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pylibcudf#

pylibcudf is a lightweight Cython wrapper around libcudf. It aims to provide a near-zero overhead interface to accessing libcudf in Python. It should be possible to achieve near-native C++ performance using Cythonized code calling pylibcudf, while also allowing fairly performant usage from Python. In addition to these requirements, pylibcudf must also integrate naturally with other Python libraries. In other words, it should interoperate fairly transparently with standard Python containers, community protocols like __cuda_array_interface__, and common vocabulary types like CuPy arrays.

General Design Principles#

To satisfy the goals of pylibcudf, we impose the following set of design principles:

  • Every public function or method should be cpdefed. This allows it to be used in both Cython and Python code. This incurs some slight overhead over cdef functions, but we assume that this is acceptable because 1) the vast majority of users will be using pure Python rather than Cython, and 2) the overhead of a cpdef function over a cdef function is on the order of a nanosecond, while CUDA kernel launch overhead is on the order of a microsecond, so these function overheads should be washed out by typical usage of pylibcudf.

  • Every variable used should be strongly typed and either be a primitive type (int, float, etc) or a cdef class. Any enums in C++ should be mirrored using cpdef enum, which will create both a C-style enum in Cython and a PEP 435-style Python enum that will automatically be used in Python.

  • All typing in code should be written using Cython syntax, not PEP 484 Python typing syntax. Not only does this ensure compatibility with Cython < 3, but even with Cython 3 PEP 484 support remains incomplete as of this writing.

  • All cudf code should interact only with pylibcudf, never with libcudf directly.

  • All imports should be relative so that pylibcudf can be easily extracted from cudf later

    • Exception: All imports of libcudf API bindings in cudf._lib.cpp should use absolute imports of cudf._lib.cpp as libcudf. We should convert the cpp directory into a proper package so that it can be imported as libcudf in that fashion. When moving pylibcudf into a separate package, it will be renamed to libcudf and only the imports will need to change.

  • Ideally, pylibcudf should depend on nothing other than rmm and pyarrow. This will allow it to be extracted into a a largely standalone library and used in environments where the larger dependency tree of cudf may be cumbersome.

Relationship to libcudf#

In general, the relationship between pylibcudf and libcudf can be understood in terms of two components, data structures and algorithms.

Data Structures#

Typically, every type in libcudf should have a mirror Cython cdef class with an attribute self.c_obj: unique_ptr[${underlying_type}] that owns an instance of the underlying libcudf type. Each type should also implement a corresponding method cdef ${cython_type} from_libcudf(${underlying_type} dt) to enable constructing the Cython object from an underlying libcudf instance. Depending on the nature of the type, the function may need to accept a unique_ptr and take ownership e.g. cdef ${cython_type} from_libcudf(unique_ptr[${underlying_type}] obj). This will typically be the case for types that own GPU data, may want to codify further.

For example, libcudf::data_type maps to pylibcudf.DataType, which looks like this (implementation omitted):

cdef class DataType:
    cdef data_type c_obj

    cpdef TypeId id(self)
    cpdef int32_t scale(self)

    @staticmethod
    cdef DataType from_libcudf(data_type dt)

This allows pylibcudf functions to accept a typed DataType parameter and then trivially call underlying libcudf algorithms by accessing the argument’s c_obj.

pylibcudf Tables and Columns#

The primary exception to the above set of rules are libcudf’s core data owning types, cudf::table and cudf::column. libcudf uses modern C++ idioms based on smart pointers to avoid resource leaks and make code exception-safe. To avoid passing around raw pointers, and to ensure that ownership semantics are clear, libcudf has separate view types corresponding to data owning types. For example, cudf::column owns data, while cudf::column_view represents an view on a column of data and cudf::mutable_column_view represents a mutable view. A column_view need not actually reference data owned by a cudf::column; any memory buffer will do. This separation allows libcudf algorithms to clearly communicate ownership expectations and allows multiple views into the same data to coexist.

While libcudf algorithms accept views as inputs, any algorithms that allocate data must return cudf::column and cudf::table objects. libcudf’s ownership model is problematic for pylibcudf, which must be able to seamlessly interoperate with data provided by other Python libraries like PyTorch or Numba. Therefore, pylibcudf employs the following strategy:

  • pylibcudf defines the gpumemoryview type, which (analogous to the Python memoryview type) represents a view into memory owned by another object that it keeps alive using Python’s standard reference counting machinery. A gpumemoryview is constructible from any object implementing the CUDA Array Interface protocol.

