[issue10376] ZipFile unzip is unbuffered
New submission from James Hutchison jamesghutchison@gmail.com:
The Unzip module is always unbuffered (tested v.3.1.2 Windows XP, 32-bit). This means that if one has to do many small reads it is a lot slower than reading a chunk of data to a buffer and then reading from that buffer. It seems logical that the unzip module should default to buffered reading and/or have a buffered argument. Likewise, the documentation should clarify that there is no buffering involved when doing a read, which runs contrary to the default behavior of a normal read.
start Zipfile read done 27432 reads done took 0.859 seconds start buffered Zipfile read done 27432 reads done took 0.072 seconds start normal read (default buffer) done 27432 reads done took 0.139 seconds start buffered normal read done 27432 took 0.137 seconds
---------- assignee: docs@python components: Documentation, IO, Library (Lib) messages: 120871 nosy: Jimbofbx, docs@python priority: normal severity: normal status: open title: ZipFile unzip is unbuffered type: performance versions: Python 2.5, Python 2.6, Python 2.7, Python 3.1
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James Hutchison jamesghutchison@gmail.com added the comment:
I should clarify that this is the zipfile constructor I am using:
zipfile.ZipFile(filename, mode='r', allowZip64=True);
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Changes by Terry J. Reedy tjreedy@udel.edu:
---------- versions: +Python 3.2, Python 3.3 -Python 2.5, Python 2.6, Python 3.1
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Changes by Xuanji Li xuanji@gmail.com:
---------- nosy: +xuanji
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Changes by Serhiy Storchaka storchaka@gmail.com:
---------- nosy: +storchaka
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Serhiy Storchaka storchaka@gmail.com added the comment:
Actually reading from the zip file is buffered (at least 4 KiB of uncompressed data at a time).
Can you give tests, scripts and data, which show the problem?
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James Hutchison jamesghutchison@gmail.com added the comment:
See attached, which will open a zipfile that contains one file and reads it a bunch of times using unbuffered and buffered idioms. This was tested on windows using python 3.2
You're in charge of coming up with a file to test it on. Sorry.
Example output:
Enter filename: test.zip Timing unbuffered read, 5 bytes at a time. 10 loops took 6.671999931335449 Timing buffered read, 5 bytes at a time (4000 byte buffer). 10 loops took 0.7350001335144043
---------- Added file: http://bugs.python.org/file25432/zipfiletest.py
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Serhiy Storchaka storchaka@gmail.com added the comment:
This is not because zipfile module is unbuffered. This is the difference between expensive function call and cheap bytes slicing. Replace `zf.open(namelist [0])` to `io.BufferedReader(zf.open(namelist [0]))` to see the effect of a good buffering. In 3.2 zipfile read() implemented not optimal, so it slower (twice), but in 3.3 it will be almost as fast as using io.BufferedReader. It is still several times more slowly than bytes slicing, but there's nothing you can do with it.
Here is a patch, which is speeds up (+20%) the reading from a zip file by small chunks. Microbenchmark:
./python -m zipfile -c test.zip python ./python -m timeit -n 1 -s "import zipfile;zf=zipfile.ZipFile('test.zip')" "with zf.open('python') as f:" " while f.read(1):pass"
Python 3.3 (vanilla): 1 loops, best of 3: 36.4 sec per loop Python 3.3 (patched): 1 loops, best of 3: 30.1 sec per loop Python 3.3 (with io.BufferedReader): 1 loops, best of 3: 30.2 sec per loop And, for comparison, Python 3.2: 1 loops, best of 3: 74.5 sec per loop
---------- components: -Documentation keywords: +patch versions: -Python 2.7, Python 3.2 Added file: http://bugs.python.org/file25530/zipfile_optimize_read.patch
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Changes by STINNER Victor victor.stinner@gmail.com:
---------- nosy: +pitrou
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Changes by Martin v. Löwis martin@v.loewis.de:
Added file: http://bugs.python.org/file25565/zipfile_optimize_read.patch
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Serhiy Storchaka storchaka@gmail.com added the comment:
Thank you, Martin, now I understood why not work Rietveld review.
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Serhiy Storchaka storchaka@gmail.com added the comment:
The patch updated to reflect Martin's stylistic comments.
Sorry for the delay, Martin. I have not received an email with your review from 2012-05-13, and only today accidentally discovered your comments in Rietveld. It seems to have been some bug in Rietveld.
---------- Added file: http://bugs.python.org/file25769/zipfile_optimize_read_2.patch
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Serhiy Storchaka storchaka@gmail.com added the comment:
Martin, now the patch is good?
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Serhiy Storchaka storchaka@gmail.com added the comment:
Any chance to commit the patch before final feature freeze?
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Changes by Antoine Pitrou pitrou@free.fr:
---------- assignee: docs@python -> nosy: +nadeem.vawda stage: -> patch review
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Nadeem Vawda nadeem.vawda@gmail.com added the comment:
Patch looks fine to me.
Antoine, can you commit this? I'm currently away from the computer that has my SSH key on it.
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Roundup Robot devnull@psf.upfronthosting.co.za added the comment:
New changeset 0e8285321659 by Antoine Pitrou in branch 'default': On behalf of Nadeem Vawda: issue #10376: micro-optimize reading from a Zipfile. http://hg.python.org/cpython/rev/0e8285321659
---------- nosy: +python-dev
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Antoine Pitrou pitrou@free.fr added the comment:
Antoine, can you commit this?
Ok, done.
---------- resolution: -> fixed stage: patch review -> committed/rejected status: open -> closed
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participants (9)
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Antoine Pitrou -
James Hutchison
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Martin v. Löwis
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Nadeem Vawda
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Roundup Robot
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Serhiy Storchaka
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STINNER Victor
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Terry J. Reedy
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Xuanji Li