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cellm — Mobile-Native LLM Serving Engine

A ground-up LLM inference engine for iOS and Android, written in Rust. Brings server-grade serving concepts — paged KV cache, continuous decode scheduling, multi-session concurrency — to phones running under 512MB RAM.

Not a wrapper around llama.cpp. Not a port of vLLM. A new runtime designed for mobile constraints from scratch.

Note

Status. The CPU and Metal backends, the iOS bindings, and the text models listed under Supported Models are stable and in active use. Metal coverage varies by model; CPU runs everything. The Android AAR and Compose demo build and package the native library, but haven't been validated or tuned on-device. Vulkan sets up the device and pipelines but still runs every op on CPU. The native .cellm VLM path and the Docker image are experimental or not yet wired up; vision via ONNX is stable. See Feature Status.

Inference Demo

Quick Links

Resource Path
Getting Started Quick Start below
Architecture & Design docs/project_architecture.md
Paged KV Cache Deep Dive docs/paged-kv-cache-foundation.md
Benchmarks docs/benchmarks/README.md
Model Conversion docs/convert-quantized-models.md
VLM (Vision) Guide docs/vlm-smolvlm-onnx.md (ONNX path; native .cellm is experimental)
iOS Demo App bindings/ios/CellmDemo
Android Bindings bindings/kotlin (builds; not yet device-validated)
WASM & WebGPU docs/wasm-backend.md
Live WASM Demo cellm-wasm ( (Research Preview)
Git Commit Messages Local LLM commit messages below
Docker (CPU tooling) Docker below (planned, not yet wired up)

Quick Start

Prerequisites

  • Rust 1.75+ (modern stable toolchain)
  • macOS / iOS for Metal acceleration (Linux/Android builds use CPU path)
  • Git LFS (for bundled sample models)

1. Build

cargo build --release

2. Run a smoke test (CPU)

cargo run --release --bin infer -- \
  --model models/smollm2-135m.cellm \
  --tokenizer models/hf/smollm2-135m/tokenizer.json \
  --prompt "Hello, how are you?" \
  --chat \
  --gen 32

3. Run with Metal (macOS/iOS)

cargo run --release --bin infer -- \
  --model models/smollm2-135m-int8.cellm \
  --tokenizer models/hf/smollm2-135m/tokenizer.json \
  --prompt "Hello" \
  --chat \
  --gen 16 \
  --backend metal

Tip: Use --chat for ChatML-style formatting. Without it, many base models behave like text-completion engines and may not answer directly.

4. Metal verification

cargo run --release --bin metal-smoke

Architecture Overview

flowchart LR
    U["User Prompt"] --> API["App/UI Request Layer"]
    API --> ORCH["CPU Orchestrator"]
    ORCH --> TOK[Tokenizer]
    TOK --> FMT["Prompt Formatter"]
    FMT --> SCH["Decode Scheduler / Batcher"]

    SCH -->|"prefill/decode jobs"| ENG["Engine Dispatcher"]
    ENG -->|backend=CPU| CPUPATH["CPU Kernels"]
    ENG -->|backend=Metal| METAL["Metal Kernels"]

    METAL --> MATMUL["QKV / MLP MatMul"]
    METAL --> ATTN["Attention + GroupKV Cache"]
    METAL --> NORM["RMSNorm / RoPE / Logits"]
    MATMUL --> SAMPLER
    ATTN --> SAMPLER
    NORM --> SAMPLER
    CPUPATH --> SAMPLER["Sampler + Stop Rules"]

    SAMPLER --> DETOK[Detokenizer]
    DETOK --> STREAM["Streaming Output"]
    STREAM --> API
    API --> U

    subgraph ModelAssets["Local Model Assets"]
      W[".cellm / .cellmd mmap Weights"]
      T["tokenizer.json + config"]
    end

    W --> ENG
    T --> TOK

    subgraph SessionState["Per-Session State"]
      KV["KV Cache (GroupKV layout)"]
      PT["Page Table / Sequence Cursor"]
      TH["Thermal + QoS Policy"]
    end

    SCH --> KV
    SCH --> PT
    ORCH --> TH
    TH --> SCH
Loading

What Makes cellm Different?

