From: Linus Torvalds <torvalds@linux-foundation.org>
Date: Mon, 04 Jan 2010 16:01:15 GMT
Subject: Re: [PATCH] grep: do not do external grep on skip-worktree entries
Message-ID: <alpine.LFD.2.00.1001040755130.3630@localhost.localdomain>
In-Reply-To: <20100104080940.GA4815@coredump.intra.peff.net>



On Mon, 4 Jan 2010, Jeff King wrote:
> 
> However, gprof reports that for the pcre dfa case, we spend more time in
> grep.c:end_of_line than we do actually running the regex. So clearly
> there are some other micro-optimizations in GNU grep that are making a
> difference, too.

Don't use gprof. You're _much_ better off using the newish Linux 'perf' 
tool. It's quite competent, and doesn't need the code to be compiled with 
-pg (which totally changes all performance characteristics).

Do something like this:

	perf record git grep qwerty

followed by

	perf report
	perf annotate grep_buffer_1

(that "perf report" gives a per-symbol overview, the "perf annotate" gives 
a disassembly with source annotations and per-instruction costs). It works 
with inlining too, so you get things like this:

	...
         :      static char *end_of_line(char *cp, unsigned long *left)
         :      {
         :              unsigned long l = *left;
         :              while (l && *cp != '\n') {
   24.76 :        476a50:       80 3b 0a                cmpb   $0xa,(%rbx)
   10.46 :        476a53:       0f 84 e7 00 00 00       je     476b40 <grep_buffer_1+0x1b0>
         :                      l--;
         :                      cp++;
   21.19 :        476a59:       48 83 c3 01             add    $0x1,%rbx
         :      }
         :
         :      static char *end_of_line(char *cp, unsigned long *left)
         :      {
         :              unsigned long l = *left;
         :              while (l && *cp != '\n') {
    0.94 :        476a5d:       49 83 ed 01             sub    $0x1,%r13
    4.85 :        476a61:       75 ed                   jne    476a50 <grep_buffer_1+0xc0>
         :
	...

and yes, it's all the per-line crap.

The perf tools are included with modern kernels in tools/perf (which also 
has a Documentation subdirectory). I can pretty much guarantee that once 
you start using it, you'll never use gprof or oprofile again.

		Linus