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

On Mon, 4 Jan 2010, Jeff King wrote:

I have to wonder, though...did anybody ever actually profile our
internal grep to find out _why_ it was so much slower than GNU grep?
Could we simply ship a better grep engine and obsolete external grep?

The internal grep is about 2.5 times slower than the external one for me. 
That's a big deal:

 - external grep:

	[torvalds@nehalem linux]$ time git grep qwerty
	...
	real	0m0.412s
	user	0m0.196s
	sys	0m0.132s

 - NO_EXTERNAL_GREP:

	[torvalds@nehalem linux]$ time ~/git/git grep qwerty
	...
	real	0m1.006s
	user	0m0.900s
	sys	0m0.096s

so that's not even close.

And "perf record" followed by "perf report" on the internal one shows 
that it's not even regexec() - we use strstr() for the trivial case:

    43.63%      git  /home/torvalds/git/git         [.] grep_buffer_1
    25.19%      git  /lib64/libc-2.11.so            [.] __strstr_sse42
     9.16%      git  /home/torvalds/git/git         [.] match_one_pattern
     4.79%      git  /lib64/libc-2.11.so            [.] __m128i_strloadu

bit it seems to be all that line-per-line crud. If we got rid of that one, 
and could do the match as a _single_ regexec() instead (at least for the 
trivial cases of just one grep expression), perhaps we'd be better off.

			Linus