metal-SingleCell

GPU-accelerated single-cell analysis on Apple Silicon.

A Metal/MLX re-implementation of rapids-singlecell — drop-in replacements for the core scanpy and squidpy functions that run on the M-series GPU. rapids-singlecell is CUDA/CuPy-only; there is no Apple-silicon path because the M-series GPU has no native sparse-matrix support. This project builds that missing sparse substrate and the scanpy/squidpy front-end on top of it.

It is a drop-in API: swap the import prefix and your existing pipeline runs on the GPU.

Documentation: metal-singlecell.readthedocs.io

import scanpy as sc
import metalsinglecell as msc          # pp / tl / gr — mirror sc.pp, sc.tl, sq.gr

adata = sc.datasets.pbmc3k()
msc.pp.normalize_total(adata, target_sum=1e4)
msc.pp.log1p(adata)
msc.pp.highly_variable_genes(adata, n_top_genes=2000)
msc.pp.pca(adata)
msc.pp.neighbors(adata)
msc.tl.leiden(adata, backend="gpu")    # Metal parallel Leiden
msc.tl.umap(adata)
sc.pl.umap(adata, color="leiden")      # plot with scanpy as usual

Functions take an AnnData, compute on the GPU, and write results back to the same slots scanpy uses (.X, .obsm, .obsp, .var, .uns), with scanpy's copy= semantics — so plotting and downstream tooling are unchanged.

Install

pip install metalsinglecell          # Python ≥ 3.11

On Apple Silicon this also pulls in mlx (the Metal GPU backend); on other platforms it installs the pure-NumPy/SciPy core, so the package imports everywhere (mlx is a Darwin/arm64-only dependency). Heavy backends (mlx, scanpy, squidpy) are lazy-imported, so it imports cleanly in any environment.

For development, clone and install editable:

git clone https://github.com/gingerii/metal-SingleCell.git
cd metal-SingleCell
pip install -e .

See envs/ for the conda environment (envs/metalsinglecell.yml), which is the recommended setup for reproducing the benchmarks.

uv users: run uv python install 3.11 before creating your venv. The system python.org 3.11 on macOS arm64 can hang indefinitely on first import scanpy (a numba/LLVM AOT-compile deadlock on framework Python builds); uv's own managed 3.11 build (3.11.13) avoids it. Then:

uv python install 3.11
uv venv --python 3.11.13 .venv
uv pip install -e ".[oracle,metal,dev]"

Tutorials

Four executable notebooks in notebooks/, mirroring the rapids-singlecell tutorials:

Notebook	Workflow
01_basic_workflow.ipynb	QC → normalize → HVG → scale → PCA → neighbors → UMAP → Leiden → markers → Harmony → diffmap (PBMC 3k)
02_pearson_residuals.ipynb	Analytic Pearson-residual normalization → PCA → clustering (PBMC 3k)
04_squidpy.ipynb	Spatial graph, Moran's I / Geary's C, co-occurrence (squidpy IMC)
brain_1M.ipynb	Full 1,000,000-cell workflow on a laptop GPU

Notebooks 1, 2 and 4 are self-contained (they auto-download their datasets). brain_1M needs the 10x 1.3M-neuron .h5 placed at data/external/1M_neurons.h5.

Hardware tested

All benchmarks below were measured on a single laptop:

Chip	Apple M3 Max
GPU	40 cores, Metal 3, ~400 GB/s unified-memory bandwidth
CPU	16 cores (12 performance + 4 efficiency)
Memory	48 GB unified
OS / runtime	macOS 14.4 · MLX 0.31.2 · Python 3.11

Speedups vs CPU

Speedup = CPU reference time ÷ our GPU time, both on the same M3 Max (vs scanpy / scikit-learn / igraph / harmonypy / bbknn / scrublet). Methodology: warm-up (defeats MLX first-call kernel compile), best-of-N, on real data. Clustering is timed algorithm-only (kNN graph pre-built for both sides). Sizes are cells: 2k–2M are the 1.3 M-neuron atlas (sub-/over-sampled) plus a 2 M-cell Xenium cohort — one data family, rows grouped by pipeline stage. bold = a standout; – = not run; (N s) = our runtime where a CPU reference is impractical at that scale.

