[PATCH v3] Add git-grep threads param

[PATCH v3] Add git-grep threads param

[Victor Leschuk]

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             |  4 ++++
 builtin/grep.c                         | 34 ++++++++++++++++++++++++++--------
 contrib/completion/git-completion.bash |  1 +
 grep.c                                 | 10 ++++++++++
 grep.h                                 |  2 ++
 6 files changed, 47 insertions(+), 8 deletions(-)

diff --git a/Documentation/config.txt b/Documentation/config.txt
index 391a0c3..1c95587 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.
+
 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..fbd4f83 100644
--- a/Documentation/git-grep.txt
+++ b/Documentation/git-grep.txt
@@ -22,6 +22,7 @@ SYNOPSIS
 	   [--color[=<when>] | --no-color]
 	   [--break] [--heading] [-p | --show-function]
 	   [-A <post-context>] [-B <pre-context>] [-C <context>]
+	   [--threads <num>]
 	   [-W | --function-context]
 	   [-f <file>] [-e] <pattern>
 	   [--and|--or|--not|(|)|-e <pattern>...]
@@ -220,6 +221,9 @@ OPTIONS
 	Show <num> leading lines, and place a line containing
 	`--` between contiguous groups of matches.
 
+--threads <num>::
+	Set number of worker threads to <num>. Default is 8.
+
 -W::
 --function-context::
 	Show the surrounding text from the previous line containing a
diff --git a/builtin/grep.c b/builtin/grep.c
index d04f440..5ef1b07 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -27,8 +27,7 @@ static char const * const grep_usage[] = {
 static int use_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
@@ -206,7 +205,8 @@ static void start_threads(struct grep_opt *opt)
 		strbuf_init(&todo[i].out, 0);
 	}
 
-	for (i = 0; i < ARRAY_SIZE(threads); i++) {
+	threads = xcalloc(opt->num_threads, sizeof(pthread_t));
+	for (i = 0; i < opt->num_threads; i++) {
 		int err;
 		struct grep_opt *o = grep_opt_dup(opt);
 		o->output = strbuf_out;
@@ -220,7 +220,7 @@ static void start_threads(struct grep_opt *opt)
 	}
 }
 
-static int wait_all(void)
+static int wait_all(struct grep_opt *opt)
 {
 	int hit = 0;
 	int i;
@@ -238,12 +238,14 @@ static int wait_all(void)
 	pthread_cond_broadcast(&cond_add);
 	grep_unlock();
 
-	for (i = 0; i < ARRAY_SIZE(threads); i++) {
+	for (i = 0; i < opt->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);
@@ -256,7 +258,7 @@ static int wait_all(void)
 }
 #else /* !NO_PTHREADS */
 
-static int wait_all(void)
+static int wait_all(struct grep_opt *opt)
 {
 	return 0;
 }
@@ -702,6 +704,8 @@ 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", &opt.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,
@@ -832,8 +836,22 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	}
 
 #ifndef NO_PTHREADS
-	if (list.nr || cached || online_cpus() == 1)
+	if (!opt.num_threads) {
+		use_threads = 0; /* User explicitely told not to use threads */
+	}
+	else if (list.nr || cached) {
+		use_threads = 0; /* Can not multi-thread object lookup */
+	}
+	else if (opt.num_threads >= 0) {
+		use_threads = 1; /* User explicitely set the number of threads */
+	}
+	else if (online_cpus() <= 1) {
 		use_threads = 0;
+	}
+	else {
+		use_threads = 1;
+		opt.num_threads = GREP_NUM_THREADS_DEFAULT;
+	}
 #else
 	use_threads = 0;
 #endif
@@ -910,7 +928,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	}
 
 	if (use_threads)
-		hit |= wait_all();
+		hit |= wait_all(&opt);
 	if (hit && show_in_pager)
 		run_pager(&opt, prefix);
 	free_grep_patterns(&opt);
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
diff --git a/grep.c b/grep.c
index 7b2b96a..b53fb14 100644
--- a/grep.c
+++ b/grep.c
@@ -40,6 +40,7 @@ void init_grep_defaults(void)
 	color_set(opt->color_selected, "");
 	color_set(opt->color_sep, GIT_COLOR_CYAN);
 	opt->color = -1;
+	opt->num_threads = -1;
 }
 
