[PATCH v4] Add git-grep threads param

[Victor Leschuk <vleschuk@gmail.com>]

Make number of git-grep worker threads a configuration parameter.
According to several tests on systems with different number of CPU cores
the hard-coded number of 8 threads is not optimal for all systems:
tuning this parameter can significantly speed up grep performance.

Signed-off-by: Victor Leschuk <vleschuk@accesssoftek.com>
---
 Documentation/config.txt               |  4 +++
 Documentation/git-grep.txt             |  9 ++++++
 builtin/grep.c                         | 56 ++++++++++++++++++++++++----------
 contrib/completion/git-completion.bash |  1 +
 4 files changed, 54 insertions(+), 16 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 391a0c3..1dd2a61 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -1447,6 +1447,10 @@ grep.extendedRegexp::
 	option is ignored when the 'grep.patternType' option is set to a value
 	other than 'default'.
 
+grep.threads::
+	Number of grep worker threads, use it to tune up performance on
+	multicore machines. Default value is 8. Set to 0 to disable threading.
+
 gpg.program::
 	Use this custom program instead of "gpg" found on $PATH when
 	making or verifying a PGP signature. The program must support the
diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index 4a44d6d..e766596 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -23,6 +23,7 @@ SYNOPSIS
 	   [--break] [--heading] [-p | --show-function]
 	   [-A <post-context>] [-B <pre-context>] [-C <context>]
 	   [-W | --function-context]
+	   [--threads <num>]
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...]
 	   [ [--[no-]exclude-standard] [--cached | --no-index | --untracked] | <tree>...]
@@ -53,6 +54,10 @@ grep.extendedRegexp::
 	option is ignored when the 'grep.patternType' option is set to a value
 	other than 'default'.
 
+grep.threads::
+	Number of grep worker threads, use it to tune up performance on
+	multicore machines. Default value is 8. Set to 0 to disable threading.
+
 grep.fullName::
 	If set to true, enable '--full-name' option by default.
 
@@ -227,6 +232,10 @@ OPTIONS
 	effectively showing the whole function in which the match was
 	found.
 
+--threads <num>::
+	Set number of worker threads to <num>. Default is 8.
+	Set to 0 to disable threading.
+
 -f <file>::
 	Read patterns from <file>, one per line.
 
diff --git a/builtin/grep.c b/builtin/grep.c
index d04f440..694553e 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -24,11 +24,11 @@ static char const * const grep_usage[] = {
 	NULL
 };
 
-static int use_threads = 1;
+#define GREP_NUM_THREADS_DEFAULT 8
+static int num_threads = -1;
 
 #ifndef NO_PTHREADS
-#define THREADS 8
-static pthread_t threads[THREADS];
+static pthread_t *threads;
 
 /* We use one producer thread and THREADS consumer
  * threads. The producer adds struct work_items to 'todo' and the
@@ -63,13 +63,13 @@ static pthread_mutex_t grep_mutex;
 
 static inline void grep_lock(void)
 {
-	if (use_threads)
+	if (num_threads)
 		pthread_mutex_lock(&grep_mutex);
 }
 
 static inline void grep_unlock(void)
 {
-	if (use_threads)
+	if (num_threads)
 		pthread_mutex_unlock(&grep_mutex);
 }
 
@@ -206,7 +206,8 @@ static void start_threads(struct grep_opt *opt)
 		strbuf_init(&todo[i].out, 0);
 	}
 
-	for (i = 0; i < ARRAY_SIZE(threads); i++) {
+	threads = xcalloc(num_threads, sizeof(pthread_t));
+	for (i = 0; i < num_threads; i++) {
 		int err;
 		struct grep_opt *o = grep_opt_dup(opt);
 		o->output = strbuf_out;
@@ -238,12 +239,14 @@ static int wait_all(void)
 	pthread_cond_broadcast(&cond_add);
 	grep_unlock();
 
-	for (i = 0; i < ARRAY_SIZE(threads); i++) {
+	for (i = 0; i < num_threads; i++) {
 		void *h;
 		pthread_join(threads[i], &h);
 		hit |= (int) (intptr_t) h;
 	}
 