Feature llama.cpp MLX ExecuTorch cellm
Language C++ C++/Python C++ Rust
KV Cache Contiguous Contiguous Contiguous Paged (Block-based)
Focus Portability Apple Native Model Export Mobile Multi-session
Scheduling Static Batch Mostly Single N/A Round-Robin Interleaved
Memory Manual/Static Managed Buffer Static Graph Dynamic Block Allocator

Project Structure

cellm/
├── crates/
│   ├── cellm-core/          # Memory arena, tensor layout, op dispatch
│   ├── cellm-model/         # Model format, configuration, weight management
│   ├── cellm-cache/         # Paged KV cache: BlockAllocator, PageTable, physical storage
│   ├── cellm-kernels/       # CPU, Metal, WASM & WebGPU compute kernels
│   ├── cellm-scheduler/     # Decode scheduler & batching logic
│   ├── cellm-wasm/          # WebAssembly bindings & JavaScript API
│   └── cellm-sdk/           # Public C FFI + high-level API for mobile consumers
├── bindings/
│   ├── ios/CellmDemo/       # SwiftUI demo app (LLM + VLM stub)
│   ├── kotlin/              # Android Kotlin/JNI bindings
│   └── swift/               # Swift Package + XCFramework build scripts
├── tools/
│   ├── infer/               # CLI inference runner (debug & validation)
│   ├── vlm-onnx-infer/      # VLM runner for SmolVLM ONNX exports
│   ├── vlm-smoke/           # SDK FFI VLM smoke test
│   ├── convert/             # HF Safetensors/GGUF/PyTorch -> .cellm converter
│   ├── bench/               # Latency & throughput benchmark harness
│   └── metal-smoke/         # Minimal Metal kernel compile + dispatch test
├── docs/                    # Architecture deep-dives, benchmarks, model guides
└── models/                  # Sample .cellm checkpoints (Git LFS)

Development Commands

Convert a Model

Convert HuggingFace Safetensors or GGUF to .cellm:

cargo run --bin convert -- \
  --input  ./models/hf/smollm2-135m \
  --output ./models/smollm2-135m.cellm \
  --dtype  f16

Quantize during conversion:

cargo run --bin convert -- \
  --input  ./models/hf/smollm2-135m \
  --output ./models/smollm2-135m-int8.cellm \
  --dtype  f16 \
  --quantize-int8-symmetric

See docs/convert-quantized-models.md for GGUF, PyTorch, and 4-bit affine workflows.

Run Benchmarks

# Quick smoke benchmark
cargo run --release --bin bench -- --model tiny

# Full LLM backend matrix (CPU vs Metal)
tools/bench/run_llm_backend_matrix.sh

Detailed benchmark reports live in docs/benchmarks/.

Run VLM (Vision-Language)

# ONNX vision + ONNX decoder (recommended)
cargo build --release -p cellm-vlm-onnx-infer

./target/release/vlm-infer \
  --model-dir models/hf/smolvlm-256m-instruct \
  --onnx-variant fp16 \
  --image models/test_images/rococo.jpg \
  --prompt "Describe this image." \
  --split-image \
  --max-new-tokens 96

Native .cellm vision + decoder is experimental:

./target/release/vlm-infer \
  --model-dir models/hf/smolvlm-256m-instruct \
  --cellm-model models/smolvlm-256m.cellm \
  --vision-backend cellm \
  --decoder-backend cellm \
  --image models/test_images/rococo.jpg \
  --prompt "Describe this image." \
  --max-new-tokens 12

See docs/vlm-smolvlm-onnx.md for full VLM docs.

iOS SwiftUI Demo

Build the XCFramework:

./scripts/build_xcframework.sh

Then open bindings/ios/CellmDemo in Xcode.

Browser / WebAssembly

Build and run the WASM engine with WebGPU acceleration:

# Build the WASM module
./scripts/build-wasm.sh --release

# Serve the demo page
python3 -m http.server 8080 --directory crates/cellm-wasm/www/

Then open http://localhost:8080 and use engine.try_init_webgpu() to enable hardware acceleration.

Docker

cellm's whole point is phone deployment (iOS/Android via Metal/Vulkan), so Docker can't run the actual mobile targets — there's no GPU passthrough for Apple's Metal API in a Linux container. Where Docker does help is the CPU-backend and tooling side of the repo: CI, headless benchmarking, model conversion pipelines, and testing without owning a Mac.

