inductive-mlxrl 0.1.1

Small single-process RL post-training for LLMs on Apple Silicon with MLX.

mlxrl

Fast on-policy MLX RL for Apple Silicon; not a general RL framework, not preference tuning, and not distributed training.

mlxrl is a small, single-process RL post-training library for LLMs on Apple Silicon. It is built around one idea: GRPO on MLX should be a fast batched rollout path with a thin loss and optimizer step on top, not a framework.

The current implementation targets QLoRA GRPO on local 4-bit MLX models. It reuses mlx-lm model loading, LoRA layers, KV caches, and sampling utilities, and keeps generation and training in one Python process with one model object.

mlxrl is pre-1.0. The correctness gates are stable, but import APIs and config fields may change before a 1.0 release.

Quickstart

git clone https://github.com/inductiveML/mlxrl.git
cd mlxrl
UV_CACHE_DIR=.uv-cache uv sync --all-groups
UV_CACHE_DIR=.uv-cache uv run mlxrl train \
  --config examples/qwen3_0_6b_grpo.toml \
  --available-memory-gb 48

For the measured 9B-on-48GB shape, use the checkpointed G=2 config:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
  --config examples/qwen35_9b_g2_checkpoint.toml \
  --available-memory-gb 48 \
  --dry-run

What Works

• Batched group rollouts with MLX-LM KV caches and sampling.
• Full-forward old-policy logprob recompute for training-time pi_old.
• Adapter-disabled reference policy on the same model object.
• GRPO, Dr. GRPO, DAPO, and GSPO loss variants.
• RLOO (REINFORCE Leave-One-Out) as a critic-free rollout objective.
• QLoRA injection on dense and heterogeneous/hybrid layer stacks.
• Qwen3.5-style hybrid support via MLX-LM auto LoRA targeting, including DeltaNet linear_attn.in_proj_* and dense attention q/k/v/o_proj.
• Per-layer gradient checkpointing through mlx_lm.tuner.trainer.grad_checkpoint for linear-attention/DeltaNet backward memory.
• Micro-batched gradient accumulation for token-mean policy losses.
• beta == 0 reference-forward skip.
• Phase 4 benchmark harness for mlxrl, mlx-tune, mlx-lm-lora, and mlx-lm.

Install

Install the PyPI distribution:

pip install inductive-mlxrl

The Python import package and CLI command are still mlxrl:

mlxrl --help

Source installs are also supported:

UV_CACHE_DIR=.uv-cache uv sync --all-groups

Run commands through the local environment:

UV_CACHE_DIR=.uv-cache uv run mlxrl --help

Python 3.11 through 3.14 is supported. Runtime dependencies are intentionally small: mlx and mlx-lm on macOS arm64, matching the MLX wheel support window. Development dependencies include pytest, ruff, pyright, mlx-tune, and mlx-lm-lora for comparison benchmarks. The PyPI distribution name is inductive-mlxrl; the import package and console script remain mlxrl.

Quick Smoke Tests

Dense Qwen3 0.6B:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase0-smoke \
  --model mlx-community/Qwen3-0.6B-4bit \
  --prompt "What is 2+2?"

Hybrid Qwen3.5 9B with rank-16 LoRA:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase0-smoke \
  --model mlx-community/Qwen3.5-9B-MLX-4bit \
  --rank 16 \
  --scale 2.0 \
  --prompt "What is 2+2?"

The smoke gate prints the model id, layer count, LoRA target keys, per-layer LoRA module counts, total/trainable parameter counts, and logits shape. It fails if any trainable leaf is not lora_a or lora_b.

Training Commands

Toy hand-computed GRPO math gate:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase1-toy-gate

Small built-in GSM8K-style run:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase1-gsm8k \
  --model mlx-community/Qwen3-0.6B-4bit \
  --steps 20 \
  --group-size 4 \
  --max-tokens 64

Config-driven run:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
  --config examples/qwen3_0_6b_grpo.toml \
  --available-memory-gb 48

The config schema validates model id, quant bits, group size, completion/prompt lengths, checkpointing granularity, iogpu.wired_limit_mb, optimizer settings, algorithm hyperparameters, KL beta, and seed before a model is loaded. CLI overrides such as --steps, --group-size, --max-tokens, --algorithm, --beta, and --seed apply on top of the file.

