JollyJack

JollyJack is a high-performance Parquet reader designed to load data directly into NumPy arrays and PyTorch tensors with minimal overhead.

Features

• Load Parquet straight into NumPy arrays or PyTorch tensors (fp16, fp32, fp64, int32, int64)
• Up to 6× faster and with lower memory use than vanilla PyArrow
• Compatibility with PalletJack
• Optional io_uring + O_DIRECT backend for I/O-bound workloads

Known limitations

• Data must not contain null values
• Destination NumPy arrays and PyTorch tensors must be column-major (Fortran-style)

Selecting a reader backend

By default, the reader uses the regular file API via parquet::ParquetFileReader. In most cases, this is the recommended choice.

An alternative reader backend based on io_uring is also available. It can provide better performance, especially for very large datasets and when used together with O_DIRECT.

To enable the alternative backend, set the JJ_READER_BACKEND environment variable to one of the following values:

• io_uring - Uses io_uring for async I/O with the page cache
• io_uring_odirect - Uses io_uring with O_DIRECT (bypasses the page cache)

Performance tuning tips

JollyJack performance is primarily determined by I/O, threading, and memory allocation behavior. The optimal configuration depends on whether your workload is I/O-bound or memory-/CPU-bound.

Threading strategy

• JollyJack can be safely called concurrently from multiple threads.
• Parallel reads usually improve throughput, but oversubscribing threads can cause contention and degrade performance.

Reuse destination arrays

• Reusing NumPy arrays or PyTorch tensors avoids repeated memory allocation.
• While allocation itself is fast, it can trigger kernel contention and degrade performance.

Large datasets (exceed filesystem cache)

For datasets larger than the available page cache, performance is typically I/O-bound.

Recommended configuration:

• use_threads = True, pre_buffer = True, JJ_READER_BACKEND = io_uring_odirect

This combination bypasses the page cache, reduces double-buffering, and allows deeper I/O queues via io_uring.

Small datasets (fit in filesystem cache)

For datasets that comfortably fit in RAM, performance is typically CPU- or memory-bound.

Recommended configuration:

• use_threads = False, pre_buffer = False, and the default reader backend (no io_uring).

Pre-buffering and cache_options

When pre_buffer=True, Arrow merges nearby column ranges and reads them into temporary buffers. The default maximum merged range is 32 MB (range_size_limit).

Arrow supports several memory allocators (mimalloc, jemalloc, system). With mimalloc (the default on most platforms), allocations above ~16 MB go straight to the OS (mmap/munmap) instead of the internal arena. This means the memory cannot be reused between calls, and each call pays the cost of mapping and zeroing fresh pages. Other allocators may behave similarly.

To avoid this, lower range_size_limit so that merged ranges fit inside the allocator's arena:

cache_options = pa.CacheOptions(
    hole_size_limit=8192,           # default
    range_size_limit=16*1024*1024,  # 16 MB, fits in mimalloc arena
    lazy=False,
)
jj.read_into_numpy(
    source=path,
    metadata=None,
    np_array=np_array,
    row_group_indices=[0],
    column_indices=range(n_columns),
    pre_buffer=True,
    cache_options=cache_options,
)

To debug allocator issues with mimalloc, run with MIMALLOC_SHOW_STATS=1 and MIMALLOC_VERBOSE=1. This prints allocation statistics at process exit.

Pre-buffering and ARROW_IO_THREADS

When pre_buffer=True, Arrow dispatches reads to its IO thread pooll, configured via the ARROW_IO_THREADS environment variable (default: 8). Tuning this value may improve performance.

Requirements

• pyarrow ~= 23.0.0

JollyJack builds on top of PyArrow. While the source package may work with newer versions, the prebuilt binary wheels are built and tested against pyarrow 23.x.

