Message 186405 - Python tracker

➜

This issue tracker has been migrated to GitHub , and is currently read-only.
For more information, see the GitHub FAQs in the Python's Developer Guide.

Author	vstinner
Recipients	asvetlov, christian.heimes, gregory.p.smith, jcea, mark.dickinson, pitrou, python-dev, rhettinger, serhiy.storchaka, skrah, tim.peters, vstinner
Date	2013年04月09日.13:40:01
SpamBayes Score	-1.0
Marked as misclassified	Yes
Message-id	<CAMpsgwaM8t1zVf5CHXENZ3PcLuyizuf_OOVUMETK2H3PW+xf0w@mail.gmail.com>
In-reply-to	<1365514473.29.0.657560354102.issue16427@psf.upfronthosting.co.za>

Content
> (dicts and sets have a sophisticated probing algorithm which takes into account the whole hash value, not the masked one). Correct, so your specific example should not be a problem since the whole hash value is different for the 6 hash values. > Now to know if that may produce slowdowns in some situations... pybench and perf.py can be used to measure performances of the patch. The speedup may not be detected, but a slowdown would be detected at least. 2013年4月9日 Antoine Pitrou <report@bugs.python.org>: > > Antoine Pitrou added the comment: > > Well, the quality of the hash function is clearly reduced: > >>>> hash("abcdefgh") & 0xff > 206 >>>> hash("abcdefgi") & 0xff > 206 >>>> hash("abcdefgj") & 0xff > 206 >>>> hash("abxxxxxx") & 0xff > 206 >>>> hash("aaaaaa11") & 0xff > 206 >>>> hash("aaaaaa12") & 0xff > 206 > > > Now to know if that may produce slowdowns in some situations... (dicts and sets have a sophisticated probing algorithm which takes into account the whole hash value, not the masked one). > > ---------- > > _______________________________________ > Python tracker <report@bugs.python.org> > <http://bugs.python.org/issue16427> > _______________________________________

Content

> (dicts and sets have a sophisticated probing algorithm which takes into account the whole hash value, not the masked one).
Correct, so your specific example should not be a problem since the
whole hash value is different for the 6 hash values.
> Now to know if that may produce slowdowns in some situations...
pybench and perf.py can be used to measure performances of the patch.
The speedup may not be detected, but a slowdown would be detected at
least.
2013年4月9日 Antoine Pitrou <report@bugs.python.org>:
>
> Antoine Pitrou added the comment:
>
> Well, the quality of the hash function is clearly reduced:
>
>>>> hash("abcdefgh") & 0xff
> 206
>>>> hash("abcdefgi") & 0xff
> 206
>>>> hash("abcdefgj") & 0xff
> 206
>>>> hash("abxxxxxx") & 0xff
> 206
>>>> hash("aaaaaa11") & 0xff
> 206
>>>> hash("aaaaaa12") & 0xff
> 206
>
>
> Now to know if that may produce slowdowns in some situations... (dicts and sets have a sophisticated probing algorithm which takes into account the whole hash value, not the masked one).
>
> ----------
>
> _______________________________________
> Python tracker <report@bugs.python.org>
> <http://bugs.python.org/issue16427>
> _______________________________________

History
Date	User	Action	Args
2013年04月09日 13:40:02	vstinner	set	recipients: + vstinner, tim.peters, rhettinger, gregory.p.smith, jcea, mark.dickinson, pitrou, christian.heimes, asvetlov, skrah, python-dev, serhiy.storchaka
2013年04月09日 13:40:02	vstinner	link	issue16427 messages
2013年04月09日 13:40:01	vstinner	create

homepage