Re: [PATCH] grep: do not do external grep on skip-worktree entries

On Mon, Jan 11, 2010 at 07:59:18AM -0800, Linus Torvalds wrote:

The meh news: this shows how grep is faster than regexec() due to being a 
smarter algorithm. For the non-fixed case (I used "qwerty.*as"), the 
numbers are

 - built-in:
	real	0m0.548s
	user	0m0.384s
	sys	0m0.152s

 - external:
	real	0m0.415s
	user	0m0.176s
	sys	0m0.160s

so it really is just 'strstr()' that is faster. But This is a 'meh', 
because I don't really care, and the new code is still way faster than the 
old one. And I'd be personally willing to just drop the external grep if 
this is the worst problem.

Just for fun, I repeated my pcre tests on what's in pu (which has
Junio's lookahead patch now). Before they didn't show any improvement
because we wasted all of our time in non-regex code. There is some
improvement in just using pcre, but I didn't get any improvement by
tweaking it:

[pu]
$ time git grep 'qwerty.*as' >/dev/null
real    0m1.007s
user    0m0.752s
sys     0m0.252s

[pu + pcre]
$ time git grep --no-ext-grep 'qwerty.*as' >/dev/null
real    0m0.864s
user    0m0.648s
sys     0m0.212s

[pu + pcre_study]
$ time git grep --no-ext-grep 'qwerty.*as' >/dev/null
real    0m0.866s
user    0m0.628s
sys     0m0.200s

[pu + pcre_dfa_exec]
$ time git grep --no-ext-grep 'qwerty.*as' >/dev/null
real    0m0.868s
user    0m0.608s
sys     0m0.256s

So pcre seems to buy us about 15%, and tweaking it gets lost in the
noise (or I am tweaking it badly, which is entirely possible). I doubt
it's worth the trouble of supporting pcre for that much.

And let me add an additional vote against strstr:

$ time git grep --no-ext-grep qwerty >/dev/null
real    0m3.285s
user    0m3.032s
sys     0m0.252s

-Peff