[RFC PATCH 2/2] grep: make default number of threads reflect runtime

["Carlo Marcelo Arenas Belón" <carenas@gmail.com>]

5b594f457a (Threaded grep, 2010-01-25) added a hardcoded number of
threads(8) to use in grep and 89f09dd34e (grep: add --threads=<num>
option and grep.threads configuration, 2015-12-15) made it configurable
through a knob as a workaround for systems where that default was not
effective.

Use instead the industry standard of 2x number of CPUs (to allow for
IO wait) for the default.

Using Debian 10 amd64 in a 2 CPU VirtualBox running in macOS 10.14.6
and that might had been representative of the original author environment
shows an overall performance improvement by avoiding thread trashing:

Test                                                origin/maint        HEAD
-----------------------------------------------------------------------------------------------
7810.1: grep worktree, fixed regex (no match)       0.52(0.43+0.49)     0.50(0.44+0.46) -3.8%
7810.2: grep worktree, fixed regex (common)         0.94(1.20+0.50)     0.91(1.24+0.44) -3.2%
7810.3: grep -I, fixed non binary regex (common)    0.98(1.24+0.51)     0.94(1.30+0.44) -4.1%
7810.4: grep -i, fixed caseless regex (common)      0.97(1.31+0.45)     0.93(1.18+0.56) -4.1%
7810.5: grep --no-index, fixed regex (common)       1.02(1.28+0.50)     0.97(1.14+0.59) -4.9%
7810.6: grep worktree, simple regex (common)        0.77(0.96+0.45)     0.73(0.88+0.48) -5.2%
7810.7: grep -I, simple non binary regex (common)   0.78(0.96+0.48)     0.73(0.94+0.43) -6.4%
7810.8: grep -i, simple caseless regex (common)     0.87(1.11+0.48)     0.82(1.16+0.38) -5.7%
7810.9: grep worktree, expensive regex              10.37(19.67+0.76)   10.20(19.46+0.76) -1.6%
7810.10: grep --cached, fixed regex                 4.48(4.37+0.10)     4.63(4.54+0.09) +3.3%
7810.11: grep --cached, expensive regex             23.74(23.61+0.11)   23.39(23.28+0.09) -1.5%

Signed-off-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
---
 Documentation/git-grep.txt | 2 +-
 builtin/grep.c             | 3 +--
 2 files changed, 2 insertions(+), 3 deletions(-)

diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index 2d27969057..5d72e03b2e 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -60,7 +60,7 @@ grep.extendedRegexp::
 
 grep.threads::
 	Number of grep worker threads to use.  If unset (or set to 0),
-	8 threads are used by default (for now).
+	2 threads per core are used by default.
 
 grep.fullName::
 	If set to true, enable `--full-name` option by default.
diff --git a/builtin/grep.c b/builtin/grep.c
index 580fd38f41..0ed8da30f8 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -32,7 +32,6 @@ static char const * const grep_usage[] = {
 
 static int recurse_submodules;
 
-#define GREP_NUM_THREADS_DEFAULT 8
 static int num_threads;
 
 static pthread_t *threads;
@@ -1068,7 +1067,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	} else if (num_threads < 0)
 		die(_("invalid number of threads specified (%d)"), num_threads);
 	else if (num_threads == 0)
-		num_threads = HAVE_THREADS ? GREP_NUM_THREADS_DEFAULT : 1;
+		num_threads = HAVE_THREADS ? online_cpus() * 2 : 1;
 
 	if (num_threads > 1) {
 		if (!HAVE_THREADS)
-- 
2.23.0.rc1