 static int parse_pattern_type_arg(const char *opt, const char *arg)
@@ -124,6 +125,14 @@ int grep_config(const char *var, const char *value, void *cb)
 			return config_error_nonbool(var);
 		return color_parse(value, color);
 	}
+
+	if (!strcmp(var, "grep.threads")) {
+		int threads = git_config_int(var, value);
+		if (threads < 0)
+			die("invalid number of threads specified (%d)", threads);
+		opt->num_threads = threads;
+		return 0;
+	}
 	return 0;
 }
 
@@ -150,6 +159,7 @@ void grep_init(struct grep_opt *opt, const char *prefix)
 	opt->pathname = def->pathname;
 	opt->regflags = def->regflags;
 	opt->relative = def->relative;
+	opt->num_threads = def->num_threads;
 
 	color_set(opt->color_context, def->color_context);
 	color_set(opt->color_filename, def->color_filename);
diff --git a/grep.h b/grep.h
index 95f197a..bb20456 100644
--- a/grep.h
+++ b/grep.h
@@ -132,6 +132,8 @@ struct grep_opt {
 	unsigned pre_context;
 	unsigned post_context;
 	unsigned last_shown;
+#define GREP_NUM_THREADS_DEFAULT 8
+	int num_threads;
 	int show_hunk_mark;
 	int file_break;
 	int heading;
-- 
2.6.2.281.g222e106.dirty

Re: [PATCH v3] Add git-grep threads param

[John Keeping]

On Mon, Oct 26, 2015 at 03:32:13PM +0300, Victor Leschuk wrote:
> 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             |  4 ++++
>  builtin/grep.c                         | 34 ++++++++++++++++++++++++++--------
>  contrib/completion/git-completion.bash |  1 +
>  grep.c                                 | 10 ++++++++++
>  grep.h                                 |  2 ++
>  6 files changed, 47 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index 391a0c3..1c95587 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.
> +
>  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..fbd4f83 100644
> --- a/Documentation/git-grep.txt
> +++ b/Documentation/git-grep.txt
> @@ -22,6 +22,7 @@ SYNOPSIS
>  	   [--color[=<when>] | --no-color]
>  	   [--break] [--heading] [-p | --show-function]
>  	   [-A <post-context>] [-B <pre-context>] [-C <context>]
> +	   [--threads <num>]

Is this the best place for this option?  I know the current list isn't
sorted in any particular way, but here you're splitting up the set of
context options (`-A`, `-B`, `-C` and `-W`).

>  	   [-W | --function-context]
>  	   [-f <file>] [-e] <pattern>
>  	   [--and|--or|--not|(|)|-e <pattern>...]
> @@ -220,6 +221,9 @@ OPTIONS
>  	Show <num> leading lines, and place a line containing
>  	`--` between contiguous groups of matches.
>  
> +--threads <num>::
> +	Set number of worker threads to <num>. Default is 8.

The same comment as above applies here.

>  -W::
>  --function-context::
>  	Show the surrounding text from the previous line containing a
> diff --git a/builtin/grep.c b/builtin/grep.c
> index d04f440..5ef1b07 100644
> --- a/builtin/grep.c
> +++ b/builtin/grep.c
> @@ -27,8 +27,7 @@ static char const * const grep_usage[] = {
>  static int use_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
> @@ -206,7 +205,8 @@ static void start_threads(struct grep_opt *opt)
>  		strbuf_init(&todo[i].out, 0);
>  	}
>  
> -	for (i = 0; i < ARRAY_SIZE(threads); i++) {
> +	threads = xcalloc(opt->num_threads, sizeof(pthread_t));
> +	for (i = 0; i < opt->num_threads; i++) {
>  		int err;
>  		struct grep_opt *o = grep_opt_dup(opt);
>  		o->output = strbuf_out;
> @@ -220,7 +220,7 @@ static void start_threads(struct grep_opt *opt)
>  	}
>  }
>  
> -static int wait_all(void)
> +static int wait_all(struct grep_opt *opt)

I'm not sure passing a grep_opt in here is the cleanest way to do this.
Options are a UI concept and all we care about here is the number of
threads.

