Merge branch 'jh/memihash-opt'

The name-hash used for detecting paths that are different only in
cases (which matter on case insensitive filesystems) has been
optimized to take advantage of multi-threading when it makes sense.

* jh/memihash-opt:
  name-hash: add test-lazy-init-name-hash to .gitignore
  name-hash: add perf test for lazy_init_name_hash
  name-hash: add test-lazy-init-name-hash
  name-hash: perf improvement for lazy_init_name_hash
  hashmap: document memihash_cont, hashmap_disallow_rehash api
  hashmap: add disallow_rehash setting
  hashmap: allow memihash computation to be continued
  name-hash: specify initial size for istate.dir_hash table

9 files changed, 848 insertions, 9 deletions

diff --git a/Documentation/technical/api-hashmap.txt b/Documentation/technical/api-hashmap.txt
index a3f020cd9e..ccc634bbd7 100644
--- a/Documentation/technical/api-hashmap.txt
+++ b/Documentation/technical/api-hashmap.txt
@@ -21,6 +21,9 @@ that the hashmap is initialized. It may also be useful for statistical purposes
 `cmpfn` stores the comparison function specified in `hashmap_init()`. In
 advanced scenarios, it may be useful to change this, e.g. to switch between
 case-sensitive and case-insensitive lookup.
++
+When `disallow_rehash` is set, automatic rehashes are prevented during inserts
+and deletes.
 
 `struct hashmap_entry`::
 
@@ -57,6 +60,7 @@ Functions
 `unsigned int strihash(const char *buf)`::
 `unsigned int memhash(const void *buf, size_t len)`::
 `unsigned int memihash(const void *buf, size_t len)`::
+`unsigned int memihash_cont(unsigned int hash_seed, const void *buf, size_t len)`::
 
 	Ready-to-use hash functions for strings, using the FNV-1 algorithm (see
 	http://www.isthe.com/chongo/tech/comp/fnv).
@@ -65,6 +69,9 @@ Functions
 `memihash` operate on arbitrary-length memory.
 +
 `strihash` and `memihash` are case insensitive versions.
++
+`memihash_cont` is a variant of `memihash` that allows a computation to be
+continued with another chunk of data.
 
 `unsigned int sha1hash(const unsigned char *sha1)`::
 
@@ -184,6 +191,21 @@ passed to `hashmap_cmp_fn` to decide whether the entry matches the key.
 +
 Returns the removed entry, or NULL if not found.
 
+`void hashmap_disallow_rehash(struct hashmap *map, unsigned value)`::
+
+	Disallow/allow automatic rehashing of the hashmap during inserts
+	and deletes.
++
+This is useful if the caller knows that the hashmap will be accessed
+by multiple threads.
++
+The caller is still responsible for any necessary locking; this simply
+prevents unexpected rehashing.  The caller is also responsible for properly
+sizing the initial hashmap to ensure good performance.
++
+A call to allow rehashing does not force a rehash; that might happen
+with the next insert or delete.
+
 `void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter)`::
 `void *hashmap_iter_next(struct hashmap_iter *iter)`::
 `void *hashmap_iter_first(struct hashmap *map, struct hashmap_iter *iter)`::
diff --git a/Makefile b/Makefile
index c80fec2920..9f8b35ad41 100644
--- a/Makefile
+++ b/Makefile
@@ -621,6 +621,7 @@ TEST_PROGRAMS_NEED_X += test-fake-ssh
 TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-hashmap
 TEST_PROGRAMS_NEED_X += test-index-version
+TEST_PROGRAMS_NEED_X += test-lazy-init-name-hash
 TEST_PROGRAMS_NEED_X += test-line-buffer
 TEST_PROGRAMS_NEED_X += test-match-trees
 TEST_PROGRAMS_NEED_X += test-mergesort
diff --git a/cache.h b/cache.h
index db4120c233..fbdf7a815a 100644
--- a/cache.h
+++ b/cache.h
@@ -343,6 +343,7 @@ struct index_state {
 extern struct index_state the_index;
 
 /* Name hashing */
+extern int test_lazy_init_name_hash(struct index_state *istate, int try_threaded);
 extern void add_name_hash(struct index_state *istate, struct cache_entry *ce);
 extern void remove_name_hash(struct index_state *istate, struct cache_entry *ce);
 extern void free_name_hash(struct index_state *istate);
diff --git a/hashmap.c b/hashmap.c
index b10b642229..7d1044eb5d 100644
--- a/hashmap.c
+++ b/hashmap.c
@@ -50,6 +50,23 @@ unsigned int memihash(const void *buf, size_t len)
 	return hash;
 }
 
