config: add new index.threads config setting

Add support for a new index.threads config setting which will be used to
control the threading code in do_read_index().  A value of 0 will tell the
index code to automatically determine the correct number of threads to use.
A value of 1 will make the code single threaded.  A value greater than 1
will set the maximum number of threads to use.

For testing purposes, this setting can be overwritten by setting the
GIT_TEST_INDEX_THREADS=<n> environment variable to a value greater than 0.

Signed-off-by: Ben Peart <benpeart@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2413,6 +2413,13 @@ imap::
 	The configuration variables in the 'imap' section are described
 	in linkgit:git-imap-send[1].
 
+index.threads::
+	Specifies the number of threads to spawn when loading the index.
+	This is meant to reduce index load time on multiprocessor machines.
+	Specifying 0 or 'true' will cause Git to auto-detect the number of
+	CPU's and set the number of threads accordingly. Specifying 1 or
+	'false' will disable multithreading. Defaults to 'true'.
+
 index.version::
 	Specify the version with which new index files should be
 	initialized.  This does not affect existing repositories.

config.c

@@ -2289,6 +2289,24 @@ int git_config_get_fsmonitor(void)
 	return 0;
 }
 
+int git_config_get_index_threads(void)
+{
+	int is_bool, val = 0;
+
+	val = git_env_ulong("GIT_TEST_INDEX_THREADS", 0);
+	if (val)
+		return val;
+
+	if (!git_config_get_bool_or_int("index.threads", &is_bool, &val)) {
+		if (is_bool)
+			return val ? 0 : 1;
+		else
+			return val;
+	}
+
+	return 0; /* auto */
+}
+
 NORETURN
 void git_die_config_linenr(const char *key, const char *filename, int linenr)
 {

config.h

@@ -250,6 +250,7 @@ extern int git_config_get_untracked_cache(void);
 extern int git_config_get_split_index(void);
 extern int git_config_get_max_percent_split_change(void);
 extern int git_config_get_fsmonitor(void);
+extern int git_config_get_index_threads(void);
 
 /* This dies if the configured or default date is in the future */
 extern int git_config_get_expiry(const char *key, const char **output);

t/README

@@ -327,6 +327,11 @@ GIT_TEST_COMMIT_GRAPH=<boolean>, when true, forces the commit-graph to
 be written after every 'git commit' command, and overrides the
 'core.commitGraph' setting to true.
 
+GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
+of the index for the whole test suite by bypassing the default number of
+cache entries and thread minimums. Setting this to 1 will make the
+index loading single threaded.
+
 Naming Tests
 ------------
 

t/t1700-split-index.sh

@@ -6,7 +6,12 @@ test_description='split index mode tests'
 
 # We need total control of index splitting here
 sane_unset GIT_TEST_SPLIT_INDEX
+
+# Testing a hard coded SHA against an index with an extension
+# that can vary from run to run is problematic so we disable
+# those extensions.
 sane_unset GIT_FSMONITOR_TEST
+sane_unset GIT_TEST_INDEX_THREADS
 
 test_expect_success 'enable split index' '
 	git config splitIndex.maxPercentChange 100 &&