Since `threads` is a global, shouldn't the number of threads be a global
as well?  Could we reuse `use_threads` here (possibly renaming it
`num_threads`)?

>  {
>  	int hit = 0;
>  	int i;
> @@ -238,12 +238,14 @@ static int wait_all(void)
>  	pthread_cond_broadcast(&cond_add);
>  	grep_unlock();
>  
> -	for (i = 0; i < ARRAY_SIZE(threads); i++) {
> +	for (i = 0; i < opt->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);
> @@ -256,7 +258,7 @@ static int wait_all(void)
>  }
>  #else /* !NO_PTHREADS */
>  
> -static int wait_all(void)
> +static int wait_all(struct grep_opt *opt)
>  {
>  	return 0;
>  }
> @@ -702,6 +704,8 @@ 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", &opt.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,
> @@ -832,8 +836,22 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
>  	}
>  
>  #ifndef NO_PTHREADS
> -	if (list.nr || cached || online_cpus() == 1)
> +	if (!opt.num_threads) {
> +		use_threads = 0; /* User explicitely told not to use threads */
> +	}
> +	else if (list.nr || cached) {
> +		use_threads = 0; /* Can not multi-thread object lookup */
> +	}
> +	else if (opt.num_threads >= 0) {
> +		use_threads = 1; /* User explicitely set the number of threads */
> +	}
> +	else if (online_cpus() <= 1) {
>  		use_threads = 0;
> +	}
> +	else {
> +		use_threads = 1;
> +		opt.num_threads = GREP_NUM_THREADS_DEFAULT;
> +	}
>  #else
>  	use_threads = 0;
>  #endif
> @@ -910,7 +928,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
>  	}
>  
>  	if (use_threads)
> -		hit |= wait_all();
> +		hit |= wait_all(&opt);
>  	if (hit && show_in_pager)
>  		run_pager(&opt, prefix);
>  	free_grep_patterns(&opt);
> 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
> diff --git a/grep.c b/grep.c
> index 7b2b96a..b53fb14 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -40,6 +40,7 @@ void init_grep_defaults(void)
>  	color_set(opt->color_selected, "");
>  	color_set(opt->color_sep, GIT_COLOR_CYAN);
>  	opt->color = -1;
> +	opt->num_threads = -1;
>  }
>  
>  static int parse_pattern_type_arg(const char *opt, const char *arg)
> @@ -124,6 +125,14 @@ int grep_config(const char *var, const char *value, void *cb)
>  			return config_error_nonbool(var);
>  		return color_parse(value, color);
>  	}
> +
> +	if (!strcmp(var, "grep.threads")) {
> +		int threads = git_config_int(var, value);
> +		if (threads < 0)
> +			die("invalid number of threads specified (%d)", threads);
> +		opt->num_threads = threads;
> +		return 0;
> +	}
>  	return 0;
>  }
>  
> @@ -150,6 +159,7 @@ void grep_init(struct grep_opt *opt, const char *prefix)
>  	opt->pathname = def->pathname;
>  	opt->regflags = def->regflags;
>  	opt->relative = def->relative;
> +	opt->num_threads = def->num_threads;
>  
>  	color_set(opt->color_context, def->color_context);
>  	color_set(opt->color_filename, def->color_filename);
> diff --git a/grep.h b/grep.h
> index 95f197a..bb20456 100644
> --- a/grep.h
> +++ b/grep.h
> @@ -132,6 +132,8 @@ struct grep_opt {
>  	unsigned pre_context;
>  	unsigned post_context;
>  	unsigned last_shown;
> +#define GREP_NUM_THREADS_DEFAULT 8
> +	int num_threads;
>  	int show_hunk_mark;
>  	int file_break;
>  	int heading;
> -- 
> 2.6.2.281.g222e106.dirty
> 
> --
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RE: [PATCH v3] Add git-grep threads param

[Victor Leschuk]

Hello John,

see comments inline.