+/*
+ * Incoporate another chunk of data into a memihash
+ * computation.
+ */
+unsigned int memihash_cont(unsigned int hash_seed, const void *buf, size_t len)
+{
+	unsigned int hash = hash_seed;
+	unsigned char *ucbuf = (unsigned char *) buf;
+	while (len--) {
+		unsigned int c = *ucbuf++;
+		if (c >= 'a' && c <= 'z')
+			c -= 'a' - 'A';
+		hash = (hash * FNV32_PRIME) ^ c;
+	}
+	return hash;
+}
+
 #define HASHMAP_INITIAL_SIZE 64
 /* grow / shrink by 2^2 */
 #define HASHMAP_RESIZE_BITS 2
@@ -87,11 +104,19 @@ static inline unsigned int bucket(const struct hashmap *map,
 	return key->hash & (map->tablesize - 1);
 }
 
+int hashmap_bucket(const struct hashmap *map, unsigned int hash)
+{
+	return hash & (map->tablesize - 1);
+}
+
 static void rehash(struct hashmap *map, unsigned int newsize)
 {
 	unsigned int i, oldsize = map->tablesize;
 	struct hashmap_entry **oldtable = map->table;
 
+	if (map->disallow_rehash)
+		return;
+
 	alloc_table(map, newsize);
 	for (i = 0; i < oldsize; i++) {
 		struct hashmap_entry *e = oldtable[i];
@@ -124,7 +149,9 @@ void hashmap_init(struct hashmap *map, hashmap_cmp_fn equals_function,
 		size_t initial_size)
 {
 	unsigned int size = HASHMAP_INITIAL_SIZE;
-	map->size = 0;
+
+	memset(map, 0, sizeof(*map));
+
 	map->cmpfn = equals_function ? equals_function : always_equal;
 
 	/* calculate initial table size and allocate the table */
diff --git a/hashmap.h b/hashmap.h
index ab7958ae33..de6022a3a9 100644
--- a/hashmap.h
+++ b/hashmap.h
@@ -12,6 +12,7 @@ extern unsigned int strhash(const char *buf);
 extern unsigned int strihash(const char *buf);
 extern unsigned int memhash(const void *buf, size_t len);
 extern unsigned int memihash(const void *buf, size_t len);
+extern unsigned int memihash_cont(unsigned int hash_seed, const void *buf, size_t len);
 
 static inline unsigned int sha1hash(const unsigned char *sha1)
 {
@@ -38,6 +39,7 @@ struct hashmap {
 	struct hashmap_entry **table;
 	hashmap_cmp_fn cmpfn;
 	unsigned int size, tablesize, grow_at, shrink_at;
+	unsigned disallow_rehash : 1;
 };
 
 struct hashmap_iter {
@@ -76,6 +78,29 @@ static inline void *hashmap_get_from_hash(const struct hashmap *map,
 	return hashmap_get(map, &key, keydata);
 }
 
+int hashmap_bucket(const struct hashmap *map, unsigned int hash);
+
+/*
+ * Disallow/allow rehashing of the hashmap.
+ * This is useful if the caller knows that the hashmap
+ * needs multi-threaded access.  The caller is still
+ * required to guard/lock searches and inserts in a
+ * manner appropriate to their usage.  This simply
+ * prevents the table from being unexpectedly re-mapped.
+ *
+ * If is up to the caller to ensure that the hashmap is
+ * initialized to a reasonable size to prevent poor
+ * performance.
+ *
+ * When value=1, prevent future rehashes on adds and deleted.
+ * When value=0, allow future rehahses.  This DOES NOT force
+ * a rehash now.
+ */
+static inline void hashmap_disallow_rehash(struct hashmap *map, unsigned value)
+{
+	map->disallow_rehash = value;
+}
+
 /* hashmap_iter functions */
 
 extern void hashmap_iter_init(struct hashmap *map, struct hashmap_iter *iter);
diff --git a/name-hash.c b/name-hash.c
index 6d9f23e932..cac313c78d 100644
--- a/name-hash.c
+++ b/name-hash.c
@@ -23,15 +23,21 @@ static int dir_entry_cmp(const struct dir_entry *e1,
 			name ? name : e2->name, e1->namelen);
 }
 
