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>

1 files changed, 91 insertions, 0 deletions

diff --git a/preload-index.c b/preload-index.c
new file mode 100644
index 0000000000..6253578c96
--- /dev/null
+++ b/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;
+}