function	2k	50k	100k	1M	2M	accuracy / notes
normalize + log1p	5.4×	3.5×	3.5×	1.7×	2.8×	Δ ≈ 1e-6
normalize_pearson_residuals	7.4×	9.8×	9.1×	–	–	exact
highly_variable_genes	5.8×	25.9×	32.9×	15.8×	49.2×	gene-overlap 1.000
regress_out	1.5×	1.7×	1.6×	–	–	Δ ≈ 1e-5
scrublet	15.1×	6.4×	–	–	–	doublet scores match
pca (sparse)	2.1×	4.3×	4.4×	4.5×	(8.0 s)	subspace 0.97–0.99
neighbors	1.4×	4.7×	4.9×	6.6×	(34.8 s)	kNN recall ~0.97 ⁂
bbknn	7.4×	1.7×	1.3×	–	–	per-batch top-k kernel
umap ‡	15.1×	29.6×	28.2×	(29 s)	(54 s)	trustworthiness 0.95
tsne	2.1×	7.0×	6.6×	(176 s)	–	trustworthiness ~0.98
diffmap	1.7×	2.9×	2.7×	3.6×	(36 s)
draw_graph	16.1×	(2.4 s)	(4.4 s)	–	–
kmeans	1.1×	3.5×	4.0×	(1.1 s)	(3.2 s)
leiden †	0.05×	4.6×	3.6×	3.2×	3.5×	Q at igraph parity
louvain †	0.14×	21.9×	21.0×	9.7×	(16.1 s)	Q at igraph parity
rank_genes_groups (t-test)	3.7×	9.6×	7.6×	–	–	top-k overlap 1.000
rank_genes_groups (logreg)	0.9×	2.2×	–	–	–
score_genes	(0.2 s)	(4.6 s)	(9.7 s)	–	–	ref not benchmarked §
harmonize	0.17×	6.3×	2.2×	2.2×	–	mixing ≥ harmonypy

Sizes are cells; 2k–2M are the 1.3 M-neuron atlas (sub-/over-sampled) and the 2 M-cell Xenium cohort — one consistent data family. 2M neighbors/umap are measured on the cached atlas PCA embedding (the full-gene path OOMs at 2M). Spatial (gr) functions are in their own table below (they run on a different, real spatial-platform size ladder).

‡ umap — the shipped hybrid layout (mlx-vis's GPU SGD optimizer driven by our shared neighbor graph) fixes the old layout's superlinear-at-scale problem (1M 188 s → 29 s) and, versus the previous GPU layout, is ~4× faster at 50k (0.66 s vs 2.58 s) and higher quality (trustworthiness 0.95 vs 0.86), while preserving the neighbors→{leiden,umap} shared-graph contract. embedding.py is now umap-learn-free.

⁂ neighbors uses a GPU brute path ≤30k (exact) and a vendored mlx-vis GPU NN-descent above (recall ~0.97), retiring the CPU-pynndescent fallback — reproducible wall time, 4.7–6.6× across 50k–1M.

† leiden / louvain rows are SBM benchmark graphs (50k–2M), on which igraph is unusually fast, so they understate the real-data advantage: on the actual 986k-neuron kNN-15 graph, Leiden is 4.8× and Louvain 58.6×. Leiden timed at n_iterations=1 (the speed operating point — see below).

§ score_genes runs on-GPU but the CPU reference errors in the current harness (a gene-list plumbing bug, not a compute failure); runtimes shown, speedup pending a harness fix.