-static struct dir_entry *find_dir_entry(struct index_state *istate,
-		const char *name, unsigned int namelen)
+static struct dir_entry *find_dir_entry__hash(struct index_state *istate,
+		const char *name, unsigned int namelen, unsigned int hash)
 {
 	struct dir_entry key;
-	hashmap_entry_init(&key, memihash(name, namelen));
+	hashmap_entry_init(&key, hash);
 	key.namelen = namelen;
 	return hashmap_get(&istate->dir_hash, &key, name);
 }
 
+static struct dir_entry *find_dir_entry(struct index_state *istate,
+		const char *name, unsigned int namelen)
+{
+	return find_dir_entry__hash(istate, name, namelen, memihash(name, namelen));
+}
+
 static struct dir_entry *hash_dir_entry(struct index_state *istate,
 		struct cache_entry *ce, int namelen)
 {
@@ -112,20 +118,493 @@ static int cache_entry_cmp(const struct cache_entry *ce1,
 	return remove ? !(ce1 == ce2) : 0;
 }
 
-static void lazy_init_name_hash(struct index_state *istate)
+static int lazy_try_threaded = 1;
+static int lazy_nr_dir_threads;
+
+#ifdef NO_PTHREADS
+
+static inline int lookup_lazy_params(struct index_state *istate)
 {
-	int nr;
+	return 0;
+}
+
+static inline void threaded_lazy_init_name_hash(
+	struct index_state *istate)
+{
+}
+
+#else
+
+#include "thread-utils.h"
+
+/*
+ * Set a minimum number of cache_entries that we will handle per
+ * thread and use that to decide how many threads to run (upto
+ * the number on the system).
+ *
+ * For guidance setting the lower per-thread bound, see:
+ *     t/helper/test-lazy-init-name-hash --analyze
+ */
+#define LAZY_THREAD_COST (2000)
+
+/*
+ * We use n mutexes to guard n partitions of the "istate->dir_hash"
+ * hashtable.  Since "find" and "insert" operations will hash to a
+ * particular bucket and modify/search a single chain, we can say
+ * that "all chains mod n" are guarded by the same mutex -- rather
+ * than having a single mutex to guard the entire table.  (This does
+ * require that we disable "rehashing" on the hashtable.)
+ *
+ * So, a larger value here decreases the probability of a collision
+ * and the time that each thread must wait for the mutex.
+ */
+#define LAZY_MAX_MUTEX   (32)
+
+static pthread_mutex_t *lazy_dir_mutex_array;
+
+/*
+ * An array of lazy_entry items is used by the n threads in
+ * the directory parse (first) phase to (lock-free) store the
+ * intermediate results.  These values are then referenced by
+ * the 2 threads in the second phase.
+ */
+struct lazy_entry {
+	struct dir_entry *dir;
+	unsigned int hash_dir;
+	unsigned int hash_name;
+};
+
+/*
+ * Decide if we want to use threads (if available) to load
+ * the hash tables.  We set "lazy_nr_dir_threads" to zero when
+ * it is not worth it.
+ */
+static int lookup_lazy_params(struct index_state *istate)
+{
+	int nr_cpus;
+
+	lazy_nr_dir_threads = 0;
+
+	if (!lazy_try_threaded)
+		return 0;
+
+	/*
+	 * If we are respecting case, just use the original