Image Backend Status Use case
cellm:cpu CPU kernels Planned CI, headless benchmarking, model conversion, testing without a Mac
cellm:vulkan Vulkan compute Research Once the Vulkan backend stabilizes (see Feature Status)
cellm:metal Metal Not possible No Metal GPU passthrough on Linux containers — Metal only runs on real macOS/iOS hardware

Unlike llama.cpp's Docker matrix (CUDA/ROCm/Vulkan/SYCL variants for full/light/server), cellm doesn't need a wide backend matrix — this is a mobile-first inference engine, not a server deployment target. A single CPU image covers the actual need for reproducible tooling.

Example Dockerfile shape:

FROM rust:1.75 AS build
WORKDIR /app
COPY . .
RUN cargo build --release --bin infer --bin convert --bin bench

FROM debian:bookworm-slim AS cpu
COPY --from=build /app/target/release/infer /app/target/release/convert /app/target/release/bench /usr/local/bin/
ENTRYPOINT ["infer"]

Usage once built:

docker run -v /path/to/models:/models cellm:cpu \
  --model /models/smollm2-135m.cellm \
  --tokenizer /models/hf/smollm2-135m/tokenizer.json \
  --prompt "Hello" --chat --gen 32

Not wired up yet — tracked as a follow-up. If you want to help, a Dockerfile + GitHub Actions workflow (mirroring .github/workflows/docker.yml-style multi-arch builds) for the cpu variant is the right first PR.

Git Commit Messages

Use cellm's local LLM to generate git commit messages from staged changes. All inference runs locally on CPU — no API keys, no data leaves your machine.

Prerequisites

  • A model converted to .cellm format (e.g., Qwen2.5 0.5B int8)
  • The matching tokenizer.json
  • ./target/release/infer built (cargo build --release)

Quick usage

# Stage your changes first
git add -A

# Generate a commit message from staged diff
./tools/git-cellm-commit.sh

# Or pipe any diff
 git diff HEAD~1 | ./tools/git-cellm-commit.sh

How it works

  1. git diff --cached captures your staged changes
  2. The diff is passed as the prompt to Qwen2.5 0.5B int8
  3. The model generates a commit message + summary locally
  4. Output is printed to stdout — pipe it anywhere

Configuration

Set these environment variables to use a different model:

Env var Default Description
CELLM_COMMIT_MODEL models/to-huggingface/qwen2.5-0.5b-int8-v1/qwen2.5-0.5b-int8-v1.cellm Path to .cellm model
CELLM_COMMIT_TOKENIZER models/to-huggingface/qwen2.5-0.5b-int8-v1/tokenizer.json Path to tokenizer.json
CELLM_COMMIT_INFER ./target/release/infer Path to the infer binary

Example with the LFM model:

CELLM_COMMIT_MODEL=models/LFM2.5-230M-int4-v2.cellm \
CELLM_COMMIT_TOKENIZER=models/to-huggingface/LFM2.5-230M/tokenizer.json \
  ./tools/git-cellm-commit.sh

Supported Models

Model Size Best For Notes
SmolLM2 135M-360M Fast smoke tests, small devices Best LLM starter model
LFM2.5 350M Long-context, efficient inference Linear attention, up to 256K context
Qwen2.5 / Qwen3.0 / Qwen3.5 0.5B-0.8B Multilingual, reasoning DeltaNet layers supported (CPU ref)
Gemma-3 1B Quality vs size tradeoff Metal path active, CPU-safe fallback
Bonsai 1.7B High-quality local chat 1-bit quantized; see docs/bonsai_1bit_analysis.md
Gemma-4 2B-4B Larger mobile workloads Experimental; see docs/gemma4_*
SmolVLM 256M Vision-language (ONNX) Native .cellm VLM path in progress
FunctionGemma 270M Mobile actions / tool use Experimental quality; see docs/function-calling-gemma.md and HF builds

Recommended first download: SmolLM2-135M

Sample checkpoints bundled in this repo (via Git LFS):

  • models/smollm2-135m-int8.cellm
  • models/smolvlm-256m-int8.cellm
  • models/qwen3.5-0.8b-int4-textonly.cellm

