each layer’s parameter bytes, including name, shape and dtype, and write one JSON per rank under iter_<iteration>/model_hash_tp*_pp*_dp*_cp*.json. Layer granularity is deliberate: a mismatch names the layer instead of just saying the model differs.

Fault injection. --ci-ft-test-actions takes a JSON array of actions such as {"at_rollout": 3, "action": "stop_cell_at_end", "cell_id": "trainer-actor-0"}, with stop_cell_at_end, start_cell_at_end and crash_before_allreduce available and cell_id naming the target cell. It is how the fault-tolerance suite kills things on purpose. See Fault Tolerance.

Aligning precision

The most common class of bug. Walk these checks before anything else.

Is the first rollout coherent?

If the very first rollout is gibberish:

• Parameters didn’t load. Megatron logs a clear load line; if it is absent, fix --load / --ref-load.
• Parameter mapping is wrong. With pp_size > 1, second-stage layer IDs are a common offset bug. Dump the parameters in the SGLang model’s load_weights and compare against the checkpoint.
• SGLang dropped buffers. Some buffers can be released during the parameter-release path; check they are re-loaded after weight sync.
• Pretrained versus instruct. If the instruct model of the same architecture works, your base model plus chat template combination is wrong.

Are log_probs and ref_log_probs equal at step 1?

They should be, because the actor and the reference are the same weights. This is exactly what the log-probs checker above asserts, so running with --ci-test turns the question into a hard failure. If KL is non-zero:

• Non-deterministic kernels. Some Transformer Engine versions need --attention-backend flash to force deterministic Flash Attention under context parallelism.
• KL below 1e-4. Kernel-level jitter, acceptable.
• KL above 1. A configuration error; re-check parallelism and precision.
• About 0.8 per token on an instruct model. Almost always a chat-template mismatch. Run the chat template verifier.

Is grad_norm reasonable?

Step 1 with num_steps_per_rollout=1 should produce a tiny gradient. If it does not, look at MoE fusion first (--moe-permute-fusion), then at whether the reward is being computed from the right key (a --label-key typo is the usual cause).

Does step 2 OOM under colocate?

The trainer’s offload and reload cycle is colliding with the engine’s static memory. Lower --sglang-mem-fraction-static to 0.7 or 0.6. See Training Backends for the layout and offload knobs.

Common kernel pitfalls

Symptom	Likely culprit
Garbled rollout, parameters loaded fine	Chat-template mismatch, or a dropped buffer in SGLang
KL non-zero at step 1	Non-deterministic fused attention; force --attention-backend flash
MoE training collapses after tens of steps	Routing not preserved; check --use-rollout-routing-replay
Gradient NaN or Inf	Bad chat template, or activation overflow in FP8
illegal memory access in SGLang	OOM in disguise; lower --sglang-mem-fraction-static
JSONDecodeError from inductor	Corrupt compile cache; set TORCHINDUCTOR_FORCE_DISABLE_CACHES=1
NCCL hang during weight sync	Raise the NCCL timeout and re-run with NCCL_DEBUG=INFO

Reading logs

Component	Where
Trainer stdout	Wherever you redirected ray job submit
Ray workers	~/.ray/session_latest/logs/worker-*.{out,err}
SGLang	Inside the Ray worker logs; verbosity via --sglang-log-level
NCCL	NCCL_DEBUG=INFO NCCL_DEBUG_FILE=/tmp/nccl_%h_%p.log

Useful environment variables while debugging:

RAY_DEDUP_LOGS=0                       # every rank's line, not one deduplicated line
NCCL_DEBUG=INFO
NCCL_DEBUG_SUBSYS=COLL,P2P
TORCHINDUCTOR_FORCE_DISABLE_CACHES=1

The per-step metrics are the real signal, and they are the same numbers the checkers above assert on: train/ppo_kl, train/pg_clipfrac, train/kl_loss, rollout/log_probs, rollout/ref_log_probs, rollout/rollout_log_probs, rollout/entropy. Trainer steps log as step <n>: {...}, and the rollout side logs one reduced dict per rollout. See Monitoring and Logging for the full metric surface.

When all else fails

• Drop to a tiny model on a known-good recipe (the Reproducibility one) to separate framework from model.
• git bisect between a known-good commit and HEAD, with the record-and-replay pattern above pinning the inputs and --debug-deterministic-collective pinning the reductions.
• Open a GitHub issue with the launch script, pip freeze, the first 200 lines of trainer stdout, and what you have already ruled out.