+	 * code to build the "istate->name_hash".  We don't
+	 * need the complexity here.
+	 */
+	if (!ignore_case)
+		return 0;
+
+	nr_cpus = online_cpus();
+	if (nr_cpus < 2)
+		return 0;
+
+	if (istate->cache_nr < 2 * LAZY_THREAD_COST)
+		return 0;
+
+	if (istate->cache_nr < nr_cpus * LAZY_THREAD_COST)
+		nr_cpus = istate->cache_nr / LAZY_THREAD_COST;
+	lazy_nr_dir_threads = nr_cpus;
+	return lazy_nr_dir_threads;
+}
+
+/*
+ * Initialize n mutexes for use when searching and inserting
+ * into "istate->dir_hash".  All "dir" threads are trying
+ * to insert partial pathnames into the hash as they iterate
+ * over their portions of the index, so lock contention is
+ * high.
+ *
+ * However, the hashmap is going to put items into bucket
+ * chains based on their hash values.  Use that to create n
+ * mutexes and lock on mutex[bucket(hash) % n].  This will
+ * decrease the collision rate by (hopefully) by a factor of n.
+ */
+static void init_dir_mutex(void)
+{
+	int j;
+
+	lazy_dir_mutex_array = xcalloc(LAZY_MAX_MUTEX, sizeof(pthread_mutex_t));
+
+	for (j = 0; j < LAZY_MAX_MUTEX; j++)
+		init_recursive_mutex(&lazy_dir_mutex_array[j]);
+}
+
+static void cleanup_dir_mutex(void)
+{
+	int j;
+
+	for (j = 0; j < LAZY_MAX_MUTEX; j++)
+		pthread_mutex_destroy(&lazy_dir_mutex_array[j]);
+
+	free(lazy_dir_mutex_array);
+}
+
+static void lock_dir_mutex(int j)
+{
+	pthread_mutex_lock(&lazy_dir_mutex_array[j]);
+}
+
+static void unlock_dir_mutex(int j)
+{
+	pthread_mutex_unlock(&lazy_dir_mutex_array[j]);
+}
+
+static inline int compute_dir_lock_nr(
+	const struct hashmap *map,
+	unsigned int hash)
+{
+	return hashmap_bucket(map, hash) % LAZY_MAX_MUTEX;
+}
+
+static struct dir_entry *hash_dir_entry_with_parent_and_prefix(
+	struct index_state *istate,
+	struct dir_entry *parent,
+	struct strbuf *prefix)
+{
+	struct dir_entry *dir;
+	unsigned int hash;
+	int lock_nr;
+
+	/*
+	 * Either we have a parent directory and path with slash(es)
+	 * or the directory is an immediate child of the root directory.
+	 */
+	assert((parent != NULL) ^ (strchr(prefix->buf, '/') == NULL));
+
+	if (parent)
+		hash = memihash_cont(parent->ent.hash,
+			prefix->buf + parent->namelen,
+			prefix->len - parent->namelen);
+	else
+		hash = memihash(prefix->buf, prefix->len);
+
+	lock_nr = compute_dir_lock_nr(&istate->dir_hash, hash);
+	lock_dir_mutex(lock_nr);
+
+	dir = find_dir_entry__hash(istate, prefix->buf, prefix->len, hash);
+	if (!dir) {
+		FLEX_ALLOC_MEM(dir, name, prefix->buf, prefix->len);
+		hashmap_entry_init(dir, hash);
+		dir->namelen = prefix->len;
+		dir->parent = parent;
+		hashmap_add(&istate->dir_hash, dir);
+
+		if (parent) {
+			unlock_dir_mutex(lock_nr);
+
+			/* All I really need here is an InterlockedIncrement(&(parent->nr)) */
+			lock_nr = compute_dir_lock_nr(&istate->dir_hash, parent->ent.hash);
+			lock_dir_mutex(lock_nr);
+			parent->nr++;
+		}
+	}
+
+	unlock_dir_mutex(lock_nr);
 
