cuML Reference

cuML is NVIDIA's GPU-accelerated machine learning library within the RAPIDS ecosystem. It provides scikit-learn-compatible APIs for classification, regression, clustering, dimensionality reduction, preprocessing, and model selection. Performance depends on algorithm, shape, dtype, fallback behavior, and transfer cost; benchmark the complete pipeline on representative data.

Full documentation: https://docs.rapids.ai/api/cuml/stable/

Table of Contents

1. Installation and Setup
2. Two Usage Modes
3. cuml.accel Accelerator Mode
4. Direct cuML API
5. Algorithm Catalog
6. Input/Output Type Handling
7. Preprocessing
8. Feature Extraction
9. Model Selection and Tuning
10. Forest Inference Library (FIL)
11. Multi-GPU with Dask
12. Model Serialization
13. Memory Management
14. Performance Optimization
15. Interoperability
16. Key Differences from sklearn
17. Common Migration Patterns

Installation and Setup

Use uv add in standalone examples; follow the user's existing project package manager when one is already configured.

uv add "cuml-cu12==26.6.*"    # For CUDA 12.x
uv add "cuml-cu13==26.6.*"    # For CUDA 13.x

cuML wheels are published directly to PyPI (since RAPIDS 25.10) — the --extra-index-url=https://pypi.nvidia.com extra index is no longer required.

Platform: Linux and WSL2 only (no native macOS or Windows). Requires: Python >= 3.11, scikit-learn >= 1.5, NVIDIA GPU with CUDA 12.x or 13.x support.

Verify:

import cuml
print(cuml.__version__)

from cuml.datasets import make_blobs
X, y = make_blobs(n_samples=1000, n_features=10)
print(f"Generated {X.shape[0]} samples on GPU")

Two Usage Modes

1. cuml.accel (Zero-Code-Change)

Transparently intercepts sklearn, umap-learn, and hdbscan calls and routes them to GPU. Falls back to CPU for unsupported operations. Best for: quick acceleration of existing sklearn code, mixed codebases, prototyping.

2. Direct cuML API

Replace from sklearn with from cuml. Maximum performance, explicit control over GPU execution. Best for: production pipelines, maximum performance, new GPU-first code.

cuml.accel Accelerator Mode

The fastest path from sklearn to GPU — no code changes required. Similar to cudf.pandas for pandas.

Activation

# Jupyter/IPython (MUST be the first cell, before any sklearn import)
%load_ext cuml.accel

import sklearn  # Now GPU-accelerated
from sklearn.cluster import KMeans  # Runs on GPU transparently

# Command line
python -m cuml.accel script.py
python -m cuml.accel -v script.py     # With info logging
python -m cuml.accel -vv script.py    # With debug logging

# Programmatic (call BEFORE importing sklearn)
import cuml
cuml.accel.install()

from sklearn.cluster import KMeans  # Now GPU-accelerated

# Environment variable
CUML_ACCEL_ENABLED=1 python script.py

How It Works

• Intercepts sklearn/umap-learn/hdbscan imports and replaces estimators with GPU versions.
• If an operation isn't supported on GPU, it silently falls back to CPU sklearn.
• Uses managed memory by default — host RAM augments GPU VRAM.
• Models pickled under cuml.accel load as standard sklearn objects in non-GPU environments.
• Accelerates 30+ algorithms across sklearn, umap-learn, and hdbscan. Recent releases (26.04-26.06) expanded coverage to preprocessing estimators (StandardScaler, MinMaxScaler, MaxAbsScaler, PolynomialFeatures, LabelEncoder) and SpectralClustering.
• Compatible with scikit-learn versions 1.5-1.8 (some estimators require >= 1.8, which enables GPU acceleration via scikit-learn's experimental array-api support).