    • This type will eventually be generalized for reuse outside of pylibcudf.

  • pylibcudf defines its own Table and Column classes.

    • A Table maintains Python references to the Columns it contains, so multiple Tables may share the same Column.

    • A Column consists of gpumemoryviews of its data buffers (which may include children for nested types) and its null mask.

  • pylibcudf.Table and pylibcudf.Column provide easy access to cudf::table_view and cudf::column_view objects viewing the same columns/memory. These can be then be used when implementing any pylibcudf algorithm in terms of the underlying libcudf algorithm. Specifically, each of these classes owns an instance of the libcudf view type and provides a method view that may be used to access a pointer to that object to be passed to libcudf.

Algorithms#

pylibcudf algorithms should look almost exactly like libcudf algorithms. Any libcudf function should be mirrored in pylibcudf with an identical signature and libcudf types mapped to corresponding pylibcudf types. All calls to libcudf algorithms should perform any requisite Python preprocessing early, then release the GIL prior to calling libcudf. For example, here is the implementation of gather:

cpdef Table gather(
    Table source_table,
    Column gather_map,
    OutOfBoundsPolicy bounds_policy
):
    cdef unique_ptr[table] c_result
    with nogil:
        c_result = move(
            cpp_copying.gather(
                source_table.view(),
                gather_map.view(),
                bounds_policy
            )
        )
    return Table.from_libcudf(move(c_result))

There are a couple of notable points from the snippet above:

  • The object returned from libcudf is immediately converted to a pylibcudf type.

  • cudf::gather accepts a cudf::out_of_bounds_policy enum parameter. OutOfBoundsPolicy is an alias for this type in pylibcudf that matches our Python naming conventions (CapsCase instead of snake_case).

Testing#

When writing pylibcudf tests, it is important to remember that all the APIs should be tested in the C++ layer in libcudf already. The primary purpose of pylibcudf tests is to ensure the correctness of the bindings; the correctness of the underlying implementation should generally be validated in libcudf. If pylibcudf tests uncover a libcudf bug, a suitable libcudf test should be added to cover this case rather than relying solely on pylibcudf testing.

pylibcudf’s conftest.py contains some standard parametrized dtype fixture lists that may in turn be used to parametrize other fixtures. Fixtures allocating data should leverage these dtype lists wherever possible to simplify testing across the matrix of important types. Where appropriate, new fixture lists may be added.

To run tests as efficiently as possible, the test suite should make generous use of fixtures. The simplest general structure to follow is for pyarrow array/table/scalar fixtures to be parametrized by one of the dtype list. Then, a corresponding pylibcudf fixture may be created using a simple from_arrow call. This approach ensures consistent global coverage across types for various tests.

In general, pylibcudf tests should prefer validating against a corresponding pyarrow implementation rather than hardcoding data. This approach is more resilient to changes to input data, particularly given the fixture strategy outlined above. Standard tools for comparing between pylibcudf and pyarrow types are provided in the utils module.

Here is an example demonstrating the above points:

import pyarrow as pa
import pyarrow.compute as pc
import pytest
from cudf._lib import pylibcudf as plc
from utils import assert_column_eq

# The pa_dtype fixture is defined in conftest.py.
@pytest.fixture(scope="module")
def pa_column(pa_dtype):
    pa.array([1, 2, 3])


@pytest.fixture(scope="module")
def column(pa_column):
    return plc.interop.from_arrow(pa_column)


def test_foo(pa_column, column):
    index = 1
    result = plc.foo(column)
    expected = pa.foo(pa_column)

    assert_column_eq(result, expected)

Some guidelines on what should be tested:

  • Tests SHOULD comprehensively cover the API, including all possible combinations of arguments required to ensure good test coverage.

  • pylibcudf SHOULD NOT attempt to stress test large data sizes, and SHOULD instead defer to libcudf tests.

    • Exception: In special cases where constructing suitable large tests is difficult in C++ (such as creating suitable input data for I/O testing), tests may be added to pylibcudf instead.

  • Nullable data should always be tested.

  • Expected exceptions should be tested. Tests should be written from the user’s perspective in mind, and if the API is not currently throwing the appropriate exception it should be updated.

    • Important note: If the exception should be produced by libcudf, the underlying libcudf API should be updated to throw the desired exception in C++. Such changes may require consultation with libcudf devs in nontrivial cases. This issue provides an overview and an indication of acceptable exception types that should cover most use cases. In rare cases a new C++ exception may need to be introduced in error.hpp. If so, this exception will also need to be mapped to a suitable Python exception in exception_handler.pxd.