+	return dir;
+}
+
+/*
+ * handle_range_1() and handle_range_dir() are derived from
+ * clear_ce_flags_1() and clear_ce_flags_dir() in unpack-trees.c
+ * and handle the iteration over the entire array of index entries.
+ * They use recursion for adjacent entries in the same parent
+ * directory.
+ */
+static int handle_range_1(
+	struct index_state *istate,
+	int k_start,
+	int k_end,
+	struct dir_entry *parent,
+	struct strbuf *prefix,
+	struct lazy_entry *lazy_entries);
+
+static int handle_range_dir(
+	struct index_state *istate,
+	int k_start,
+	int k_end,
+	struct dir_entry *parent,
+	struct strbuf *prefix,
+	struct lazy_entry *lazy_entries,
+	struct dir_entry **dir_new_out)
+{
+	int rc, k;
+	int input_prefix_len = prefix->len;
+	struct dir_entry *dir_new;
+
+	dir_new = hash_dir_entry_with_parent_and_prefix(istate, parent, prefix);
+
+	strbuf_addch(prefix, '/');
+
+	/*
+	 * Scan forward in the index array for index entries having the same
+	 * path prefix (that are also in this directory).
+	 */
+	if (strncmp(istate->cache[k_start + 1]->name, prefix->buf, prefix->len) > 0)
+		k = k_start + 1;
+	else if (strncmp(istate->cache[k_end - 1]->name, prefix->buf, prefix->len) == 0)
+		k = k_end;
+	else {
+		int begin = k_start;
+		int end = k_end;
+		while (begin < end) {
+			int mid = (begin + end) >> 1;
+			int cmp = strncmp(istate->cache[mid]->name, prefix->buf, prefix->len);
+			if (cmp == 0) /* mid has same prefix; look in second part */
+				begin = mid + 1;
+			else if (cmp > 0) /* mid is past group; look in first part */
+				end = mid;
+			else
+				die("cache entry out of order");
+		}
+		k = begin;
+	}
+
+	/*
+	 * Recurse and process what we can of this subset [k_start, k).
+	 */
+	rc = handle_range_1(istate, k_start, k, dir_new, prefix, lazy_entries);
+
+	strbuf_setlen(prefix, input_prefix_len);
+
+	*dir_new_out = dir_new;
+	return rc;
+}
+
+static int handle_range_1(
+	struct index_state *istate,
+	int k_start,
+	int k_end,
+	struct dir_entry *parent,
+	struct strbuf *prefix,
+	struct lazy_entry *lazy_entries)
+{
+	int input_prefix_len = prefix->len;
+	int k = k_start;
+
+	while (k < k_end) {
+		struct cache_entry *ce_k = istate->cache[k];
+		const char *name, *slash;
+
+		if (prefix->len && strncmp(ce_k->name, prefix->buf, prefix->len))
+			break;
+
+		name = ce_k->name + prefix->len;
+		slash = strchr(name, '/');
+
+		if (slash) {
+			int len = slash - name;
+			int processed;
+			struct dir_entry *dir_new;
+
+			strbuf_add(prefix, name, len);
+			processed = handle_range_dir(istate, k, k_end, parent, prefix, lazy_entries, &dir_new);
+			if (processed) {
+				k += processed;
+				strbuf_setlen(prefix, input_prefix_len);
+				continue;
+			}
+
+			strbuf_addch(prefix, '/');
+			processed = handle_range_1(istate, k, k_end, dir_new, prefix, lazy_entries);
+			k += processed;
+			strbuf_setlen(prefix, input_prefix_len);
+			continue;
+		}
+
+		/*
+		 * It is too expensive to take a lock to insert "ce_k"
+		 * into "istate->name_hash" and increment the ref-count
+		 * on the "parent" dir.  So we defer actually updating
+		 * permanent data structures until phase 2 (where we
+		 * can change the locking requirements) and simply
+		 * accumulate our current results into the lazy_entries
+		 * data array).
+		 *
+		 * We do not need to lock the lazy_entries array because
+		 * we have exclusive access to the cells in the range
+		 * [k_start,k_end) that this thread was given.
+		 */
+		lazy_entries[k].dir = parent;
+		if (parent) {
+			lazy_entries[k].hash_name = memihash_cont(
+				parent->ent.hash,
+				ce_k->name + parent->namelen,
+				ce_namelen(ce_k) - parent->namelen);
+			lazy_entries[k].hash_dir = parent->ent.hash;
+		} else {
+			lazy_entries[k].hash_name = memihash(ce_k->name, ce_namelen(ce_k));
+		}
+
+		k++;
+	}
+
+	return k - k_start;
+}
+
+struct lazy_dir_thread_data {
+	pthread_t pthread;
+	struct index_state *istate;
+	struct lazy_entry *lazy_entries;
