GNU bug report logs - #19358
[PATCH 1/3] grep: use Aho-Corasick algorithm to search multiple fixed words

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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 1/3] grep: use Aho-Corasick algorithm to search multiple fixed
 words
Date: Sat, 13 Dec 2014 00:58:02 +0900

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

Hi,

Searching multiple fixed words, grep uses Commentz-Walter algorithm, but
it is very slow for a worst case e.g. as following.  It is O(m*n).

  - input: yes `printf %040d` | head -10000000
  - word1: x0000000000000000000
  - word2: x

This change uses Aho-Corasick algorithm instead of Commentz-Walter
algorithm to search multiple fixed words.  It uses a function to build
tries which has been already defined for Commentz-Walter algorithm in
kwset.c and which has been already high quality.

I see 7x speed-up even for a typical case on Fedora 21 with a 3.2GHz i5
by this change.

First without this change (best-of-5 trials):

    find /usr/share/doc/ -type f |
    LC_ALL=C time -p xargs.sh src/grep -Ff /usr/share/dict/linux.words >/dev/null
        real 11.37      user 11.03      sys 0.24

Next with this change (best-of-5 trials):

    find /usr/share/doc/ -type f |
    LC_ALL=C  time -p xargs.sh src/grep -Ff /usr/share/dict/linux.words >/dev/null
        real 1.49       user 1.31       sys 0.15

I also wrote two additional patches.

Second, If search multiple fixed words, grep immediately returns without
longest match if not needed.  Without this change, grep tries longest
match for multiple words even if not needed.

Third, use memchr2 for two patterns of a character.

Thanks,
Norihiro

[0001-grep-use-Aho-Corasick-algorithm-to-search-multiple-f.patch (text/plain, attachment)]

[0002-grep-immediately-return-without-longest-match-to-sea.patch (text/plain, attachment)]

[0003-use-memchr2-for-two-patterns-of-a-character.patch (text/plain, attachment)]

Message #8 received at 19358 <at> debbugs.gnu.org (full text, mbox):

From: Jim Meyering <jim <at> meyering.net>
To: Norihiro Tanaka <noritnk <at> kcn.ne.jp>
Cc: 19358 <at> debbugs.gnu.org
Subject: Re: bug#19358: [PATCH 1/3] grep: use Aho-Corasick algorithm to search
 multiple fixed words
Date: Wed, 5 Aug 2015 21:12:33 -0700

On Fri, Dec 12, 2014 at 7:58 AM, Norihiro Tanaka <noritnk <at> kcn.ne.jp> wrote:
> Hi,
>
> Searching multiple fixed words, grep uses Commentz-Walter algorithm, but
> it is very slow for a worst case e.g. as following.  It is O(m*n).
>
>   - input: yes `printf %040d` | head -10000000
>   - word1: x0000000000000000000
>   - word2: x
>
> This change uses Aho-Corasick algorithm instead of Commentz-Walter
> algorithm to search multiple fixed words.  It uses a function to build
> tries which has been already defined for Commentz-Walter algorithm in
> kwset.c and which has been already high quality.
>
> I see 7x speed-up even for a typical case on Fedora 21 with a 3.2GHz i5
> by this change.
>
> First without this change (best-of-5 trials):
>
>     find /usr/share/doc/ -type f |
>     LC_ALL=C time -p xargs.sh src/grep -Ff /usr/share/dict/linux.words >/dev/null
>         real 11.37      user 11.03      sys 0.24
>
> Next with this change (best-of-5 trials):
>
>     find /usr/share/doc/ -type f |
>     LC_ALL=C  time -p xargs.sh src/grep -Ff /usr/share/dict/linux.words >/dev/null
>         real 1.49       user 1.31       sys 0.15

Nice patch.  I see a similar performance improvement.

> I also wrote two additional patches.
>
> Second, If search multiple fixed words, grep immediately returns without
> longest match if not needed.  Without this change, grep tries longest
> match for multiple words even if not needed.

That sounds like a very good idea and the change looks correct.

> Third, use memchr2 for two patterns of a character.

This sounds useful, but I discovered that the new code is not
triggered even once by any of our tests. As such, I suspect that
it is not justified to add these new conditionals in code that is
often inlined.  Do you have experiments that demonstrate how
that final patch helps?

Thank you,
Jim

Message #11 received at 19358 <at> debbugs.gnu.org (full text, mbox):

From: Norihiro Tanaka <noritnk <at> kcn.ne.jp>
To: Jim Meyering <jim <at> meyering.net>
Cc: 19358 <at> debbugs.gnu.org
Subject: Re: bug#19358: [PATCH 1/3] grep: use Aho-Corasick algorithm to search
 multiple fixed words
Date: Thu, 06 Aug 2015 22:10:08 +0900

On Wed, 5 Aug 2015 21:12:33 -0700
Jim Meyering <jim <at> meyering.net> wrote:

> This sounds useful, but I discovered that the new code is not
> triggered even once by any of our tests. As such, I suspect that
> it is not justified to add these new conditionals in code that is
> often inlined.  Do you have experiments that demonstrate how
> that final patch helps?

Thanks for reviewing.  It is effective in only special case.  Could you
try following test for not patched and patched?

  $ printf 'a\nb\n' >pat
  $ yes $(printf %040d 0) | head -10000000 >in
  $ time -p src/grep -Ff pat in

In my machine, patched version is about 10% faster than not patched.

Message #16 received at 19358-done <at> debbugs.gnu.org (full text, mbox):

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Norihiro Tanaka <noritnk <at> kcn.ne.jp>
Cc: 19358-done <at> debbugs.gnu.org
Subject: Re: grep: use Aho-Corasick algorithm to search multiple fixed words
Date: Thu, 2 Jun 2016 15:45:27 -0700

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

Sorry that patch took so long to review. I installed it, along with the 
attached followup patches which are mostly just minor style things (plus 
fixing the attribution for a patch that I forgot to specify --author for).

I didn't get as much performance improvement on my platform, so I toned 
down the NEWS item a bit. Still, wow. It is a 2.5x performance 
improvement for that test case, and it's asymptotically better. Thanks.

[0001-grep-minor-cleanups-for-F-Aho-Corasick.patch (application/x-patch, attachment)]

[0002-grep-simplify-F-Aho-Corasick-a-bit.patch (application/x-patch, attachment)]

[0003-maint-correct-attribution.patch (application/x-patch, attachment)]

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