Spatial (gr) functions

Measured on a real spatial-platform ladder — a different data family from the single-cell table above, so it gets its own columns. Speedup = squidpy CPU wall time ÷ ours; matched graph / thresholds / n_perms=100. (N s) = our GPU wall time where squidpy becomes impractical (>100k: permutation/pairwise references run for minutes-to-hours or OOM).

function	Visium 2.7k	Stereo-seq 19k	Xenium 63k	MERFISH 81k	Xenium-breast 253k	accuracy
spatial_autocorr (Moran)	80×	62×	57×	49×	(7.5 s)	vs squidpy
spatial_autocorr (Geary)	79×	47×	42×	41×	(9.7 s)	vs squidpy
co_occurrence	13.0×	18.8×	17.0×	16.6×	(82 s)	correlation 1.000
calculate_niche	11.9×	110×	89×	123×	(0.14 s)	composition-matched
ligrec	15.3×	5.6×	4.6×	7.0×	(0.65 s)	10 pairs, n_perms=100
spatial_neighbors ◆	1.5×	3.9×	2.1×	3.2×	1.1×	exact, Jaccard ≥0.997 ◆

All five win and hold with scale, and keep running at 253k where squidpy's permutation/pairwise references become impractical. co_occurrence also scales past the n² memory wall via a tiled device-atomic-histogram kernel.

◆ spatial_neighbors was formerly the one loss (brute-force O(n²), OOM past ~120k); a uniform-grid spatial index (see What was solved) made it exact and O(n), clearing the wall (2M cells in 1.27 s). NNDescent is not applicable here — on 2-D coordinates its recall collapses to ~6%.

Clustering crosses over with cell count: Louvain wins from ~50k up (58.6× on the real 986k-neuron graph), and Leiden — after an O(degree) SIMD-group kernel rewrite + vertex pruning — went from a catastrophic 0.05× to a 4.1× end-to-end speedup, winning at every scale ≥50k (4.8× vs igraph on the real 986k graph) — see What was solved.

Leiden operating points. msc.tl.leiden defaults to n_iterations=2 (scanpy parity) — igraph-parity modularity at ~2.6×; n_iterations=1 is the ~4.8× speed point at ~1% lower modularity. Pick per run.

Accuracy

Every accelerated function is validated against its CPU reference: normalize/Pearson exact (Δ≈1e-6), HVG gene-overlap 1.000, PCA subspace 0.97–0.99, rank-genes top-k overlap 1.000, Leiden modularity at igraph parity (Q 0.8586 vs 0.8588 at n_iterations=2; Q 0.8504 at the n_iterations=1 speed point), kNN recall ≈0.99, co-occurrence correlation 1.000 vs squidpy. An asserting tests/ suite covers drop-in defaults, GPU-parity, numerical-accuracy guards, streaming/out-of-core, and harmonize quality, run in CI (a CPU lane + a self-hosted Metal-GPU lane).