Known Fallback Triggers (Runs on CPU Instead)

• Sparse input data (most algorithms)
• Callable parameters (e.g., callable init for KMeans)
• Certain parameter values: n_components="mle" for PCA, positive=True for linear models, warm starts
• Unsupported distance metrics for neighbors algorithms
• Multi-output targets for Random Forest
• String/object dtypes — must pre-encode with LabelEncoder first

Numerical Precision

GPU results are numerically equivalent but may differ at floating-point precision level due to parallel reduction order. Compare model quality via scores (accuracy, R2, etc.), not raw coefficient values.

Direct cuML API

Replace sklearn imports with cuml imports. The API is identical — fit/predict/transform.

from cuml.cluster import DBSCAN
from cuml.datasets import make_blobs

# Create data directly on GPU
X, y = make_blobs(n_samples=100_000, centers=5, n_features=10, random_state=42)

# Fit — runs on GPU
model = DBSCAN(eps=1.0, min_samples=5)
model.fit(X)
print(model.labels_)

from cuml import LinearRegression
from cuml.datasets import make_regression
from cuml.model_selection import train_test_split

X, y = make_regression(n_samples=100_000, n_features=50, noise=0.1)
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2)

model = LinearRegression()
model.fit(X_train, y_train)
predictions = model.predict(X_test)
score = model.score(X_test, y_test)
print(f"R2 score: {score:.4f}")

Algorithm Catalog

Clustering

Regression

Classification

Dimensionality Reduction and Manifold Learning

Nearest Neighbors

Time Series

Metrics (GPU-Accelerated)

Regression: r2_score, mean_squared_error, mean_absolute_error, mean_squared_log_error, median_absolute_error

Classification: accuracy_score, log_loss, roc_auc_score, precision_recall_curve, confusion_matrix

Clustering: adjusted_rand_score, silhouette_score, silhouette_samples, homogeneity_score, completeness_score, v_measure_score, mutual_info_score

Other: trustworthiness, pairwise_distances, pairwise_kernels

Model Explainability

Input/Output Type Handling

Supported Input Types

cuML accepts: NumPy arrays, CuPy arrays, cuDF DataFrames/Series, pandas DataFrames/Series, Numba device arrays, PyTorch tensors (via __cuda_array_interface__).

NumPy and pandas inputs are automatically transferred to GPU. For best performance, pass CuPy arrays or cuDF DataFrames to avoid transfers.

Controlling Output Type

import cuml

# Global setting
cuml.set_global_output_type('cupy')  # Options: 'input', 'cupy', 'numpy', 'cudf', 'pandas'

# Context manager
with cuml.using_output_type('cudf'):
    result = model.predict(X)  # Returns cudf Series

# Per-estimator
model = cuml.KMeans(output_type='cupy')

Performance ranking (fastest to slowest output type):

1. cupy — no host transfers, most efficient
2. cudf — slight overhead for some shapes
3. numpy / pandas — device-to-host transfer cost

Best practice: Use cupy or cudf for intermediate results. Only convert to numpy/pandas at the end for visualization or export.

Preprocessing

cuML provides GPU-accelerated versions of all common sklearn preprocessors.

Scalers and Transformers

from cuml.preprocessing import StandardScaler, MinMaxScaler, RobustScaler
from cuml.preprocessing import Normalizer, PowerTransformer, QuantileTransformer
from cuml.preprocessing import Binarizer, PolynomialFeatures, KBinsDiscretizer

scaler = StandardScaler()
X_scaled = scaler.fit_transform(X)

Encoders

from cuml.preprocessing import LabelEncoder, OneHotEncoder, LabelBinarizer, TargetEncoder

le = LabelEncoder()
y_encoded = le.fit_transform(y)

ohe = OneHotEncoder(sparse_output=False)
X_encoded = ohe.fit_transform(X_categorical)

Imputers

from cuml.preprocessing import SimpleImputer, MissingIndicator

imputer = SimpleImputer(strategy='mean')
X_imputed = imputer.fit_transform(X)