For DeltaNet / linear-attention models, enable per-layer checkpointing:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase1-gsm8k \
  --model mlx-community/Qwen3.5-9B-MLX-4bit \
  --rank 16 \
  --scale 2.0 \
  --checkpoint-completion-forward \
  --steps 1 \
  --group-size 2 \
  --max-tokens 256

Despite the historical CLI name, --checkpoint-completion-forward now enables per-transformer-block checkpointing at model setup. The old whole-model mx.checkpoint(...) wrapper was removed because it does not cap DeltaNet's per-layer scan memory.

Phase 2 rollout equivalence check:

UV_CACHE_DIR=.uv-cache uv run mlxrl phase2-equivalence \
  --model mlx-community/Qwen3-0.6B-4bit \
  --group-size 4 \
  --max-tokens 32 \
  --compile-decode-step \
  --batch-groups

Import API

Minimal model setup:

from mlxrl.policy import LoRAConfig, load_policy_with_lora

model, tokenizer, report = load_policy_with_lora(
    model_id="mlx-community/Qwen3.5-9B-MLX-4bit",
    config=LoRAConfig(
        rank=16,
        scale=2.0,
        dropout=0.0,
        grad_checkpoint=True,
    ),
)

One optimizer step:

import mlx.optimizers as optim

from mlxrl.algo import GRPOAlgorithm
from mlxrl.train import batch_from_rollouts, optimizer_step

optimizer = optim.Adam(learning_rate=1e-5)
algorithm = GRPOAlgorithm()
batch = batch_from_rollouts(
    model=model,
    completions=completions,
    rewards=rewards,
    group_size=4,
    pad_token_id=pad_token_id,
    algorithm=algorithm,
    compute_reference=beta != 0.0,
)
metrics = optimizer_step(
    model=model,
    optimizer=optimizer,
    batch=batch,
    beta=beta,
    pad_token_id=pad_token_id,
    algorithm=algorithm,
    use_checkpoint=True,
    micro_batch_size=2,
)

micro_batch_size=0 keeps the original whole-batch path. Micro-batching is currently exact for token-mean policy losses: base GRPO, DAPO, GSPO token mode, RLOO, and Dr. GRPO with loss_reduction="token_mean". Sequence-reduced losses should keep micro_batch_size=0.

Policy Semantics

• The base model is frozen before LoRA injection.
• Only LoRA adapter leaves are trainable.
• Reference logprobs are computed by temporarily disabling adapters on the same model object; there is no second reference model in memory.
• Old-policy logprobs are recomputed with a full forward for the training batch. Rollout-time logprobs are captured for inspection, but 4-bit sequential decode and full-forward prefill are not numerically identical on hybrid/quantized models, so recompute remains the default training semantics.
• When beta == 0, the reference forward is skipped and the policy logprobs are used as a zero-KL placeholder.
• PPO, DPO, and ORPO are intentionally out of scope. PPO needs a separate critic and value forward; DPO/ORPO are offline preference objectives with no rollout phase. mlxrl is critic-free, on-policy, and rollout-based by design.

Algorithms

Concrete algorithms implement the small Algorithm protocol: compute advantages, optionally filter a prepared batch, then compute a loss from policy, old-policy, and reference logprobs. rollout/, policy/, and train/ do not import concrete algorithm implementations.

algorithm	defining behavior
GRPO	group-normalized rewards, token-level importance ratio
Dr. GRPO	centered or normalized rewards with decoupled length reduction
DAPO	asymmetric low/high clipping plus optional dynamic zero-advantage group filtering
GSPO	sequence-level, length-normalized importance ratio and clipping
RLOO	leave-one-out group baseline, no critic, no std-normalized advantage

Memory Preflight

mlxrl train can estimate memory before loading the model:

UV_CACHE_DIR=.uv-cache uv run mlxrl train \
  --config examples/qwen3_0_6b_grpo.toml \
  --available-memory-gb 48 \
  --dry-run