What was solved

• A GPU sparse substrate for Apple Silicon — CSR container, SpMM, segmented reductions, and custom mx.fast.metal_kernel kernels (QC, sparse PCA, a register-based top-k for kNN), since the M-series GPU has no native sparse support.
• The full scanpy pp/tl + squidpy gr surface — ~30 functions as a drop-in AnnData API.
• Coloring-free parallel clustering — the first parallel Louvain/Leiden on Metal. Replacing graph-coloring local-moving with cuGraph-style synchronous moves + a random-commit rule took Louvain to 58.6× on the real 986k-neuron graph. Leiden was then rebuilt with O(degree) SIMD-group move/refine kernels (simd_sum/simd_any, no atomics or grid barrier — retiring the old O(degree²) rescans) plus vertex pruning and batched host-sync, for a 4.1× end-to-end speedup (11.9 s → 2.9 s on the 986k graph) at equal-or-better modularity — winning at every scale ≥50k.
• A fused co-occurrence kernel — a tiled device-atomic histogram that matches squidpy exactly, runs 13–19× faster across real Visium/Stereo-seq/Xenium/MERFISH data (measured), and scales past the n² memory wall to 250k+ cells where squidpy is impractical.
• A GPU uniform-grid spatial index for spatial_neighbors — a cell-list kNN (custom Metal kernel, one thread/query scanning its 3×3 / 3×3×3 cell block) that replaced the brute-force O(n²) path: exact (grid-completeness guarantee + fp32 fallback) and O(n), beating squidpy's KD-tree at every scale (1.1–3.9×) and doing 2M cells in 1.27 s where brute OOM'd past ~120k. It also fixed a latent fp16-ranking bug that corrupted neighbors on large-magnitude coordinates.
• GPU-native kNN, t-SNE, and Harmony — the neighbor graph (>30k) now builds on a GPU NN-descent (vendored from mlx-vis), retiring the CPU-pynndescent fallback (recall-matched at ~0.97, 4.7–6.6× faster across 50k–1M, reproducible wall time); t-SNE runs entirely on the Metal GPU (mlx-vis, FFT-interpolation repulsion, ~7× at 50k–100k); and Harmony integration was rewritten to run every step on-GPU — an analytic block-inverse correction (no linear solver), a GPU k-means init, and GPU L2-norms — turning a 0.07–0.59× loss into a 6.3× win at 50k / 2.2× at 100k, mixing quality (iLISI) matching or beating harmonypy.
• Out-of-core front-end (streaming from disk) — the sparse front-end streams row-blocks from a chunked zarr store, so the full 1.3 M-neuron atlas (2.6 B non-zeros) — which OOMs in-core — completes in ~300 s at 25.6 GB peak on a 48 GB laptop. See Out-of-core.
• Validated at atlas scale — a complete 1 M-cell workflow (and every function through 2 M cells) runs end-to-end on a 48 GB laptop; the full 28k-gene 1.3 M-neuron atlas runs out-of-core.

Out-of-core: atlas-scale on a laptop

The sparse front-end runs on datasets whose full expression matrix does not fit in unified memory, by streaming cell-axis row-blocks from a chunked on-disk zarr store — peak memory is bounded by one block, not the cell count.

# one-time: convert an .h5ad / 10x .h5 to a chunked-zarr store
python -m metalsinglecell.backed  1M_neurons.h5  atlas.zarr  --block-rows 100000

import anndata as ad, metalsinglecell as msc
adata = ad.read_zarr("atlas.zarr")     # backed, not fully loaded
msc.pp.calculate_qc_metrics(adata)     # each pp step detects the backed .X and streams
msc.pp.normalize_total(adata); msc.pp.log1p(adata)
msc.pp.highly_variable_genes(adata); msc.pp.scale(adata)
msc.pp.pca(adata, svd_solver="covariance_eigh")   # single streaming covariance pass → dense eigh
# → adata.obsm["X_pca"]; downstream (neighbors/UMAP/clustering) fits in memory and runs as usual

Full 1.3M-neuron atlas (1,306,127 × 27,998, 2.6 B non-zeros) — the case that OOMs in-core:

	in-core	out-of-core (streaming)
front-end (QC→normalize→log1p→HVG→scale→PCA)	OOMs (>48 GB)	completes
peak memory	—	25.6 GB (bounded by block_rows)
wall time (end-to-end)	—	~300 s

Design notes: streaming is opt-in — it activates only when .X is a backed zarr store, and every in-core default is unchanged. Results are bit-exact vs in-core for the linear ops (QC / normalize / log1p) and match to subspace ≥ 0.999 for PCA. Out-of-core PCA uses a covariance-eigh solver — one streaming pass accumulates the gene×gene covariance, then a dense fp64 eigendecomposition (the same choice rapids-singlecell made for Dask PCA); randomized/Lanczos are in-core only. An optional post-log1p checkpoint (msc.pp.materialize) trades disk for repeated compute. Downstream (neighbors/UMAP/clustering) runs in memory as usual — the n × 50 embedding is small even at atlas scale.