Pipeline and Composition

from cuml.compose import ColumnTransformer, make_column_transformer
from cuml.preprocessing import StandardScaler, OneHotEncoder

preprocessor = make_column_transformer(
    (StandardScaler(), ['age', 'income']),
    (OneHotEncoder(), ['category', 'region']),
)
X_processed = preprocessor.fit_transform(df)

Preprocessing Functions

scale(), minmax_scale(), maxabs_scale(), robust_scale(), normalize(), binarize(), add_dummy_feature(), label_binarize()

Feature Extraction

from cuml.feature_extraction.text import TfidfVectorizer, CountVectorizer, HashingVectorizer

tfidf = TfidfVectorizer(max_features=10000)
X_tfidf = tfidf.fit_transform(corpus)

Model Selection and Tuning

Train/Test Split

from cuml.model_selection import train_test_split

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2, random_state=42)

Cross-Validation

from cuml.model_selection import KFold

kf = KFold(n_splits=5, shuffle=True, random_state=42)
for train_idx, test_idx in kf.split(X):
    X_train, X_test = X[train_idx], X[test_idx]
    # ...

Hyperparameter Tuning

For GPU-efficient hyperparameter search, use dask-ml's GridSearchCV/RandomizedSearchCV rather than sklearn's — sklearn's version causes excessive CPU-GPU data transfers per fold.

from dask_ml.model_selection import RandomizedSearchCV
from cuml.ensemble import RandomForestClassifier

param_distributions = {
    'max_depth': [8, 12, 16, 20],
    'n_estimators': [100, 200, 500],
    'max_features': [0.5, 0.75, 1.0],
}

search = RandomizedSearchCV(
    RandomForestClassifier(),
    param_distributions,
    n_iter=25,
    cv=5,
    random_state=42,
)
search.fit(X_train, y_train)
print(f"Best score: {search.best_score_:.4f}")
print(f"Best params: {search.best_params_}")

Dataset Generators

from cuml.datasets import make_blobs, make_classification, make_regression

X, y = make_blobs(n_samples=100_000, centers=5, n_features=20, random_state=42)
X, y = make_classification(n_samples=100_000, n_features=50, n_informative=25)
X, y = make_regression(n_samples=100_000, n_features=50, noise=0.1)

Forest Inference Library

FIL provides GPU inference for supported tree-based models trained in other frameworks. Its value depends on model structure and inference batch size, so compare warm and end-to-end latency with the deployment baseline.

from cuml.fil import ForestInference

# Load from XGBoost, LightGBM, or sklearn saved models
fil_model = ForestInference.load("xgboost_model.ubj", is_classifier=True)

# Optional: optimize for specific batch size
fil_model.optimize()

# Predict on GPU
predictions = fil_model.predict(X_test)
probas = fil_model.predict_proba(X_test)

Supports: XGBoost, LightGBM, sklearn Random Forests, any Treelite-compatible model.

This is especially valuable when you have a model already trained on CPU and want to speed up inference without retraining.

Multi-GPU with Dask

For datasets too large for a single GPU or when you want to use multiple GPUs.

from dask.distributed import Client
from dask_cuda import LocalCUDACluster

# One Dask worker per GPU
cluster = LocalCUDACluster(
    rmm_pool_size="12GB",
    enable_cudf_spill=True,
)
client = Client(cluster)

# Create distributed data
from cuml.dask.datasets import make_blobs
X, y = make_blobs(
    n_samples=1_000_000,
    n_features=20,
    centers=5,
    n_parts=len(client.scheduler_info()['workers']) * 2,  # 2 partitions per worker
)

# Use Dask estimator
from cuml.dask.cluster import KMeans
kmeans = KMeans(n_clusters=5)
kmeans.fit(X)
labels = kmeans.predict(X)

# Convert to single-GPU model for serialization
single_model = kmeans.get_combined_model()

client.close()
cluster.close()

Available Multi-GPU Estimators (cuml.dask)

