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

[Jeff King <peff@peff.net>]

On Sat, Dec 24, 2011 at 05:55:14PM +0700, Nguyen Thai Ngoc Duy wrote:

> On Sat, Dec 24, 2011 at 2:07 PM, Jeff King <peff@peff.net> wrote:
> > The case where we would most expect the setup cost to be drowned out is
> > using a more complex regex, grepping tree objects. There we have a
> > baseline of:
> >
> >  $ time git grep 'a.*c' HEAD >/dev/null
> >  real    0m5.684s
> >  user    0m5.472s
> >  sys     0m0.196s
> >
> >  $ time git ls-tree --name-only -r HEAD |
> >      xargs git grep 'a.*c' HEAD -- >/dev/null
> >  real    0m10.906s
> >  user    0m10.725s
> >  sys     0m0.240s
> >
> > Here, we still almost double our time. It looks like we don't use the
> > same pathspec matching code in this case. But we do waste a lot of extra
> > time zlib-inflating the trees in "ls-tree", only to do it separately in
> > "grep".
> 
> I assume this is gree_tree(), we have another form of pathspec
> matching here: tree_entry_interesting() and it's still a bunch of
> strcmp inside. Does strcmp show up in perf report?

Yes, but not nearly as high. The top of the report is:

  +  32.16%    git  libc-2.13.so        [.] re_search_internal
  +  17.82%    git  libz.so.1.2.3.4     [.] 0xe986
  +   7.81%    git  git                 [.] look_ahead
  +   6.24%    git  libc-2.13.so        [.] __strncmp_sse42
  +   4.08%    git  git                 [.] tree_entry_interesting
  +   3.27%    git  git                 [.] end_of_line
  +   2.63%    git  libz.so.1.2.3.4     [.] adler32
  +   1.93%    git  libz.so.1.2.3.4     [.] inflate

where the strncmps are from[1]:

  -   6.24%    git  libc-2.13.so        [.] __strncmp_sse42
     - __strncmp_sse42
        + 80.92% grep_tree
        + 19.08% tree_entry_interesting

So we're spending maybe 10% of our time on pathspecs, but most of it is
going to zlib and the actual regex search.

-Peff

[1] Note that this is with -O2, so some of that is from inlined calls.