lmz

Smaller checkpoints. Byte for byte.

Lossless compression built for model weights. zstd -1 takes 22.7% off a Llama-3.1-8B BF16 checkpoint. lmz takes 34.7% — and 64.6% off the directory as Hugging Face actually ships it, which is 13 GB more than zstd on one 8B model.

Nothing is approximated. Every byte comes back.

And the decoder now runs on the GPU — 111 GB/s from an ordinary archive against a 28.8 GB/s PCIe link, shipped in the wheel. Jump to it.

pip install lmzip
lmz compress ./Llama-3.1-8B-Instruct/

What it saves

Real checkpoints, every round-trip verified byte-identical.

On BF16 weights lmz beats the published state of the art, and sits 0.3 points off the bound no lossless coder of any kind can pass:

Three things a general compressor structurally cannot do, which is where most of the margin comes from: store a tensor once when a directory ships it twice, code a checkpoint as the difference from the one before it, and split on a float's own bit-fields instead of byte boundaries.

What it costs to do that

A 1.12 GiB BF16 shard, RAM-backed, including all I/O and per-chunk checksums:

zstd -1 still compresses about 1.5× faster than lmz does, and saves twelve points less. Past four threads lmz is not CPU-bound any more: it runs into the memory bus at about 2 GiB/s, and it reaches that at four threads where it used to need eight. On real storage the disk arrives before either of them, and the archive is a third smaller, so a storage-bound load moves a third fewer bytes.

On a GPU

pip install lmzip ships a CUDA decoder. On an RTX 5080 it decodes lmz's own rANS at 111 GB/s out of an archive written today, and 418 GB/s when the frequency table is shared across chunks — both verified byte-identical to the CPU decoder over 936 MB of real BF16 planes.

The ratio to the link is the point. PCIe Gen4 x16 delivers 28.8 GB/s, so a decoder 3.9× faster than that makes compression on the path into VRAM free, and every point of lmz's ratio becomes a point of load bandwidth. The fused whole-BF16 kernel in scratchpad/gpu/ measures the end of it: cold disk to VRAM, plain safetensors 0.373 s against lmz's 0.256 — 1.46× faster, converting 98% of the ratio into load speed.

from lmz import gpu

gpu.available()                                    # (True, '') -- or why not
gpu.decode_batch(streams, offsets, nstr, plane)    # a batch in, plaintext out

A batch, not a stream: lmz's 8 interleaved rANS states are 8 lanes of work, so one stream never fills a GPU however large it is, and many streams at once do.

The first thing it does on any machine is decode a stream that machine just encoded and check the CPU decoder agrees; a device that disagrees is not used, and lmz doctor names it. The kernel is clean under compute-sanitizer and compiles for sm_75 through sm_121, but it has only ever been run on one card's silicon, so it verifies rather than assumes.

If you have a GPU that is not a Blackwell, this is worth thirty seconds:

lmz doctor --gpu-verify

It decodes thirty awkward distributions and batch shapes and checks lmz's own CPU decoder agrees with every byte — no data file, no network, no login: the streams are built by lmz's own encoder, so the oracle travels with the question. Paste the block into an issue. A pass is evidence too, and right now there is one card's worth of it.

No GPU? A free Colab T4 is the card that matters most —

Turing runs different generated code: it has no cp.async instruction, so the intrinsic falls back to a synchronous copy. Counting them says which architectures share the verified path:

That fallback is no longer code nothing has run. compute_75 emits PTX and no cubin, so the driver has to JIT it — which means Turing's generated code can be run on a card that is not a Turing. It decodes 936 MB byte-identically at every block size, on both kernels, and is clean under memcheck, racecheck and synccheck; so are sm_80, sm_86, sm_89 and sm_90. What is left open for Turing is a number and a scheduler, not whether it decodes. Shared memory excludes nobody either: a T4's 64 KiB holds the per-chunk tables at 64 threads a block, which is the fastest row measured here.

CUDA is optional in every direction. The wheel is pure Python, carries a .cu and no CUDA, and installing needs no toolkit. nvcc, if it is there, is used once to build the decoder into the package directory — the same bargain the SIMD kernel already makes with a C compiler, and nothing is installed system-wide. No nvcc or no card means the CPU path, unchanged. lmz doctor says which you have.

That holds even when the driver itself is broken. A CUDA driver that is half-removed or mid-upgrade leaves libcuda.so.1 on disk with an initialiser that faults, and loading it takes the whole process down with no return code involved — so lmz does its first load in a child process that is allowed to die, and reports it. A segmentation fault in your program is not a fallback.

Nothing in lmz decompress routes to it yet, deliberately: the useful thing to do with a GPU decode is to leave the result in VRAM, and deciding when belongs to the layer above — see the GPU residency handover for that boundary and for the work still between here and a residency engine.

Where it is not worth it

Stated plainly, because a compressor that only advertises its wins should not be believed:

Reading a compressed file transparently can never beat a plain one by more than 1/(1−saved) — you still have to read the archive. That is 1.5× on BF16 and 1.05× on Q4_K, so on a fast SSD the mount is slower. It buys disk, not speed.

Also included

lmz add ./my-model/ && lmz mount ~/models   # read a compressed model as ordinary
                                            #   files; llama.cpp needs no patch
lmz fs ~/.lmz/data ~/data                   # a read-write compressed filesystem;
                                            #   32.1% where btrfs+zstd gets 18.9%

Buy me a coffee

lmz is free, MIT-licensed and unfunded. If it saved you disk or bandwidth —

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Documentation

• How it works — why a float array defeats a general-purpose compressor, and the bit-level choices that close the gap
• Measured results — every number, with its conditions
• Using lmz — command line, Python API, the mount and the filesystem
• Limitations — where it does not pay, and what the 99 tests check
• Vectorising the coder — how the encoder reached arm64, the one piece of work still open, and the six that were tried and measured out flat
• GPU residency handover — the GPU decoder runs at 418 GB/s against a 28.8 GB/s PCIe link, so on that path compression is free by 14×; what shipped as lmz.gpu, the three pieces of work still between it and a residency layer, and where lmz's job ends

Python 3.10+, no runtime dependencies. zstd comes from the standard library on 3.14+; a C compiler, if present, is used once to build the SIMD kernel into the package directory, and nvcc, if present, does the same once for the CUDA decoder — nothing is installed system-wide and neither is required. Runs straight from a checkout with ./lmz-cli if you would rather not install it at all.

Check what is active with lmz doctor.