+	int k_start;
+	int k_end;
+};
+
+static void *lazy_dir_thread_proc(void *_data)
+{
+	struct lazy_dir_thread_data *d = _data;
+	struct strbuf prefix = STRBUF_INIT;
+	handle_range_1(d->istate, d->k_start, d->k_end, NULL, &prefix, d->lazy_entries);
+	strbuf_release(&prefix);
+	return NULL;
+}
+
+struct lazy_name_thread_data {
+	pthread_t pthread;
+	struct index_state *istate;
+	struct lazy_entry *lazy_entries;
+};
+
+static void *lazy_name_thread_proc(void *_data)
+{
+	struct lazy_name_thread_data *d = _data;
+	int k;
+
+	for (k = 0; k < d->istate->cache_nr; k++) {
+		struct cache_entry *ce_k = d->istate->cache[k];
+		ce_k->ce_flags |= CE_HASHED;
+		hashmap_entry_init(ce_k, d->lazy_entries[k].hash_name);
+		hashmap_add(&d->istate->name_hash, ce_k);
+	}
+
+	return NULL;
+}
+
+static inline void lazy_update_dir_ref_counts(
+	struct index_state *istate,
+	struct lazy_entry *lazy_entries)
+{
+	int k;
+
+	for (k = 0; k < istate->cache_nr; k++) {
+		if (lazy_entries[k].dir)
+			lazy_entries[k].dir->nr++;
+	}
+}
+
+static void threaded_lazy_init_name_hash(
+	struct index_state *istate)
+{
+	int nr_each;
+	int k_start;
+	int t;
+	struct lazy_entry *lazy_entries;
+	struct lazy_dir_thread_data *td_dir;
+	struct lazy_name_thread_data *td_name;
+
+	k_start = 0;
+	nr_each = DIV_ROUND_UP(istate->cache_nr, lazy_nr_dir_threads);
+
+	lazy_entries = xcalloc(istate->cache_nr, sizeof(struct lazy_entry));
+	td_dir = xcalloc(lazy_nr_dir_threads, sizeof(struct lazy_dir_thread_data));
+	td_name = xcalloc(1, sizeof(struct lazy_name_thread_data));
+
+	init_dir_mutex();
+
+	/*
+	 * Phase 1:
+	 * Build "istate->dir_hash" using n "dir" threads (and a read-only index).
+	 */
+	for (t = 0; t < lazy_nr_dir_threads; t++) {
+		struct lazy_dir_thread_data *td_dir_t = td_dir + t;
+		td_dir_t->istate = istate;
+		td_dir_t->lazy_entries = lazy_entries;
+		td_dir_t->k_start = k_start;
+		k_start += nr_each;
+		if (k_start > istate->cache_nr)
+			k_start = istate->cache_nr;
+		td_dir_t->k_end = k_start;
+		if (pthread_create(&td_dir_t->pthread, NULL, lazy_dir_thread_proc, td_dir_t))
+			die("unable to create lazy_dir_thread");
+	}
+	for (t = 0; t < lazy_nr_dir_threads; t++) {
+		struct lazy_dir_thread_data *td_dir_t = td_dir + t;
+		if (pthread_join(td_dir_t->pthread, NULL))
+			die("unable to join lazy_dir_thread");
+	}
+
+	/*
+	 * Phase 2:
+	 * Iterate over all index entries and add them to the "istate->name_hash"
+	 * using a single "name" background thread.
+	 * (Testing showed it wasn't worth running more than 1 thread for this.)
+	 *
+	 * Meanwhile, finish updating the parent directory ref-counts for each
+	 * index entry using the current thread.  (This step is very fast and
+	 * doesn't need threading.)
+	 */
+	td_name->istate = istate;
+	td_name->lazy_entries = lazy_entries;
+	if (pthread_create(&td_name->pthread, NULL, lazy_name_thread_proc, td_name))
+		die("unable to create lazy_name_thread");
+
+	lazy_update_dir_ref_counts(istate, lazy_entries);
+
+	if (pthread_join(td_name->pthread, NULL))
+		die("unable to join lazy_name_thread");
+
+	cleanup_dir_mutex();
+
+	free(td_name);
+	free(td_dir);
+	free(lazy_entries);
+}
+
+#endif
+
+static void lazy_init_name_hash(struct index_state *istate)
+{
 	if (istate->name_hash_initialized)
 		return;
 	hashmap_init(&istate->name_hash, (hashmap_cmp_fn) cache_entry_cmp,
 			istate->cache_nr);
-	hashmap_init(&istate->dir_hash, (hashmap_cmp_fn) dir_entry_cmp, 0);
-	for (nr = 0; nr < istate->cache_nr; nr++)
-		hash_index_entry(istate, istate->cache[nr]);
+	hashmap_init(&istate->dir_hash, (hashmap_cmp_fn) dir_entry_cmp,
+			istate->cache_nr);
+
+	if (lookup_lazy_params(istate)) {
+		hashmap_disallow_rehash(&istate->dir_hash, 1);
+		threaded_lazy_init_name_hash(istate);
+		hashmap_disallow_rehash(&istate->dir_hash, 0);
+	} else {
+		int nr;
+		for (nr = 0; nr < istate->cache_nr; nr++)
+			hash_index_entry(istate, istate->cache[nr]);
+	}
+
 	istate->name_hash_initialized = 1;
 }
 
