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

From: René Scharfe <[email-masked]>
Date: Tue, 06 Dec 2011 23:48:26 +0100

Am 02.12.2011 17:15, schrieb René Scharfe:
> How about adding a parameter to control the number of threads 
> (--threads?) instead that defaults to eight (or five) for the worktree 
> and one for the rest? That would also make benchmarking easier.

Like this:

-- >8 --
Subject: grep: add parameter --threads

Allow the number of threads to be specified by the user.  This makes
benchmarking the performance impact of different numbers of threads
much easier.

Move the code for thread handling after argument parsing.  This allows
to change the default number of threads based on the kind of search
(worktree etc.) later on.

Signed-off-by: Rene Scharfe <[email-masked]>
---
Applies on top of your second patch.

 Documentation/git-grep.txt |    4 ++
 builtin/grep.c             |   75 +++++++++++++++++++++++--------------------
 2 files changed, 44 insertions(+), 35 deletions(-)

diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
index 15d6711..47ac188 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -227,6 +227,10 @@ OPTIONS
 	Do not output matched lines; instead, exit with status 0 when
 	there is a match and with non-zero status when there isn't.
 
+--threads <n>::
+	Run <n> search threads in parallel.  Default is 8.  This option
+	is ignored if git was built without support for POSIX threads.
+
 <tree>...::
 	Instead of searching tracked files in the working tree, search
 	blobs in the given trees.
diff --git a/builtin/grep.c b/builtin/grep.c
index 65b1ffe..0bda900 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -24,11 +24,10 @@ static char const * const grep_usage[] = {
 	NULL
 };
 
-static int use_threads = 1;
-
 #ifndef NO_PTHREADS
 #define THREADS 8
-static pthread_t threads[THREADS];
+static pthread_t *threads;
+static int nr_threads = -1;
 
 static void *load_sha1(const unsigned char *sha1, unsigned long *size,
 		       const char *name);
@@ -76,13 +75,13 @@ static pthread_mutex_t grep_mutex;
 
 static inline void grep_lock(void)
 {
-	if (use_threads)
+	if (nr_threads > 0)
 		pthread_mutex_lock(&grep_mutex);
 }
 
 static inline void grep_unlock(void)
 {
-	if (use_threads)
+	if (nr_threads > 0)
 		pthread_mutex_unlock(&grep_mutex);
 }
 
@@ -91,13 +90,13 @@ static pthread_mutex_t read_sha1_mutex;
 
 static inline void read_sha1_lock(void)
 {
-	if (use_threads)
+	if (nr_threads > 0)
 		pthread_mutex_lock(&read_sha1_mutex);
 }
 
 static inline void read_sha1_unlock(void)
 {
-	if (use_threads)
+	if (nr_threads > 0)
 		pthread_mutex_unlock(&read_sha1_mutex);
 }
 
@@ -254,6 +253,8 @@ static void start_threads(struct grep_opt *opt)
 {
 	int i;
 
+	threads = xcalloc(nr_threads, sizeof(pthread_t));
+
 	pthread_mutex_init(&grep_mutex, NULL);
 	pthread_mutex_init(&read_sha1_mutex, NULL);
 	pthread_mutex_init(&grep_attr_mutex, NULL);
@@ -265,7 +266,7 @@ static void start_threads(struct grep_opt *opt)
 		strbuf_init(&todo[i].out, 0);
 	}
 
-	for (i = 0; i < ARRAY_SIZE(threads); i++) {
+	for (i = 0; i < nr_threads; i++) {
 		int err;
 		struct grep_opt *o = grep_opt_dup(opt);
 		o->output = strbuf_out;
@@ -296,7 +297,7 @@ static int wait_all(void)
 	pthread_cond_broadcast(&cond_add);
 	grep_unlock();
 