Feature Status

  • Paged KV Cache - Fixed-size block allocation with BlockAllocator & PageTable
  • Multi-session Scheduler - Round-robin interleaved decoding
  • 4-bit Affine Dequantization - Native MLX/HF packed weight support
  • Multimodal Vision (ONNX) - SmolVLM via ONNX runtime; stable, see docs/vlm-smolvlm-onnx.md
  • Native .cellm VLM path - ViT/SigLIP encoder + linear projector (experimental, see docs/vlm-smolvlm-onnx.md)
  • Accelerated Math - Metal + WASM SIMD + WebGPU compute kernels
  • WebAssembly Support - Run LLMs in the browser with wasm-bindgen
  • High-Performance CLI - Conversion, benchmarking, debug inference
  • Git Commit Messages - Generate commit messages from local LLM via tools/git-cellm-commit.sh
  • Vulkan Support - Device/pipeline/buffer management is in place, but every compute op still falls back to CPU (research)
  • Android Integration - Kotlin/JNI bindings, AAR, and Compose demo app build against a cross-compiled .so; on-device validation and tuning still open
  • Qwen iOS Porting - Optimize Qwen inference for native iOS
  • Docker (CPU tooling image) - Reproducible CI/benchmarking image for infer/convert/bench (see Docker)

Documentation Index

Topic Doc
Architecture & crate design docs/project_architecture.md
Paged KV cache internals docs/paged-kv-cache-foundation.md
Scheduler & continuous batching docs/phase4-continuous-batching.md
Model conversion & quantization docs/convert-quantized-models.md
On-device function calling docs/function-calling-gemma.md
TurboQuant KV compression docs/turboquant_dataflow.md
VLM / SmolVLM ONNX guide docs/vlm-smolvlm-onnx.md
VLM sequence tracking docs/cellm-vlm-sequence.md
Qwen3.5 / DeltaNet docs/qwen3_5-deltanet.md
Metal acceleration notes docs/LFM_Metal_Acceleration.md
Benchmark history & raw runs docs/benchmarks/
Data flow diagrams docs/data_flow.md
Format specification docs/format.md
Inference graph docs/inference_graph.md
WASM & WebGPU Backend docs/wasm-backend.md

Troubleshooting

Metal is not being used

# 1. Verify Metal device access
cargo run --release --bin metal-smoke

# 2. Verify infer picks Metal
./target/release/infer \
  --model models/smollm2-135m-int8.cellm \
  --tokenizer models/hf/smollm2-135m/tokenizer.json \
  --prompt "hello" --gen 8 --backend metal

In restricted/sandboxed shells, Metal device discovery can fail. infer --backend metal now errors instead of silently falling back to CPU.

SmolLM2 360M needs non-interleaved RoPE

CELLM_LLAMA_ROPE_INTERLEAVED=0 ./target/release/infer ...

Gemma-3 Metal quality knobs

Default keeps norm/RoPE/logits on CPU-safe path for quality parity. Opt-in Metal paths:

CELLM_GEMMA_USE_METAL_NORM=1   # enable Metal RMSNorm
CELLM_GEMMA_USE_METAL_ROPE=1   # enable Metal RoPE
CELLM_GEMMA_USE_METAL_LOGITS=1 # enable Metal final logits matvec

Llama graph path (experimental speed)

CELLM_LLAMA_ENABLE_GRAPH=1 ./target/release/infer ...

Model-specific env flags

Model Flag Purpose
SmolLM2 360M CELLM_LLAMA_ROPE_INTERLEAVED=0 Correct RoPE layout
Llama CELLM_LLAMA_USE_METAL_NORM=1 Force Metal norm
Llama CELLM_LLAMA_USE_METAL_ROPE=1 Force Metal RoPE
Qwen VLM CELLM_VLM_TOKENIZER=... Set tokenizer path for vlm-smoke

For more debug flags and backend-specific notes, see the per-model docs in docs/.

Citation

If you use cellm in your research, please cite:

@software{asante2026cellm,
  author       = {Asante, Jeffrey},
  title        = {cellm: A Mobile-Native LLM Serving Engine},
  year         = {2026},
  url          = {https://github.com/jeffasante/cellm},
  note         = {Rust inference engine with paged KV cache and multi-session scheduling}
}

License

Apache License, Version 2.0 (LICENSE-APACHE)

About

A from-scratch LLM inference runtime for iOS/Android, targeting phones with under 512MB RAM. It's research-grade, not a wrapper around llama.cpp or a port of vLLM.

jeffasante.github.io/cellm/

Topics

androidinferenceioskv-cachellmmetalmobileon-device-airusttransformervulkanwasmwebgpu

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