• Clustering: KMeans, DBSCAN
• Linear models: LinearRegression, Ridge, Lasso, ElasticNet
• Ensemble: RandomForestClassifier, RandomForestRegressor
• Decomposition: PCA, TruncatedSVD
• Manifold: UMAP (inference only)
• Neighbors: NearestNeighbors, KNeighborsClassifier, KNeighborsRegressor
• Naive Bayes: MultinomialNB
• Preprocessing: LabelEncoder, LabelBinarizer, OneHotEncoder

Model Serialization

import pickle

# Save cuML model
with open("model.pkl", "wb") as f:
    pickle.dump(model, f, protocol=5)

# Load cuML model
with open("model.pkl", "rb") as f:
    model = pickle.load(f)

• Models trained under cuml.accel can be pickled and loaded as standard sklearn objects in non-GPU environments.
• Dask distributed models must be converted first: single_model = dask_model.get_combined_model().
• joblib also works for serialization.

Memory Management

RMM (RAPIDS Memory Manager)

import rmm

# Pre-allocate a memory pool for faster allocation
rmm.reinitialize(pool_allocator=True, initial_pool_size=2**32)  # 4 GB pool

Aligning with cuDF and CuPy

When using cuML alongside cuDF and CuPy, align all libraries on the same RMM allocator:

import rmm
from rmm.allocators.cupy import rmm_cupy_allocator
import cupy
cupy.cuda.set_allocator(rmm_cupy_allocator)

cuml.accel Memory

cuml.accel uses managed memory by default (host RAM augments GPU VRAM). Disable with --disable-uvm flag if experiencing slowdowns. Managed memory does NOT work on WSL2 or when RMM is externally configured.

Best Practices

• Use float32 instead of float64 when precision allows — halves memory, doubles throughput.
• Keep data on GPU throughout the pipeline — avoid NumPy/pandas round-trips.
• For datasets larger than GPU memory: use Dask multi-GPU or chunk processing.
• Pre-allocate RMM pools to avoid fragmentation.

Performance Optimization

Benchmarking

1. Verify whether cuml.accel used a GPU implementation or fell back to scikit-learn.
2. Compare the same estimator parameters, train/test split, random seed, and output semantics.
3. Warm the CUDA context and any lazy compilation before timed repetitions.
4. Report fit, transform/predict, and end-to-end times separately, including conversion and transfer costs paid by the application.
5. Record rows, features, sparsity, dtype, estimator parameters, CPU/GPU models, and software versions.
6. For stochastic or approximate algorithms, compare quality metrics as well as time.

Key Optimization Tips

1. Use float32 when the model's accuracy and stability permit it. Validate metrics after changing precision; architecture-specific throughput ratios are not a correctness argument.

2. Keep data on GPU. Pass CuPy arrays or cuDF DataFrames. Every NumPy/pandas conversion triggers a device-host transfer.

3. Use enough work to amortize setup and transfer. Do not rely on a fixed row threshold; features, sparsity, estimator, batch size, and reuse all matter.

4. Benchmark the actual feature shape. Width affects compute, memory traffic, and temporary storage differently for each estimator.

5. First call has JIT overhead. Benchmark on subsequent calls, not the first.

6. Use RMM pools when allocation overhead or fragmentation is visible in profiling. Pools amortize allocator costs but reserve device memory and should be sized deliberately.

7. Use dask-ml for hyperparameter tuning, not sklearn's GridSearchCV — it avoids excessive CPU-GPU transfers.

8. Evaluate FIL for supported tree-model inference. Benchmark the target model and production batch sizes against the existing serving path.

Interoperability

• cuDF: Zero-copy input. cuDF DataFrames accepted directly by all estimators.
• CuPy: Zero-copy via __cuda_array_interface__. Most efficient intermediate format.
• NumPy/pandas: Accepted as input (auto-transferred to GPU). Output type configurable.
• PyTorch: Tensors accepted via array interface.
• sklearn: API-compatible. Models interconvertible. cuml.accel for transparent acceleration.
• XGBoost/LightGBM: FIL provides GPU inference for externally-trained tree models.
• Dask: Native distributed support via cuml.dask module.