+/*
+ * A test routine for t/helper/ sources.
+ *
+ * Returns the number of threads used or 0 when
+ * the non-threaded code path was used.
+ *
+ * Requesting threading WILL NOT override guards
+ * in lookup_lazy_params().
+ */
+int test_lazy_init_name_hash(struct index_state *istate, int try_threaded)
+{
+	lazy_nr_dir_threads = 0;
+	lazy_try_threaded = try_threaded;
+
+	lazy_init_name_hash(istate);
+
+	return lazy_nr_dir_threads;
+}
+
 void add_name_hash(struct index_state *istate, struct cache_entry *ce)
 {
 	if (istate->name_hash_initialized)
diff --git a/t/helper/.gitignore b/t/helper/.gitignore
index d6e8b36798..758ed2e8fa 100644
--- a/t/helper/.gitignore
+++ b/t/helper/.gitignore
@@ -11,6 +11,7 @@
 /test-genrandom
 /test-hashmap
 /test-index-version
+/test-lazy-init-name-hash
 /test-line-buffer
 /test-match-trees
 /test-mergesort
diff --git a/t/helper/test-lazy-init-name-hash.c b/t/helper/test-lazy-init-name-hash.c
new file mode 100644
index 0000000000..6368a89345
--- /dev/null
+++ b/t/helper/test-lazy-init-name-hash.c
@@ -0,0 +1,264 @@
+#include "cache.h"
+#include "parse-options.h"
+
+static int single;
+static int multi;
+static int count = 1;
+static int dump;
+static int perf;
+static int analyze;
+static int analyze_step;
+
+/*
+ * Dump the contents of the "dir" and "name" hash tables to stdout.
+ * If you sort the result, you can compare it with the other type
+ * mode and verify that both single and multi produce the same set.
+ */
+static void dump_run(void)
+{
+	struct hashmap_iter iter_dir;
+	struct hashmap_iter iter_cache;
+
+	/* Stolen from name-hash.c */
+	struct dir_entry {
+		struct hashmap_entry ent;
+		struct dir_entry *parent;
+		int nr;
+		unsigned int namelen;
+		char name[FLEX_ARRAY];
+	};
+
+	struct dir_entry *dir;
+	struct cache_entry *ce;
+
+	read_cache();
+	if (single) {
+		test_lazy_init_name_hash(&the_index, 0);
+	} else {
+		int nr_threads_used = test_lazy_init_name_hash(&the_index, 1);
+		if (!nr_threads_used)
+			die("non-threaded code path used");
+	}
+
+	dir = hashmap_iter_first(&the_index.dir_hash, &iter_dir);
+	while (dir) {
+		printf("dir %08x %7d %s\n", dir->ent.hash, dir->nr, dir->name);
+		dir = hashmap_iter_next(&iter_dir);
+	}
+
+	ce = hashmap_iter_first(&the_index.name_hash, &iter_cache);
+	while (ce) {
+		printf("name %08x %s\n", ce->ent.hash, ce->name);
+		ce = hashmap_iter_next(&iter_cache);
+	}
+
+	discard_cache();
+}
+
+/*
+ * Run the single or multi threaded version "count" times and
+ * report on the time taken.
+ */
+static uint64_t time_runs(int try_threaded)
+{
+	uint64_t t0, t1, t2;
+	uint64_t sum = 0;
+	uint64_t avg;
+	int nr_threads_used;
+	int i;
+
+	for (i = 0; i < count; i++) {
+		t0 = getnanotime();
+		read_cache();
+		t1 = getnanotime();
+		nr_threads_used = test_lazy_init_name_hash(&the_index, try_threaded);
+		t2 = getnanotime();
+
+		sum += (t2 - t1);
+
+		if (try_threaded && !nr_threads_used)
+			die("non-threaded code path used");
+
+		if (nr_threads_used)
+			printf("%f %f %d multi %d\n",
+				   ((double)(t1 - t0))/1000000000,
+				   ((double)(t2 - t1))/1000000000,
+				   the_index.cache_nr,
+				   nr_threads_used);
+		else
+			printf("%f %f %d single\n",
+				   ((double)(t1 - t0))/1000000000,
+				   ((double)(t2 - t1))/1000000000,
+				   the_index.cache_nr);
+		fflush(stdout);
+
+		discard_cache();
+	}
+
+	avg = sum / count;
+	if (count > 1)
+		printf("avg %f %s\n",
+			   (double)avg/1000000000,
+			   (try_threaded) ? "multi" : "single");
+
+	return avg;
+}
+
+/*
+ * Try a series of runs varying the "istate->cache_nr" and
+ * try to find a good value for the multi-threaded criteria.
+ */
+static void analyze_run(void)
+{
