Archive for Dezember, 2011

XML parser performance in CPython 3.3 and PyPy 1.7

Freitag, Dezember 16th, 2011

In a recent article, I compared the performance of MiniDOM and the three ElementTree implementations ElementTree, cElementTree and lxml.etree for parsing XML in CPython 3.3. Given the utterly poor performance of the pure Python library MiniDOM in this competition, I decided to give it another chance and tried the same in PyPy 1.7. Because lxml.etree and cElementTree are not available on this platform, I only ran the tests with plain ElementTree and MiniDOM. I also report the original benchmark results for CPython below for comparison.

Parser performance of XML libraries in CPython 3.3 and PyPy 1.7

While I also provide numbers regarding the memory usage of each library in this comparison, they are not directly comparable between PyPy and CPython because of the different memory management of both platforms and because the overall memory that PyPy uses right from the start is much larger than for CPython. So the relative increase in memory may or may not be an accurate way to tell what each runtime does with the memory. However, it appears that PyPy manages to kill at least the severe memory problems of MiniDOM, as the total amount of memory used for the larger files is several times smaller than that used by CPython.

Memory usage of XML trees in CPython 3.3 and PyPy 1.7

So, what do I take from this benchmark? If you have legacy MiniDOM code lying around, you want PyPy to run it. It exhibits several times better performance in terms of memory and runtime. It also performs substantially better for ElementTree than the plain Python ElementTree in CPython.

However, for fast XML processing in general, the better performance of PyPy even for plain Python ElementTree is not really all that interesting, because it is still several times slower than cElementTree or lxml.etree in CPython. That means that you will often be able to process multiple files in CPython in the time that you need for just one in PyPy, even if your actual application code that does the processing manages to get a substantial JIT speed-up in PyPy. Even worse, the GIL in PyPy will keep your code from getting a parallel speedup that you usually get with multi-threaded processing in lxml and CPython, e.g. in a web server setting.

So, as always, the decision depends on what your actual application does and which library it uses. Do your own benchmarks.

XML parser performance in Python 3.3

Freitag, Dezember 9th, 2011

For a recent bug ticket about MiniDOM, I collected some performance numbers that compare it to ElementTree, cElementTree and lxml.etree under a recent CPython 3.3 developer build, all properly compiled and optimised for Linux 64bit, using os.fork() and the resource module to get a clean measure of the memory usage for the in-memory tree. Here are the numbers:

Parsing hamlet.xml in English, 274KB:

Memory usage: 7284
xml.etree.ElementTree.parse done in 0.104 seconds
Memory usage: 14240 (+6956)
xml.etree.cElementTree.parse done in 0.022 seconds
Memory usage: 9736 (+2452)
lxml.etree.parse done in 0.014 seconds
Memory usage: 11028 (+3744)
minidom tree read in 0.152 seconds
Memory usage: 30360 (+23076)

Parsing the old testament in English (ot.xml, 3.4MB) into memory:

Memory usage: 20444
xml.etree.ElementTree.parse done in 0.385 seconds
Memory usage: 46088 (+25644)
xml.etree.cElementTree.parse done in 0.056 seconds
Memory usage: 32628 (+12184)
lxml.etree.parse done in 0.041 seconds
Memory usage: 37500 (+17056)
minidom tree read in 0.672 seconds
Memory usage: 110428 (+89984)

A 25MB XML file with Slavic Unicode text content:

Memory usage: 57368
xml.etree.ElementTree.parse done in 3.274 seconds
Memory usage: 223720 (+166352)
xml.etree.cElementTree.parse done in 0.459 seconds
Memory usage: 154012 (+96644)
lxml.etree.parse done in 0.454 seconds
Memory usage: 135720 (+78352)
minidom tree read in 6.193 seconds
Memory usage: 604860 (+547492)

And a contrived 4.5MB XML file with a lot more structure than data and no whitespace at all:

Memory usage: 11600
xml.etree.ElementTree.parse done in 3.374 seconds
Memory usage: 203420 (+191820)
xml.etree.cElementTree.parse done in 0.192 seconds
Memory usage: 36444 (+24844)
lxml.etree.parse done in 0.131 seconds
Memory usage: 62648 (+51048)
minidom tree read in 5.935 seconds
Memory usage: 527684 (+516084)

I also took the last file and pretty printed it, thus adding lots of indentation whitespace that increased the file size to 6.2MB. Here are the numbers for that:

Memory usage: 13308
xml.etree.ElementTree.parse done in 4.178 seconds
Memory usage: 222088 (+208780)
xml.etree.cElementTree.parse done in 0.478 seconds
Memory usage: 103056 (+89748)
lxml.etree.parse done in 0.199 seconds
Memory usage: 101860 (+88552)
minidom tree read in 8.705 seconds
Memory usage: 810964 (+797656)

Yes, 2MB of whitespace account for almost 300MB more memory in MiniDOM.

Here are the graphs:

XML tree memory usage in Python 3.3 for lxml, ElementTree, cElementTree and MiniDOM

XML perser performance in Python 3.3 for lxml, ElementTree, cElementTree and MiniDOM

I think it is pretty clear that minidom has basically left the scale, whereas cElementTree and lxml.etree are pretty close to each other. lxml tends to be a tad faster, and cElementTree tends to use a little less memory.