End-to-End RAPIDS Pipeline

import cudf
import cuml
from cuml.preprocessing import StandardScaler
from cuml.ensemble import RandomForestClassifier
from cuml.model_selection import train_test_split

# Load data on GPU
df = cudf.read_parquet("data.parquet")
X = df.drop("target", axis=1)
y = df["target"]

# Split
X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2)

# Preprocess
scaler = StandardScaler()
X_train = scaler.fit_transform(X_train)
X_test = scaler.transform(X_test)

# Train
model = RandomForestClassifier(n_estimators=100, max_depth=16)
model.fit(X_train, y_train)

# Evaluate
score = model.score(X_test, y_test)
print(f"Accuracy: {score:.4f}")

All of this runs entirely on GPU — from Parquet read to model evaluation — with zero CPU-GPU transfers.

Key Differences from sklearn

1. Platform: Linux and WSL2 only. No native macOS or Windows.

2. Sparse data: Most cuML algorithms do not support sparse matrices (Lasso and ElasticNet gained sparse input support in 26.06). Under cuml.accel, sparse inputs fall back to CPU.

3. String data: Must be pre-encoded to numeric. No native string column support in estimators.

4. Multi-output: Not supported for Random Forest.

5. Warm starts: Not supported for most algorithms.

6. Some sklearn parameters ignored: n_jobs (GPU handles parallelism), positive=True, specific solver choices.

7. Numerical precision: Results equivalent in quality but may differ at floating-point level. Compare scores, not raw coefficients.

8. Memory: Limited by GPU VRAM (typically 8-80 GB). Use managed memory or Dask for larger datasets.

9. Missing fitted attributes: Some sklearn attributes not computed under cuml.accel (e.g., HDBSCAN exemplars_, LinearRegression rank_).

Common Migration Patterns

Pattern 1: Zero-Effort (cuml.accel)

# Add one line at top of notebook:
%load_ext cuml.accel

from sklearn.cluster import KMeans  # Now GPU-accelerated
from sklearn.decomposition import PCA  # Now GPU-accelerated
# Everything else stays exactly the same

Pattern 2: Direct Import Swap

# Before
from sklearn.ensemble import RandomForestClassifier
from sklearn.preprocessing import StandardScaler
from sklearn.model_selection import train_test_split

# After
from cuml.ensemble import RandomForestClassifier
from cuml.preprocessing import StandardScaler
from cuml.model_selection import train_test_split

Pattern 3: Full RAPIDS Pipeline (cuDF + cuML)

import cudf
from cuml.preprocessing import StandardScaler, LabelEncoder
from cuml.ensemble import RandomForestClassifier
from cuml.model_selection import train_test_split

# Load and preprocess entirely on GPU
df = cudf.read_parquet("data.parquet")
le = LabelEncoder()
df["category_encoded"] = le.fit_transform(df["category"])

X = df[["feature1", "feature2", "category_encoded"]].to_cupy()
y = df["target"].to_cupy()

X_train, X_test, y_train, y_test = train_test_split(X, y, test_size=0.2)

scaler = StandardScaler()
X_train = scaler.fit_transform(X_train)
X_test = scaler.transform(X_test)

model = RandomForestClassifier(n_estimators=200, max_depth=16)
model.fit(X_train, y_train)
print(f"Accuracy: {model.score(X_test, y_test):.4f}")

Pattern 4: GPU Inference for CPU-Trained Models

from cuml.fil import ForestInference

# Load a supported XGBoost/LightGBM/sklearn model for GPU inference
fil_model = ForestInference.load("my_xgboost_model.ubj", is_classifier=True)
predictions = fil_model.predict(X_test)