Some guidelines on how best to use pytests.

  • By default, fixtures producing device data containers should be of module scope and treated as immutable by tests. Allocating data on the GPU is expensive and slows tests. Almost all pylibcudf operations are out of place operations, so module-scoped fixtures should not typically be problematic to work with. Session-scoped fixtures would also work, but they are harder to reason about since they live in a different module, and if they need to change for any reason they could affect an arbitrarily large number of tests. Module scope is a good balance.

  • Where necessary, mutable fixtures should be named as such (e.g. mutable_col) and be of function scope. If possible, they can be implemented as simply making a copy of a corresponding module-scope immutable fixture to avoid duplicating the generation logic.

Tests should be organized corresponding to pylibcudf modules, i.e. one test module for each pylibcudf module.

The following sections of the cuDF Python testing guide also generally apply to pylibcudf unless superseded by any statements above:

Miscellaneous Notes#

Cython Scoped Enums#

Cython 3 introduced support for scoped enumerations. However, this support has some bugs as well as some easy pitfalls. Our usage of enums is intended to minimize the complexity of our code while also working around Cython’s limitations.

Warning

The guidance in this section may change often as Cython is updated and our understanding of best practices evolves.

  • All pxd files that declare a C++ enum should use cpdef enum class declarations.

    • Reason: This declaration makes the C++ enum available in Cython code while also transparently creating a Python enum.

  • Any pxd file containing only C++ declarations must still have a corresponding pyx file if any of the declarations are scoped enums.

    • Reason: The creation of the Python enum requires that Cython actually generate the necessary Python C API code, which will not happen if only a pxd file is present.

  • If a C++ enum will be part of a pylibcudf module’s public API, then it should be imported (not cimported) directly into the pyx file and aliased with a name that matches our Python class naming conventions (CapsCase) instead of our C++ naming convention (snake_case).

  • Reason: We want to expose the enum to both Python and Cython consumers of the module. As a side effect, this aliasing avoids this Cython bug.

  • Note: Once the above Cython bug is resolved, the enum should also be aliased into the pylibcudf pxd file when it is cimported so that Python and Cython usage will match.

Here is an example of appropriate enum usage.

# cpp/copying.pxd
cdef extern from "cudf/copying.hpp" namespace "cudf" nogil:
    # cpdef here so that we export both a cdef enum class and a Python enum.Enum.
    cpdef enum class out_of_bounds_policy(bool):
        NULLIFY
        DONT_CHECK


# cpp/copying.pyx
# This file is empty, but is required to compile the Python enum in cpp/copying.pxd


# pylibcudf/copying.pxd

# cimport the enum using the exact name
# Once https://github.com/cython/cython/issues/5609 is resolved,
# this import should instead be
# from cudf._lib.cpp.copying cimport out_of_bounds_policy as OutOfBoundsPolicy
from cudf._lib.cpp.copying cimport out_of_bounds_policy


# pylibcudf/copying.pyx
# Access cpp.copying members that aren't part of this module's public API via
# this module alias
from cudf._lib.cpp cimport copying as cpp_copying
from cudf._lib.cpp.copying cimport out_of_bounds_policy

# This import exposes the enum in the public API of this module.
# It requires a no-cython-lint tag because it will be unused: all typing of
# parameters etc will need to use the Cython name `out_of_bounds_policy` until
# the Cython bug is resolved.
from cudf._lib.cpp.copying import \
    out_of_bounds_policy as OutOfBoundsPolicy  # no-cython-lint

Handling overloaded functions in libcudf#

As a C++ library, libcudf makes extensive use of function overloading. For example, both of the following functions exist in libcudf:

std::unique_ptr<table> empty_like(table_view const& input_table);
std::unique_ptr<column> empty_like(column_view const& input);

However, Cython does not directly support overloading in this way, instead following Pythonic semantics where every function name must uniquely identify the function. Therefore, Cython’s fused types should be used when implementing pylibcudf wrappers of overloaded functions like the above. Fused types are Cython’s version of generic programming and in this case amount to writing templated functions that compile into separate copies corresponding to the different C++ overloads. For the above functions, the equivalent Cython function is

ctypedef fused ColumnOrTable:
    Table
    Column

cpdef ColumnOrTable empty_like(ColumnOrTable input)

Cython supports specializing the contents of fused-type functions based on the argument types, so any type-specific logic may be encoded using the appropriate conditionals. See the pylibcudf source for examples of how to implement such functions.

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