-	for (i = 0; i < ARRAY_SIZE(threads); i++) {
+	for (i = 0; i < nr_threads; i++) {
 		void *h;
 		pthread_join(threads[i], &h);
 		hit |= (int) (intptr_t) h;
@@ -309,6 +310,8 @@ static int wait_all(void)
 	pthread_cond_destroy(&cond_write);
 	pthread_cond_destroy(&cond_result);
 
+	free(threads);
+
 	return hit;
 }
 #else /* !NO_PTHREADS */
@@ -410,7 +413,7 @@ static int grep_sha1(struct grep_opt *opt, const unsigned char *sha1,
 	name = strbuf_detach(&pathbuf, NULL);
 
 #ifndef NO_PTHREADS
-	if (use_threads) {
+	if (nr_threads > 0) {
 		grep_sha1_async(opt, name, sha1);
 		return 0;
 	} else
@@ -472,7 +475,7 @@ static int grep_file(struct grep_opt *opt, const char *filename)
 	name = strbuf_detach(&buf, NULL);
 
 #ifndef NO_PTHREADS
-	if (use_threads) {
+	if (nr_threads > 0) {
 		grep_file_async(opt, name, filename);
 		return 0;
 	} else
@@ -895,6 +898,13 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			PARSE_OPT_OPTARG, NULL, (intptr_t)default_pager },
 		OPT_BOOLEAN(0, "ext-grep", &external_grep_allowed__ignored,
 			    "allow calling of grep(1) (ignored by this build)"),
+#ifdef NO_PTHREADS
+		OPT_INTEGER(0, "threads", &nr_threads,
+			"handle <n> files in parallel (ignored by this build)"),
+#else
+		OPT_INTEGER(0, "threads", &nr_threads,
+			"handle <n> files in parallel"),
+#endif
 		{ OPTION_CALLBACK, 0, "help-all", &options, NULL, "show usage",
 		  PARSE_OPT_HIDDEN | PARSE_OPT_NOARG, help_callback },
 		OPT_END()
@@ -995,7 +1005,7 @@ 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;
+		nr_threads = 0;
 	}
 
 	if (!opt.pattern_list)
@@ -1003,28 +1013,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	if (!opt.fixed && opt.ignore_case)
 		opt.regflags |= REG_ICASE;
 
-#ifndef NO_PTHREADS
-	if (online_cpus() == 1)
-		use_threads = 0;
-#else
-	use_threads = 0;
-#endif
-
-	opt.use_threads = use_threads;
-
-#ifndef NO_PTHREADS
-	if (use_threads) {
-		if (opt.pre_context || opt.post_context || opt.file_break ||
-		    opt.funcbody)
-			skip_first_line = 1;
-		start_threads(&opt);
-	}
-#else
-	use_threads = 0;
-#endif
-
-	compile_grep_patterns(&opt);
-
 	/* Check revs and then paths */
 	for (i = 0; i < argc; i++) {
 		const char *arg = argv[i];
@@ -1056,6 +1044,23 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	pathspec.max_depth = opt.max_depth;
 	pathspec.recursive = 1;
 
+#ifdef NO_PTHREADS
+	nr_threads = 0;
+#else
+	if (nr_threads == -1)
+		nr_threads = (online_cpus() > 1) ? THREADS : 0;
+
+	if (nr_threads > 0) {
+		opt.use_threads = 1;
+		if (opt.pre_context || opt.post_context || opt.file_break ||
+		    opt.funcbody)
+			skip_first_line = 1;
+		start_threads(&opt);
+	}
+#endif
+
+	compile_grep_patterns(&opt);
+
 	if (show_in_pager && (cached || list.nr))
 		die(_("--open-files-in-pager only works on the worktree"));
 
@@ -1100,7 +1105,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		hit = grep_objects(&opt, &pathspec, &list);
 	}
 
-	if (use_threads)
+	if (nr_threads > 0)
 		hit |= wait_all();
 	if (hit && show_in_pager)
 		run_pager(&opt, prefix);
-- 
1.7.8