>> @@ -22,6 +22,7 @@ SYNOPSIS
>>          [--color[=<when>] | --no-color]
>>          [--break] [--heading] [-p | --show-function]
>>          [-A <post-context>] [-B <pre-context>] [-C <context>]
>> +        [--threads <num>]

> Is this the best place for this option?  I know the current list isn't
> sorted in any particular way, but here you're splitting up the set of
> context options (`-A`, `-B`, `-C` and `-W`).

Agree, I'll move the option both here and in documentation.

>> -static int wait_all(void)
>> +static int wait_all(struct grep_opt *opt)

> I'm not sure passing a grep_opt in here is the cleanest way to do this.
> Options are a UI concept and all we care about here is the number of
> threads.

> Since `threads` is a global, shouldn't the number of threads be a global
> as well?  Could we reuse `use_threads` here (possibly renaming it
> `num_threads`)?

This thought also crossed my mind, however we already pass grep_opt to start_threads() function,
so I think passing it to wait_all() is not that ugly, and kind of symmetric. And I do not like the idea
of duplicating same information in different places. What do you think?

--
Best Regards,
Victor

RE: [PATCH v3] Add git-grep threads param

[Victor Leschuk]

Hello Linus, 

>> 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:

> Did you also compare cold-cache filesystem performance?

>  One of the reasons for doing threaded grep is for CPU scaling. But another is for IO scaling. If your git tree is over NFS, doing grep eight threads at a time if likely going to make things much faster even if you are on a single CPU.

Yes, I have performed tests on cold-cache FS and it looks like number of threads affects performance. Here are the results for grepping linux kernel repo on a 4-core machine (similar test was conducted on 8-core machine):

Threads: 4 Time: 39.13
Threads: 8 Time: 34.39
Threads: 16 Time: 31.46
Threads: 32 Time: 27.40

Here is test scenario:

#!/bin/bash
TIMEFORMAT=%R
GIT=/home/del/git-dev/bin/git
TESTS=10
for n in 4 8 16 32; do
    echo -n "Threads: $n Time: "
    for i in $(seq 1 $TESTS); do
        echo 3 > /proc/sys/vm/drop_caches
        time $GIT grep --threads $n -e '#define' --and \( -e MAX_PATH -e PATH_MAX \)  >/dev/null
    done 2>&1 | awk -v ntests=${TESTS} '{sum+=$1} END{printf "%.2f\n", sum/ntests}'
done

Note: With hot-cache grepping with 4 threads gives fastest results on both 4-core and 8-core machines.

Thus I think it can be useful for users to be able to tune the threads number according to their needs.

Re: [PATCH v3] Add git-grep threads param

[John Keeping]

On Mon, Oct 26, 2015 at 10:25:41PM -0700, Victor Leschuk wrote:
> >> @@ -22,6 +22,7 @@ SYNOPSIS
> >>          [--color[=<when>] | --no-color]
> >>          [--break] [--heading] [-p | --show-function]
> >>          [-A <post-context>] [-B <pre-context>] [-C <context>]
> >> +        [--threads <num>]
> 
> > Is this the best place for this option?  I know the current list isn't
> > sorted in any particular way, but here you're splitting up the set of
> > context options (`-A`, `-B`, `-C` and `-W`).
> 
> Agree, I'll move the option both here and in documentation.
> 
> >> -static int wait_all(void)
> >> +static int wait_all(struct grep_opt *opt)
> 
> > I'm not sure passing a grep_opt in here is the cleanest way to do this.
> > Options are a UI concept and all we care about here is the number of
> > threads.
> 
> > Since `threads` is a global, shouldn't the number of threads be a global
> > as well?  Could we reuse `use_threads` here (possibly renaming it
> > `num_threads`)?
> 
> This thought also crossed my mind, however we already pass grep_opt to
> start_threads() function, so I think passing it to wait_all() is not
> that ugly, and kind of symmetric. And I do not like the idea of
> duplicating same information in different places. What do you think?

