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Subject: |
Improve performance when -P (PCRE) is used in UTF-8 locales |
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Date: |
Fri, 12 Sep 2014 03:24:49 +0200 |
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User-agent: |
Mutt/1.5.23-6361-vl-r59709 (2014-07-25) |
With the patch that fixes bug 18266, grep -P works again on binary
files (with invalid UTF-8 sequences), but it is now significantly
slower than old versions (which could yield undefined behavior).
Timings with the Debian packages on my personal svn working copy
(binary + text files):
2.18-2 0.9s with -P, 0.4s without -P
2.20-3 11.6s with -P, 0.4s without -P
On this example, that's a 13x slowdown! Though the performance issue
would better be fixed in libpcre3, I suppose that it is not so simple
and won't occur any time soon. Things could be done in grep:
1. Ignore -P when the pattern would have the same meaning without -P
(patterns could also be transformed, e.g. "a\d+b" -> "a[0-9]\+b",
at least for the simplest cases).
2. Call PCRE in the C locale when this is equivalent.
3. Transform invalid bytes to null bytes in-place before the PCRE
call. This changes the current semantic, but:
* the semantic on invalid bytes has never been specified, AFAIK;
* the best *practical* behavior may not be the current one
(I personally prefer to be able to match invalid bytes, just
like one can match top-bit-set characters in the C locale, and
seeing such invalid bytes as equivalent to null bytes would
not be a problem for most users, IMHO -- things can also be
configurable).
--
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
100% accessible validated (X)HTML - Blog: <https://www.vinc17.net/blog/>
Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)
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Subject: |
Re: bug#18454: Improve performance when -P (PCRE) is used in UTF-8 locales |
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Date: |
Tue, 23 Nov 2021 19:36:11 -0800 |
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Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.3.0 |
On 9/30/14 12:39, Paul Eggert wrote:
GNU grep is smart
enough to start matching at character boundaries without checking the
validity of the input data. This helps it run faster. However, because
libpcre requires a validity prepass, grep -P must slow down and do the
validity check one way or another. Grep does this only when libpcre is
used, and that's one reason grep -P is slower than plain grep.
Now that Grep master on Savannah has been changed to use PCRE2 instead
of PCRE, the 'grep -P' performance problem seems to have been fixed, in
that the following commands now take about the same amount of time:
grep -P zzzyyyxxx 10840.pdf
pcre2grep -U zzzyyyxxx 10840.pdf
where the file is from <http://research.nhm.org/pdfs/10840/10840.pdf>.
Formerly, 'grep -P' was about 10x slower on this test.
My guess is that the grep -P performance boost comes from bleeding-edge
grep using PCRE2's PCRE2_MATCH_INVALID_UTF option.
I'm closing this old bug report <https://bugs.gnu.org/18454>. We can
always reopen it if there are still performance issues that I've missed.
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