This project takes a Python XML/HTML/SVG writer originally implemented in Python and migrates it to C++ with two popular interfaces, firstly pybind11 and secondly as a traditional C extension. The aim was to measure the performance of both interfaces and expose the trade offs between runtime performance and cost of development.
A XML writer makes it easy to generate well formed and correctly encoded XML and XHTML thus:
with XmlWrite.XhtmlStream() as xS:
with XmlWrite.Element(xS, 'head'):
with XmlWrite.Element(xS, 'title'):
xS.characters('Virtual Library')
with XmlWrite.Element(xS, 'body'):
with XmlWrite.Element(xS, 'p'):
xS.characters(u'Moved to ')
with XmlWrite.Element(xS, 'a', {'href' : 'http://example.org/'}):
xS.characters('example.org')
xS.characters(' since >"2015".')
Then xS.getvalue() gives this:
<?xml version='1.0' encoding="utf-8"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html lang="en" xml:lang="en" xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Virtual Library</title>
</head>
<body>
<p>Moved to <a href="http://example.org/">example.org</a> since >"2015".</p>
</body>
</html>
The nature of this code is that many small objects are constructed that mostly have short lifetimes. If implemented in C/C++ then the cost of crossing the boundary from Python to C/C++ can be significant. For each element the Python interpreter makes at least five calls to a C/C++ implementation: __new__, __init__, __enter__, __exit__, __del__.
This project is based on this pybind11 example.
The main aim of this project was to establish the performance of:
- The original pure Python implementation.
- The new C++ baseline implementation.
- The C++ baseline implementation with a pybind11 interface.
- The C++ baseline implementation with a traditional CPython extension interface.
pytest-benchmark was used to measure the Python test code. The C++ baseline implementation was benchmarked within a C++ executable.
This measured the cost of creating XHTML of varying sizes. There are two tests for each size as the document is created with no attributes on each element, then with some attributes (see BENCHMARK_ATTRIBUTES in tests/unit/_test_XmlWrite.py). The value of the second test is that a far bigger payload must be transported and converted between Python and C/C++.
We reduce the execution time to μs per element written with these size of documents:
- A "Small" document with 128 XML elements. About 61 kB without element attributes or 100 kB with attributes.
- A "Large" document with 2560 XML elements. About 1 MB without element attributes or 2 MB with attributes.
- A "Very large" document with 32768 XML elements. About 15 MB without element attributes or 24 MB with attributes.
The time to write each element in μs is shown below, first with no element attributes:
So pybind11 is about twice as fast a pure Python, C++ is twice as fast again and the CPython extension is around the C++ time plus 15 to 25% So CPython interface provides significantly less friction than the pybind11 one.
The story is similar when writing out attributes on each element where a much bigger payload has to be transferred from Python to C++:
Subtracting the execution time of the underlying C++ code gives the 'friction' caused by the pybind11 and C extension:
The C extension gives about 1/4 the friction of the pybnd11 one.
The pybind11 interface and the C++ code took about two to three days to write. The C Extension on top of the existing C++ code took about four to five days to write.
- The pure C++ implementation is about four times faster than the pure Python one.
- pybind11 slows this C++ implementation down by a factor of two.
- The C extension slows this C++ implementation down by a factor of aroung 1.2.
- The 'friction' caused by the C extension is about 1/4 that of pybind11.
Of course these figures are only reflective of this particular problem.
Wrote a C extension that uses the same C++ code as pybind11.
Added automatic migration of python documentation strings with pydoc2cppdoc.py.
Made public around: Tue 5 Dec 2017 11:45:14 GMT
commit 517f5267709029fe9f651bf3e0b88655a40ae052
Author: Paul Ross <[email protected]>
Date: Tue Dec 5 11:44:17 2017 +0000
Last private commit.
commit f4267ff0eefe9a99c27a9b84ff22087e1ff29f1c
Author: paulross <[email protected]>
Date: Mon Nov 27 09:19:37 2017 +0000
Initial commit.
On Unix (Linux, OS X)
- clone this repository
pip install ./xmlwriter
On Windows (Requires Visual Studio 2015)
-
For Python 3.5:
- clone this repository
pip install ./xmlwriter
-
For earlier versions of Python, including Python 2.7:
Pybind11 requires a C++11 compliant compiler (i.e. Visual Studio 2015 on Windows). Running a regular
pip installcommand will detect the version of the compiler used to build Python and attempt to build the extension with it. We must force the use of Visual Studio 2015.- clone this repository
"%VS140COMNTOOLS%\..\..\VC\vcvarsall.bat" x64set DISTUTILS_USE_SDK=1set MSSdk=1pip install ./xmlwriter
Note that this requires the user building
xmlwriterto have registry edition rights on the machine, to be able to run thevcvarsall.batscript.
On Windows, the Visual C++ 2015 redistributable packages are a runtime requirement for this project. It can be found here.
If you use the Anaconda python distribution, you may require the Visual Studio runtime as a platform-dependent runtime requirement for you package:
requirements:
build:
- python
- setuptools
- pybind11
run:
- python
- vs2015_runtime # [win]Documentation for the example project is generated using Sphinx. Sphinx has the ability to automatically inspect the signatures and documentation strings in the extension module to generate beautiful documentation in a variety formats. The following command generates HTML-based reference documentation; for other formats please refer to the Sphinx manual:
cd xmlwriter/docsmake html
pybind11 is provided under a BSD-style license that can be found in the LICENSE file. By using, distributing, or contributing to this project, you agree to the terms and conditions of this license.
import cXmlWrite
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