The grep_opt in start_threads() is being passed through to run(), so it
seems slightly different to me.  If the threads were being setup in
grep.c (as opposed to builtin/grep.c) then I'd agree that it belongs in
grep_opt, but since this is local to this particular user of the grep
infrastructure adding num_threads to the grep_opt structure at all feels
wrong to me.

Note that I wasn't suggesting passing num_threads as a parameter to
wait_all(), but rather having it as global state that is accessed by
wait_all() in the same way as the `threads` array.

If we rename use_threads to num_threads and just use that, then we only
have the information in one place don't we?

RE: [PATCH v3] Add git-grep threads param

[Victor Leschuk]

Hello John,

>> This thought also crossed my mind, however we already pass grep_opt to
>> start_threads() function, so I think passing it to wait_all() is not
>> that ugly, and kind of symmetric. And I do not like the idea of
>> duplicating same information in different places. What do you think?

> The grep_opt in start_threads() is being passed through to run(), so it
> seems slightly different to me.  If the threads were being setup in
> grep.c (as opposed to builtin/grep.c) then I'd agree that it belongs in
> grep_opt, but since this is local to this particular user of the grep
> infrastructure adding num_threads to the grep_opt structure at all feels
> wrong to me.

> Note that I wasn't suggesting passing num_threads as a parameter to
> wait_all(), but rather having it as global state that is accessed by
> wait_all() in the same way as the `threads` array.

> If we rename use_threads to num_threads and just use that, then we only
> have the information in one place don't we?

Yeah, I understood your idea. So we parse config_value directly to 

static int num_threads; /* old use_threads */

And use it internally in builtin/grep.c. I think you are right.

Looks like grep_cmd_config() is the right place to parse it. Something like:

--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -267,6 +267,8 @@ static int wait_all(struct grep_opt *opt)
 static int grep_cmd_config(const char *var, const char *value, void *cb)
 {
        int st = grep_config(var, value, cb);
+       if (thread_config(var, value, cb) < 0)
+               st = -1;
        if (git_color_default_config(var, value, cb) < 0)
                st = -1;
        return st;

What do you think?

--
Best Regards,
Victor

Re: [PATCH v3] Add git-grep threads param

[John Keeping]

On Tue, Oct 27, 2015 at 06:54:16AM -0700, Victor Leschuk wrote:
> Hello John,
> 
> >> This thought also crossed my mind, however we already pass grep_opt to
> >> start_threads() function, so I think passing it to wait_all() is not
> >> that ugly, and kind of symmetric. And I do not like the idea of
> >> duplicating same information in different places. What do you think?
> 
> > The grep_opt in start_threads() is being passed through to run(), so it
> > seems slightly different to me.  If the threads were being setup in
> > grep.c (as opposed to builtin/grep.c) then I'd agree that it belongs in
> > grep_opt, but since this is local to this particular user of the grep
> > infrastructure adding num_threads to the grep_opt structure at all feels
> > wrong to me.
> 
> > Note that I wasn't suggesting passing num_threads as a parameter to
> > wait_all(), but rather having it as global state that is accessed by
> > wait_all() in the same way as the `threads` array.
> 
> > If we rename use_threads to num_threads and just use that, then we only
> > have the information in one place don't we?
> 
> Yeah, I understood your idea. So we parse config_value directly to 
> 
> static int num_threads; /* old use_threads */

Presumably this is:

	static int num_threads = -1;

so that the default behaviour continues to work correctly.

> And use it internally in builtin/grep.c. I think you are right.
> 
> Looks like grep_cmd_config() is the right place to parse it. Something like:
> 
> --- a/builtin/grep.c
> +++ b/builtin/grep.c
> @@ -267,6 +267,8 @@ static int wait_all(struct grep_opt *opt)
>  static int grep_cmd_config(const char *var, const char *value, void *cb)
>  {
>         int st = grep_config(var, value, cb);
> +       if (thread_config(var, value, cb) < 0)
> +               st = -1;
>         if (git_color_default_config(var, value, cb) < 0)
>                 st = -1;
>         return st;
> 
> What do you think?

I'd be tempted to open code the "grep.threads" case in this function
rather than introducing a helper for a single variable, but I don't
think it matters either way.  This looks good.