The estimator is calibrated to measured anchors: 6.245 GB for Qwen3-0.6B/G4/prompt≈19/T256, 25.9 GB for Qwen3.5-9B/G2/seq609/per-layer-checkpointed, 45.9 GB for Qwen3.5-9B/G4/seq609/per-layer-checkpointed, and 36 GB for Qwen3.5-9B/G2/seq128/no-checkpoint. For hybrid 9B no-checkpoint long-sequence configs, it reports an OOM-risk lower bound rather than a fake precise peak. For an obviously too-large Qwen3.5-9B/G8/prompt97/T512/no-checkpoint config on 48 GB, it flags the run and suggests the measured-boundary fallback around G4/T512/checkpointed.

Benchmarks

Local M4 Max Phase 4 snapshot:

• 454 rollout tok/s on Qwen3-0.6B GRPO with G=4 and 256-token completions.
• 0.283 end-to-end it/s with full mlxrl training semantics.
• 3.2x faster rollout and 2.2x higher end-to-end it/s than mlx-tune v0.5.1 on the same run shape.
• 1.3x faster rollout than sequential mlx-lm generation at G=4.

These are the two-pass means from benchmarks/results/gate5_full_reconciled.md, run with MLX 0.31.2, MLX-LM 0.31.3, mlx-community/Qwen3-0.6B-4bit, 100 measured steps with 5 warmup steps discarded:

target	comparison	rollout tok/s	grad s/step	samples/s	it/s	peak GB
mlxrl	apples-to-apples GRPO	454.1	1.282	1.133	0.283	6.25
mlx-lm	generation-only, G=1	347.0	-	1.355	-	0.52
mlx-lm-g4	generation-only, sequential G=4	349.7	-	1.366	-	0.52
mlx-tune	package-speed reference	142.2	0.502	0.519	0.130	6.16
mlx-lm-lora	package-speed reference	557.9	0.592	1.648	0.412	5.32

mlx-lm-lora reports higher raw package-speed throughput in this snapshot, but its benchmarked path is not the same training problem as mlxrl's live old-policy/reference semantics and completion-loss masking. That is the honest case where mlxrl is not faster; the apples-to-apples comparison label is reserved for mlxrl's own semantic path. On the 9B Noether real workload, the checkpointed MLX path measured about 6x faster than the previous torch-MPS path; that workload is separate from the public Phase 4 package-speed harness.

Run the Phase 4 harness:

UV_CACHE_DIR=.uv-cache uv run python benchmarks/run_phase4.py run \
  --targets mlxrl,mlx-lm,mlx-tune,mlx-lm-lora \
  --model mlx-community/Qwen3-0.6B-4bit \
  --steps 100 \
  --warmup-steps 5 \
  --group-size 4 \
  --max-tokens 256 \
  --passes 2 \
  --output benchmarks/results/phase4.jsonl \
  --summary benchmarks/results/phase4.md \
  --allow-missing-baselines

The harness reports synchronized rollout tok/s, gradient seconds per step, samples/s, it/s, and peak MLX memory. mlx-lm targets are generation-only; external package targets are useful speed references but may not match mlxrl training semantics.

Development

See CONTRIBUTING.md and DESIGN.md before adding algorithms or changing rollout/logprob semantics.

Run the quality gates:

UV_CACHE_DIR=.uv-cache uv run pytest
UV_CACHE_DIR=.uv-cache uv run ruff check .
UV_CACHE_DIR=.uv-cache uv run pyright

MLX lazy evaluation matters. Any mx.eval(...) or mx.synchronize() in this repo should mark a real boundary: sampled token append/EOS checks, logprob freezing before adapter mutation, per-micro-batch graph release, optimizer updates, or benchmark timing boundaries.

Layout

mlxrl/
  rollout/   # batched group generation
  policy/    # model loading, LoRA setup, logprob passes
  algo/      # GRPO-family advantages and losses
  train/     # value_and_grad and optimizer integration
  data/      # toy GSM8K data and rewards
  cli.py
tests/
benchmarks/

Non-Goals

• No inference server or second model copy.
• No CUDA or torch fallback.
• No distributed training.
• No broad RL framework abstractions beyond the small algorithm interface.