Add cache preload facility

This can do the lstat() storm in parallel, giving potentially much
improved performance for cold-cache cases or things like NFS that have
weak metadata caching.

Just use "read_cache_preload()" instead of "read_cache()" to force an
optimistic preload of the index stat data.  The function takes a
pathspec as its argument, allowing us to preload only the relevant
portion of the index.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -413,6 +413,15 @@ data writes properly, but can be useful for filesystems that do not use
 journalling (traditional UNIX filesystems) or that only journal metadata
 and not file contents (OS X's HFS+, or Linux ext3 with "data=writeback").
 
+core.preloadindex::
+	Enable parallel index preload for operations like 'git diff'
++
+This can speed up operations like 'git diff' and 'git status' especially
+on filesystems like NFS that have weak caching semantics and thus
+relatively high IO latencies.  With this set to 'true', git will do the
+index comparison to the filesystem data in parallel, allowing
+overlapping IO's.
+
 alias.*::
 	Command aliases for the linkgit:git[1] command wrapper - e.g.
 	after defining "alias.last = cat-file commit HEAD", the invocation

Makefile

@@ -496,6 +496,7 @@ LIB_OBJS += write_or_die.o
 LIB_OBJS += ws.o
 LIB_OBJS += wt-status.o
 LIB_OBJS += xdiff-interface.o
+LIB_OBJS += preload-index.o
 
 BUILTIN_OBJS += builtin-add.o
 BUILTIN_OBJS += builtin-annotate.o

builtin-commit.c

@@ -225,18 +225,18 @@ static char *prepare_index(int argc, const char **argv, const char *prefix)
 
 	if (interactive) {
 		interactive_add(argc, argv, prefix);
-		if (read_cache() < 0)
+		if (read_cache_preload(NULL) < 0)
 			die("index file corrupt");
 		commit_style = COMMIT_AS_IS;
 		return get_index_file();
 	}
 
-	if (read_cache() < 0)
-		die("index file corrupt");
-
 	if (*argv)
 		pathspec = get_pathspec(prefix, argv);
 
+	if (read_cache_preload(pathspec) < 0)
+		die("index file corrupt");
+
 	/*
 	 * Non partial, non as-is commit.
 	 *

builtin-diff-files.c

@@ -59,8 +59,8 @@ int cmd_diff_files(int argc, const char **argv, const char *prefix)
 	    (rev.diffopt.output_format & DIFF_FORMAT_PATCH))
 		rev.combine_merges = rev.dense_combined_merges = 1;
 
-	if (read_cache() < 0) {
+	if (read_cache_preload(rev.diffopt.paths) < 0) {
-		perror("read_cache");
+		perror("read_cache_preload");
 		return -1;
 	}
 	result = run_diff_files(&rev, options);

builtin-diff.c

@@ -134,8 +134,8 @@ static int builtin_diff_index(struct rev_info *revs,
 	    revs->max_count != -1 || revs->min_age != -1 ||
 	    revs->max_age != -1)
 		usage(builtin_diff_usage);
-	if (read_cache() < 0) {
+	if (read_cache_preload(revs->diffopt.paths) < 0) {
-		perror("read_cache");
+		perror("read_cache_preload");
 		return -1;
 	}
 	return run_diff_index(revs, cached);
@@ -234,8 +234,8 @@ static int builtin_diff_files(struct rev_info *revs, int argc, const char **argv
 		revs->combine_merges = revs->dense_combined_merges = 1;
 
 	setup_work_tree();
-	if (read_cache() < 0) {
+	if (read_cache_preload(revs->diffopt.paths) < 0) {
-		perror("read_cache");
+		perror("read_cache_preload");
 		return -1;
 	}
 	result = run_diff_files(revs, options);

cache.h

@@ -262,6 +262,7 @@ static inline void remove_name_hash(struct cache_entry *ce)
 
 #define read_cache() read_index(&the_index)
 #define read_cache_from(path) read_index_from(&the_index, (path))
+#define read_cache_preload(pathspec) read_index_preload(&the_index, (pathspec))
 #define is_cache_unborn() is_index_unborn(&the_index)
 #define read_cache_unmerged() read_index_unmerged(&the_index)
 #define write_cache(newfd, cache, entries) write_index(&the_index, (newfd))
@@ -368,6 +369,7 @@ extern int init_db(const char *template_dir, unsigned int flags);
 
