MPS BitsAndBytes

Real 4-bit and 8-bit quantization for PyTorch on Apple Silicon (M1/M2/M3/M4).

Full bitsandbytes-compatible API with Metal GPU acceleration for running large models on your Mac.

Features

Format	Bits	Memory Savings	Best For
NF4	4-bit	~75%	LLM weights (normally distributed)
FP4	4-bit	~75%	Alternative with better dynamic range
FP8 E4M3	8-bit	~50%	Better precision than INT8
INT8	8-bit	~50%	General purpose

Plus:

• Metal GPU kernels - Fused dequant+matmul, no Python overhead
• Double quantization - Extra ~10% savings on scales
• 8-bit Optimizers - Adam8bit, AdamW8bit, Lion8bit, SGD8bit
• Paged Optimizers - CPU offloading for larger models
• Quantized Embeddings - Embedding4bit, Embedding8bit
• Sparse Operations - spmm_coo, spmm_coo_int8
• LLM.int8 - OutlierAwareLinear with col+row quantization
• HuggingFace compatible - BitsAndBytesConfig API works out of the box
• QLoRA training - Freeze quantized weights, train LoRA adapters

Installation

pip install mps-bitsandbytes

Or from source:

git clone https://github.com/mpsops/mps-bitsandbytes
cd mps-bitsandbytes
pip install -e .

Quick Start

4-bit Quantization (NF4 - Recommended for LLMs)

import torch
from mps_bitsandbytes import Linear4bit, BitsAndBytesConfig, quantize_model

# Convert a single layer
linear = torch.nn.Linear(4096, 4096).half().to('mps')
linear_4bit = Linear4bit.from_linear(linear)  # NF4 by default

# Or use FP4
linear_fp4 = Linear4bit.from_linear(linear, quant_type='fp4')

# Quantize entire model
config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type="nf4",
    bnb_4bit_compute_dtype=torch.float16,
)
model = quantize_model(your_model, quantization_config=config, device='mps')

8-bit Quantization (FP8 or INT8)

from mps_bitsandbytes import Linear8bit, LinearFP8

# INT8 (traditional)
linear_int8 = Linear8bit.from_linear(linear)

# FP8 E4M3 (better precision)
linear_fp8 = LinearFP8.from_linear(linear)

8-bit Optimizers

Memory-efficient optimizers that store momentum/variance in 8-bit:

from mps_bitsandbytes import Adam8bit, AdamW8bit, Lion8bit, SGD8bit

# Drop-in replacement for torch optimizers
optimizer = Adam8bit(model.parameters(), lr=1e-3)
optimizer = AdamW8bit(model.parameters(), lr=1e-3, weight_decay=0.01)
optimizer = Lion8bit(model.parameters(), lr=1e-4)
optimizer = SGD8bit(model.parameters(), lr=0.1, momentum=0.9)

Paged Optimizers

Offload optimizer states to CPU for training larger models:

from mps_bitsandbytes import PagedAdam, PagedAdamW, PagedLion

# States are stored on CPU, copied to GPU during step()
optimizer = PagedAdamW(model.parameters(), lr=1e-3, page_to_cpu=True)

Quantized Embeddings

Reduce embedding table memory by 50-75%:

from mps_bitsandbytes import Embedding4bit, Embedding8bit, EmbeddingNF4, EmbeddingFP4

# Convert existing embedding
embed = torch.nn.Embedding(50000, 4096).half().to('mps')
embed_4bit = Embedding4bit.from_embedding(embed)  # NF4 by default
embed_fp4 = EmbeddingFP4.from_embedding(embed)    # FP4
embed_8bit = Embedding8bit.from_embedding(embed)  # INT8

Functional API

from mps_bitsandbytes import (
    # 4-bit
    quantize_nf4, dequantize_nf4, matmul_nf4,
    quantize_fp4, dequantize_fp4, matmul_fp4,
    # 8-bit
    quantize_fp8_e4m3, dequantize_fp8_e4m3, matmul_fp8_e4m3,
    quantize_rowwise, dequantize_rowwise, matmul_int8,
    # Col+Row INT8 (LLM.int8 style)
    quantize_colrow, dequantize_colrow, matmul_colrow,
    # Double quantization
    double_quant, dequant_absmax,
    # Sparse
    spmm_coo, spmm_coo_int8, sparse_coo_from_dense, quantize_sparse_coo,
)

# NF4
weight = torch.randn(4096, 4096, device='mps', dtype=torch.float16)
packed, absmax = quantize_nf4(weight, block_size=64)
output = matmul_nf4(input, packed, absmax)