Installation

pip install jollyjack

How to use

Generating a sample Parquet file

import jollyjack as jj
import pyarrow.parquet as pq
import pyarrow as pa
import numpy as np

from pyarrow import fs

chunk_size = 3
n_row_groups = 2
n_columns = 5
n_rows = n_row_groups * chunk_size
path = "my.parquet"

data = np.random.rand(n_rows, n_columns).astype(np.float32)
pa_arrays = [pa.array(data[:, i]) for i in range(n_columns)]
schema = pa.schema([(f"column_{i}", pa.float32()) for i in range(n_columns)])
table = pa.Table.from_arrays(pa_arrays, schema=schema)
pq.write_table(
    table,
    path,
    row_group_size=chunk_size,
    use_dictionary=False,
    write_statistics=True,
    store_schema=False,
    write_page_index=True,
)

Generating a NumPy array to read into

# Create an array of zeros
np_array = np.zeros((n_rows, n_columns), dtype="f", order="F")

Reading an entire file into a NumPy array

pr = pq.ParquetReader()
pr.open(path)

row_begin = 0
row_end = 0

for rg in range(pr.metadata.num_row_groups):
    row_begin = row_end
    row_end = row_begin + pr.metadata.row_group(rg).num_rows

    # To define which subset of the NumPy array we want read into,
    # we need to create a view which shares underlying memory with the target NumPy array
    subset_view = np_array[row_begin:row_end, :]
    jj.read_into_numpy(
        source=path,
        metadata=pr.metadata,
        np_array=subset_view,
        row_group_indices=[rg],
        column_indices=range(pr.metadata.num_columns),
    )

# Alternatively
with fs.LocalFileSystem().open_input_file(path) as f:
    jj.read_into_numpy(
        source=f,
        metadata=None,
        np_array=np_array,
        row_group_indices=range(pr.metadata.num_row_groups),
        column_indices=range(pr.metadata.num_columns),
    )

Reading columns in reverse order

with fs.LocalFileSystem().open_input_file(path) as f:
    jj.read_into_numpy(
        source=f,
        metadata=None,
        np_array=np_array,
        row_group_indices=range(pr.metadata.num_row_groups),
        column_indices={
            i: pr.metadata.num_columns - i - 1 for i in range(pr.metadata.num_columns)
        },
    )

Reading column 3 into multiple destination columns

with fs.LocalFileSystem().open_input_file(path) as f:
    jj.read_into_numpy(
        source=f,
        metadata=None,
        np_array=np_array,
        row_group_indices=range(pr.metadata.num_row_groups),
        column_indices=((3, 0), (3, 1)),
    )

Sparse reading

np_array = np.zeros((n_rows, n_columns), dtype="f", order="F")
with fs.LocalFileSystem().open_input_file(path) as f:
    jj.read_into_numpy(
        source=f,
        metadata=None,
        np_array=np_array,
        row_group_indices=[0],
        row_ranges=[slice(0, 1), slice(4, 6)],
        column_indices=range(pr.metadata.num_columns),
    )
print(np_array)

Using cache options

np_array = np.zeros((n_rows, n_columns), dtype="f", order="F")
cache_options = pa.CacheOptions(
    hole_size_limit=8192,           # default
    range_size_limit=16*1024*1024,  # 16 MB, fits in mimalloc arena
    lazy=False,
)
with fs.LocalFileSystem().open_input_file(path) as f:
    jj.read_into_numpy(
        source=f,
        metadata=None,
        np_array=np_array,
        row_group_indices=[0],
        row_ranges=[slice(0, 1), slice(4, 6)],
        column_indices=range(pr.metadata.num_columns),
        cache_options=cache_options,
        pre_buffer=True,
    )
print(np_array)

Generating a PyTorch tensor to read into

import torch

# Create a tensor and transpose it to get Fortran-style order
tensor = torch.zeros(n_columns, n_rows, dtype=torch.float32).transpose(0, 1)

Reading an entire file into a PyTorch tensor

pr = pq.ParquetReader()
pr.open(path)

jj.read_into_torch(
    source=path,
    metadata=pr.metadata,
    tensor=tensor,
    row_group_indices=range(pr.metadata.num_row_groups),
    column_indices=range(pr.metadata.num_columns),
    pre_buffer=True,
    use_threads=True,
)

print(tensor)