 /* Initialize and use the cache information */
 extern int read_index(struct index_state *);
+extern int read_index_preload(struct index_state *, const char **pathspec);
 extern int read_index_from(struct index_state *, const char *path);
 extern int is_index_unborn(struct index_state *);
 extern int read_index_unmerged(struct index_state *);
@@ -458,6 +460,7 @@ extern size_t packed_git_limit;
 extern size_t delta_base_cache_limit;
 extern int auto_crlf;
 extern int fsync_object_files;
+extern int core_preload_index;
 
 enum safe_crlf {
 	SAFE_CRLF_FALSE = 0,

config.c

@@ -490,6 +490,11 @@ static int git_default_core_config(const char *var, const char *value)
 		return 0;
 	}
 
+	if (!strcmp(var, "core.preloadindex")) {
+		core_preload_index = git_config_bool(var, value);
+		return 0;
+	}
+
 	/* Add other config variables here and to Documentation/config.txt. */
 	return 0;
 }

environment.c

@@ -43,6 +43,9 @@ unsigned whitespace_rule_cfg = WS_DEFAULT_RULE;
 enum branch_track git_branch_track = BRANCH_TRACK_REMOTE;
 enum rebase_setup_type autorebase = AUTOREBASE_NEVER;
 
+/* Parallel index stat data preload? */
+int core_preload_index = 0;
+
 /* This is set by setup_git_dir_gently() and/or git_default_config() */
 char *git_work_tree_cfg;
 static char *work_tree;

preload-index.c

@@ -0,0 +1,91 @@
+/*
+ * Copyright (C) 2008 Linus Torvalds
+ */
+#include "cache.h"
+#include <pthread.h>
+
+/*
+ * Mostly randomly chosen maximum thread counts: we
+ * cap the parallelism to 20 threads, and we want
+ * to have at least 500 lstat's per thread for it to
+ * be worth starting a thread.
+ */
+#define MAX_PARALLEL (20)
+#define THREAD_COST (500)
+
+struct thread_data {
+	pthread_t pthread;
+	struct index_state *index;
+	const char **pathspec;
+	int offset, nr;
+};
+
+static void *preload_thread(void *_data)
+{
+	int nr;
+	struct thread_data *p = _data;
+	struct index_state *index = p->index;
+	struct cache_entry **cep = index->cache + p->offset;
+
+	nr = p->nr;
+	if (nr + p->offset > index->cache_nr)
+		nr = index->cache_nr - p->offset;
+
+	do {
+		struct cache_entry *ce = *cep++;
+		struct stat st;
+
+		if (ce_stage(ce))
+			continue;
+		if (ce_uptodate(ce))
+			continue;
+		if (!ce_path_match(ce, p->pathspec))
+			continue;
+		if (lstat(ce->name, &st))
+			continue;
+		if (ie_match_stat(index, ce, &st, 0))
+			continue;
+		ce_mark_uptodate(ce);
+	} while (--nr > 0);
+	return NULL;
+}
+
+static void preload_index(struct index_state *index, const char **pathspec)
+{
+	int threads, i, work, offset;
+	struct thread_data data[MAX_PARALLEL];
+
+	if (!core_preload_index)
+		return;
+
+	threads = index->cache_nr / THREAD_COST;
+	if (threads < 2)
+		return;
+	if (threads > MAX_PARALLEL)
+		threads = MAX_PARALLEL;
+	offset = 0;
+	work = (index->cache_nr + threads - 1) / threads;
+	for (i = 0; i < threads; i++) {
+		struct thread_data *p = data+i;
+		p->index = index;
+		p->pathspec = pathspec;
+		p->offset = offset;
+		p->nr = work;
+		offset += work;
+		if (pthread_create(&p->pthread, NULL, preload_thread, p))
+			die("unable to create threaded lstat");
+	}
+	for (i = 0; i < threads; i++) {
+		struct thread_data *p = data+i;
+		if (pthread_join(p->pthread, NULL))
+			die("unable to join threaded lstat");
+	}
+}
+
+int read_index_preload(struct index_state *index, const char **pathspec)
+{
+	int retval = read_index(index);
+
+	preload_index(index, pathspec);
+	return retval;
+}