# Double quantization (quantize the scales too)
absmax_quant, absmax_scales = double_quant(absmax)

Memory Savings

Model	FP16	INT8/FP8	NF4/FP4
7B params	14 GB	7 GB	3.5 GB
13B params	26 GB	13 GB	6.5 GB
70B params	140 GB	70 GB	35 GB

HuggingFace Integration

from transformers import AutoModelForCausalLM
from mps_bitsandbytes import BitsAndBytesConfig, quantize_model

model = AutoModelForCausalLM.from_pretrained(
    "meta-llama/Llama-2-7b-hf",
    torch_dtype=torch.float16,
)

config = BitsAndBytesConfig(
    load_in_4bit=True,
    bnb_4bit_quant_type="nf4",
    bnb_4bit_compute_dtype=torch.float16,
)
model = quantize_model(model, quantization_config=config, device='mps')

QLoRA Training

from mps_bitsandbytes import BitsAndBytesConfig, quantize_model, Adam8bit
from peft import get_peft_model, LoraConfig

# Load in 4-bit
config = BitsAndBytesConfig(load_in_4bit=True, bnb_4bit_quant_type="nf4")
model = AutoModelForCausalLM.from_pretrained("model_name", torch_dtype=torch.float16)
model = quantize_model(model, quantization_config=config, device='mps')

# Add LoRA adapters (train in fp16 while base stays quantized)
lora_config = LoraConfig(r=8, lora_alpha=16, target_modules=["q_proj", "v_proj"])
model = get_peft_model(model, lora_config)

# Use 8-bit optimizer for extra memory savings
optimizer = Adam8bit(model.parameters(), lr=1e-4)
trainer.train()

API Reference

Linear Modules

Class	Format	Use Case
Linear4bit	NF4 or FP4	LLM inference, QLoRA
Linear8bit	INT8	General quantization
LinearFP8	FP8 E4M3	Better precision 8-bit
OutlierAwareLinear	INT8 + FP16	LLM.int8 mixed precision
SwitchBackLinear	INT8	Training with quantized forward

Embedding Modules

Class	Format	Memory Savings
Embedding4bit	NF4 (default)	~75%
EmbeddingNF4	NF4	~75%
EmbeddingFP4	FP4	~75%
Embedding8bit	INT8	~50%

Optimizers

Class	Description
Adam8bit	Adam with 8-bit states
AdamW8bit	AdamW with 8-bit states
Lion8bit	Lion optimizer with 8-bit momentum
SGD8bit	SGD with 8-bit momentum
PagedAdam	Adam with CPU offloading
PagedAdamW	AdamW with CPU offloading
PagedLion	Lion with CPU offloading

Functional API

4-bit (NF4/FP4):

• quantize_nf4(tensor, block_size=64) / quantize_fp4(...)
• dequantize_nf4(packed, absmax, ...) / dequantize_fp4(...)
• matmul_nf4(input, weight_packed, weight_absmax, bias=None) / matmul_fp4(...)

8-bit:

• quantize_fp8_e4m3(tensor) - FP8 quantization
• quantize_rowwise(tensor) - INT8 row-wise quantization
• quantize_colrow(tensor) - INT8 col+row quantization (LLM.int8)
• matmul_fp8_e4m3(...) / matmul_int8(...) / matmul_colrow(...)

Double Quantization:

• double_quant(absmax, double_quant_block=256) - Quantize scales
• dequant_absmax(absmax_quant, absmax_scales) - Restore scales

Sparse Operations:

• sparse_coo_from_dense(tensor) - Convert to COO format
• spmm_coo(row_idx, col_idx, values, dense, rows, cols) - Sparse matmul
• spmm_coo_int8(...) - INT8 sparse matmul
• quantize_sparse_coo(row_idx, col_idx, values) - Quantize sparse values

Utilities:

• is_available() - Check MPS availability
• has_native_kernels() - Check Metal kernels loaded
• get_memory_footprint(model) - Calculate memory usage

Comparison with bitsandbytes

Feature	bitsandbytes (CUDA)	mps-bitsandbytes
NF4/FP4	CUDA	Metal
INT8/FP8	CUDA	Metal
Double quant	CUDA	Metal
8-bit Optimizers	CUDA	Pure PyTorch
Paged Optimizers	CUDA	Pure PyTorch
Quantized Embeddings	CUDA	Pure PyTorch
Sparse matmul	CUDA	Pure PyTorch
LLM.int8 (col+row)	CUDA	Pure PyTorch
Platform	NVIDIA	Apple Silicon

Demo

# Chat with a quantized LLM
python demo/chat.py

License

MIT

Credits

• bitsandbytes - Original CUDA implementation
• QLoRA - NF4 quantization paper