+	free(threads);
+
 	pthread_mutex_destroy(&grep_mutex);
 	pthread_mutex_destroy(&grep_read_mutex);
 	pthread_mutex_destroy(&grep_attr_mutex);
@@ -262,10 +265,22 @@ static int wait_all(void)
 }
 #endif
 
+static int grep_threads_config(const char *var, const char *value, void *cb)
+{
+	if (!strcmp(var, "grep.threads")) {
+		num_threads = git_config_int(var, value);
+		if (num_threads < 0)
+			die("Invalid number of threads specified (%d)", num_threads);
+	}
+	return 0;
+}
+
 static int grep_cmd_config(const char *var, const char *value, void *cb)
 {
 	int st = grep_config(var, value, cb);
-	if (git_color_default_config(var, value, cb) < 0)
+	if (grep_threads_config(var, value, cb) < 0)
+		st = -1;
+	else if (git_color_default_config(var, value, cb) < 0)
 		st = -1;
 	return st;
 }
@@ -294,7 +309,7 @@ static int grep_sha1(struct grep_opt *opt, const unsigned char *sha1,
 	}
 
 #ifndef NO_PTHREADS
-	if (use_threads) {
+	if (num_threads) {
 		add_work(opt, GREP_SOURCE_SHA1, pathbuf.buf, path, sha1);
 		strbuf_release(&pathbuf);
 		return 0;
@@ -323,7 +338,7 @@ static int grep_file(struct grep_opt *opt, const char *filename)
 		strbuf_addstr(&buf, filename);
 
 #ifndef NO_PTHREADS
-	if (use_threads) {
+	if (num_threads) {
 		add_work(opt, GREP_SOURCE_FILE, buf.buf, filename, filename);
 		strbuf_release(&buf);
 		return 0;
@@ -702,6 +717,8 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			N_("show <n> context lines before matches")),
 		OPT_INTEGER('A', "after-context", &opt.post_context,
 			N_("show <n> context lines after matches")),
+		OPT_INTEGER(0, "threads", &num_threads,
+			N_("use <n> worker threads")),
 		OPT_NUMBER_CALLBACK(&opt, N_("shortcut for -C NUM"),
 			context_callback),
 		OPT_BOOL('p', "show-function", &opt.funcname,
@@ -801,7 +818,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		opt.output_priv = &path_list;
 		opt.output = append_path;
 		string_list_append(&path_list, show_in_pager);
-		use_threads = 0;
+		num_threads = 0;
 	}
 
 	if (!opt.pattern_list)
@@ -832,14 +849,21 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	}
 
 #ifndef NO_PTHREADS
-	if (list.nr || cached || online_cpus() == 1)
-		use_threads = 0;
+	if (list.nr || cached) {
+		num_threads = 0; /* Can not multi-thread object lookup */
+	}
+	else if (num_threads < 0 && online_cpus() <= 1) {
+		num_threads = 0; /* User didn't set threading option and we have <= 1 of hardware cores */
+	}
+	else if (num_threads < 0) {
+		num_threads = GREP_NUM_THREADS_DEFAULT;
+	}
 #else
-	use_threads = 0;
+	num_threads = 0;
 #endif
 
 #ifndef NO_PTHREADS
-	if (use_threads) {
+	if (num_threads) {
 		if (!(opt.name_only || opt.unmatch_name_only || opt.count)
 		    && (opt.pre_context || opt.post_context ||
 			opt.file_break || opt.funcbody))
@@ -909,7 +933,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		hit = grep_objects(&opt, &pathspec, &list);
 	}
 
-	if (use_threads)
+	if (num_threads)
 		hit |= wait_all();
 	if (hit && show_in_pager)
 		run_pager(&opt, prefix);
diff --git a/contrib/completion/git-completion.bash b/contrib/completion/git-completion.bash
index 482ca84..390d9c0 100644
--- a/contrib/completion/git-completion.bash
+++ b/contrib/completion/git-completion.bash
@@ -1310,6 +1310,7 @@ _git_grep ()
 			--full-name --line-number
 			--extended-regexp --basic-regexp --fixed-strings
 			--perl-regexp
+			--threads
 			--files-with-matches --name-only
 			--files-without-match
 			--max-depth
-- 
2.6.2.308.g3b8f10c.dirty