GNU bug report logs - #21696
[PATCH 1/2] grep: improvement of performance of grep -Fw

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Norihiro Tanaka <noritnk <at> kcn.ne.jp>
To: <bug-grep <at> gnu.org>
Subject: [PATCH 1/2] grep: improvement of performance of grep -Fw
Date: Sat, 17 Oct 2015 10:13:36 +0900

[Message part 1 (text/plain, inline)]

I found that grep -Fw is extremely slow in spite of whether in
multibyte locales or not.

$ yes 'abcdefg hijklmn opqrstu vwxyz' | head -100000 >k
$ time -p env LC_ALL=C grep -Fw vwxy k
real 14.03
user 12.51
sys 0.74
$ time -p env LC_ALL=ja_JP.eucJP grep -Fw vwxy k
real 14.29
user 12.67
sys 0.50

$ time -p env LC_ALL=C grep -w vwxy k
real 0.11
user 0.01
sys 0.09
$ time -p env LC_ALL=ja_JP.eucJP grep -w vwxy k
real 0.89
user 0.71
sys 0.15

First patch fixes the problem.  Second patch changes as using grep
matcher for grep -Fw in single byte locales.

In single byte locales, DFA (not regex) is also used for words matching,
and it is very fast as above result.

[0001-grep-improvement-of-performance-of-grep-Fw.patch (text/plain, attachment)]

[0002-grep-use-grep-matcher-for-grep-Fw-in-single-byte-loc.patch (text/plain, attachment)]

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