bug#17576: [PATCH] dfa: speed-up at initial state

[Jim Meyering]

On Fri, May 23, 2014 at 8:31 PM, Norihiro Tanaka <address@hidden> wrote:
> Thanks to Jim and for the new release.  Let's just start with, for next
> release I want to add further improvement to it.
>
> In dfaexec, DFA state is always 0 until have found potential match.  So
> we can improve matching there by continuing to use the transition table
> without replacing it.
>
> I tested the patch, and got about 3x speed-up.
>
> $ yes j | head -10000000 >k
> $ env LC_ALL=C time -p src/grep '\(a\|b\)' k
>   Before: real 1.12   user 1.06   sys 0.04
>   After : real 0.39   user 0.34   sys 0.04
>
> I also tested for non-utf8 multibyte locale.
> $ env LC_ALL=ja_JP.eucJP time -p src/grep '\(a\|b\)' k
>   Before: real 1.41   user 1.35   sys 0.05
>   After : real 0.38   user 0.32   sys 0.06
...

Thank you for the patch, but with a 2.30GHz i7-3615QM
using 1600MHz DDR3, I see a 40% decrease in performance
in that first case:

  $ for i in grep src/grep; do env LC_ALL=C time -f %e $i '\(a\|b\)' k; done
  0.40
  0.65

and no change in the latter case.
In each case, I used an input file 10 times larger:

   $ yes j | head -100000000 >k

I've also compared on an i5-4250U w/1600MHz DDR3, where there
is still a performance decrease, but only 19% this time (each time
is the best of 5 trials):

  0.43
  0.53