When to use the CPU version instead

We benchmarked honestly; the GPU does not win everywhere on this hardware. Use the CPU implementation (scanpy / squidpy / backend="igraph") when:

• Leiden / Louvain below ~50k cells. igraph's lazy-sequential optimizer is extremely fast on small graphs; the GPU only wins once parallelism outweighs launch/coloring overhead (keep the default backend="igraph" for small data).
• Harmony integration below ~50k cells. After the GPU rewrite Harmony wins at scale (6.3× at 50k, 2.2× at 100k — the speedup peaks near 50k because our correction is superlinear in N while harmonypy is near-linear), but at a few-thousand cells it still loses (~0.15×) — the clustering loop is launch-latency-bound and CPU harmonypy is sub-second there anyway. Mixing quality matches/beats harmonypy at every scale.

(kNN, t-SNE, and spatial_neighbors were formerly CPU-favored and no longer are — see the tables and What was solved.)

Rule of thumb: the M3 GPU wins on parallel-arithmetic, bandwidth-bound work (and on clustering / kNN / integration once the data is large enough to amortize launch overhead), and loses on launch-latency-bound work at small N. Against an already-optimized CPU reference (numba / igraph / pynndescent), a GPU kernel on M3 typically wins ~1.5–2× (the bandwidth ratio) on raw arithmetic; the large wins come from removing genuine algorithmic waste.

vs rapids-singlecell (NVIDIA)

rapids-singlecell's headline speedups (e.g. 470× Leiden) are GPU-vs-CPU on a datacenter GPU (A100/3090) with ~2–5× the memory bandwidth and fully-fused CUDA. We do not match those absolute numbers. What this project provides is the only Apple-silicon path for this workflow, with honest laptop-scale speedups and an identical drop-in API — develop and run atlas-scale single-cell analysis on a Mac, no CUDA required.

Credits & acknowledgements

metal-SingleCell is an independent, unaffiliated project. It would not exist without the work it builds on, and the API and workflows are modeled directly on these libraries — please cite them:

• rapids-singlecell (MIT, part of scverse®) — the GPU single-cell API this project mirrors for Apple Silicon, and the source of the tutorials and out-of-core design reproduced here. Dicks, S. et al. GPU-accelerated single-cell analysis at scale with rapids-singlecell. arXiv:2603.02402 (2026). doi:10.48550/arXiv.2603.02402.
• mlx-vis (Apache-2.0) — the pure-MLX Apple-Silicon GPU NNDescent (approximate k-NN graph), t-SNE, and UMAP-layout code that this project's neighbor-graph, t-SNE, and UMAP-layout paths use (vendored under src/metalsinglecell/_vendor/mlx_vis/, with NOTICE). Xiao, H. mlx-vis: GPU-Native Dimensionality Reduction on Apple Silicon. arXiv:2603.04035 (2026). doi:10.48550/arXiv.2603.04035.
• scanpy — Wolf, F. A., Angerer, P. & Theis, F. J. SCANPY: large-scale single-cell gene expression data analysis. Genome Biology 19, 15 (2018).
• squidpy — Palla, G. et al. Squidpy: a scalable framework for spatial omics analysis. Nature Methods 19, 171–178 (2022).
• MLX (Apple) — the array framework powering the Metal GPU kernels.

The pp / tl / gr function names and signatures intentionally follow scanpy / squidpy / rapids-singlecell so existing pipelines port with a one-line import change. This project is not endorsed by or affiliated with scverse, NVIDIA, or Apple.

License

BSD-3-Clause (see pyproject.toml). rapids-singlecell, scanpy, and squidpy are released under permissive licenses (MIT / BSD-3-Clause); MLX is MIT.

Status

Functionally complete and validated (pp / tl / gr + tutorials + benchmark), with an out-of-core streaming front-end for datasets larger than memory, a GPU-native kNN / t-SNE / Harmony path, and an asserting tests/ suite run in CI (CPU + self-hosted Metal-GPU lanes). Version 0.1.0. The full benchmark with methodology is in results/validation/RESULTS_v_benchmark.md.