Re: [PATCH v2 3/3] grep: disable threading in all but worktree case

> I tried to get some timings for this, but ran across some quite
> surprising results. Here's a simple grep of the linux-2.6 working tree,
> using a single-threaded grep:
>
>  $ time git grep SIMPLE >/dev/null
>  real    0m0.439s
>  user    0m0.272s
>  sys     0m0.160s
>
> and then the same thing, via xargs, without even turning on
> parallelization. This should give us a measurement of the overhead for
> going through xargs at all. We'd expect it to be slower, but not too
> much so:
>
>  $ time git ls-files | xargs git grep SIMPLE -- >/dev/null
>  real    0m11.989s
>  user    0m11.769s
>  sys     0m0.268s
>
> Twenty-five times slower! Running 'perf' reports the culprit as pathspec
> matching:
>
>  +  63.23%    git  git                 [.] match_pathspec_depth
>  +  28.60%    git  libc-2.13.so        [.] __strncmp_sse42
>  +   2.22%    git  git                 [.] strncmp@plt
>  +   1.67%    git  git                 [.] kwsexec
>
> where the strncmps are called as part of match_pathspec_depth. So over
> 90% of the CPU time is spent on matching the pathspecs, compared to less
> than 2% actually grepping.
>
> Which really makes me wonder if our pathspec matching could stand to be
> faster. True, giving a bunch of single files is the least efficient way
> to use pathspecs, but that's pretty amazingly slow.