+	uint64_t t1s, t1m, t2s, t2m;
+	int cache_nr_limit;
+	int nr_threads_used;
+	int i;
+	int nr;
+
+	read_cache();
+	cache_nr_limit = the_index.cache_nr;
+	discard_cache();
+
+	nr = analyze;
+	while (1) {
+		uint64_t sum_single = 0;
+		uint64_t sum_multi = 0;
+		uint64_t avg_single;
+		uint64_t avg_multi;
+
+		if (nr > cache_nr_limit)
+			nr = cache_nr_limit;
+
+		for (i = 0; i < count; i++) {
+			read_cache();
+			the_index.cache_nr = nr; /* cheap truncate of index */
+			t1s = getnanotime();
+			test_lazy_init_name_hash(&the_index, 0);
+			t2s = getnanotime();
+			sum_single += (t2s - t1s);
+			the_index.cache_nr = cache_nr_limit;
+			discard_cache();
+
+			read_cache();
+			the_index.cache_nr = nr; /* cheap truncate of index */
+			t1m = getnanotime();
+			nr_threads_used = test_lazy_init_name_hash(&the_index, 1);
+			t2m = getnanotime();
+			sum_multi += (t2m - t1m);
+			the_index.cache_nr = cache_nr_limit;
+			discard_cache();
+
+			if (!nr_threads_used)
+				printf("    [size %8d] [single %f]   non-threaded code path used\n",
+					   nr, ((double)(t2s - t1s))/1000000000);
+			else
+				printf("    [size %8d] [single %f] %c [multi %f %d]\n",
+					   nr,
+					   ((double)(t2s - t1s))/1000000000,
+					   (((t2s - t1s) < (t2m - t1m)) ? '<' : '>'),
+					   ((double)(t2m - t1m))/1000000000,
+					   nr_threads_used);
+			fflush(stdout);
+		}
+		if (count > 1) {
+			avg_single = sum_single / count;
+			avg_multi = sum_multi / count;
+			if (!nr_threads_used)
+				printf("avg [size %8d] [single %f]\n",
+					   nr,
+					   (double)avg_single/1000000000);
+			else
+				printf("avg [size %8d] [single %f] %c [multi %f %d]\n",
+					   nr,
+					   (double)avg_single/1000000000,
+					   (avg_single < avg_multi ? '<' : '>'),
+					   (double)avg_multi/1000000000,
+					   nr_threads_used);
+			fflush(stdout);
+		}
+
+		if (nr >= cache_nr_limit)
+			return;
+		nr += analyze_step;
+	}
+}
+
+int cmd_main(int argc, const char **argv)
+{
+	const char *usage[] = {
+		"test-lazy-init-name-hash -d (-s | -m)",
+		"test-lazy-init-name-hash -p [-c c]",
+		"test-lazy-init-name-hash -a a [--step s] [-c c]",
+		"test-lazy-init-name-hash (-s | -m) [-c c]",
+		"test-lazy-init-name-hash -s -m [-c c]",
+		NULL
+	};
+	struct option options[] = {
+		OPT_BOOL('s', "single", &single, "run single-threaded code"),
+		OPT_BOOL('m', "multi", &multi, "run multi-threaded code"),
+		OPT_INTEGER('c', "count", &count, "number of passes"),
+		OPT_BOOL('d', "dump", &dump, "dump hash tables"),
+		OPT_BOOL('p', "perf", &perf, "compare single vs multi"),
+		OPT_INTEGER('a', "analyze", &analyze, "analyze different multi sizes"),
+		OPT_INTEGER(0, "step", &analyze_step, "analyze step factor"),
+		OPT_END(),
+	};
+	const char *prefix;
+	uint64_t avg_single, avg_multi;
+
+	prefix = setup_git_directory();
+
+	argc = parse_options(argc, argv, prefix, options, usage, 0);
+
+	/*
+	 * istate->dir_hash is only created when ignore_case is set.
+	 */
+	ignore_case = 1;
+
+	if (dump) {
+		if (perf || analyze > 0)
+			die("cannot combine dump, perf, or analyze");
+		if (count > 1)
+			die("count not valid with dump");
+		if (single && multi)
+			die("cannot use both single and multi with dump");
+		if (!single && !multi)
+			die("dump requires either single or multi");
+		dump_run();
+		return 0;
+	}
+
+	if (perf) {
+		if (analyze > 0)
+			die("cannot combine dump, perf, or analyze");
+		if (single || multi)
+			die("cannot use single or multi with perf");
+		avg_single = time_runs(0);
+		avg_multi = time_runs(1);
+		if (avg_multi > avg_single)
+			die("multi is slower");
+		return 0;
+	}
+
+	if (analyze) {
+		if (analyze < 500)
+			die("analyze must be at least 500");
+		if (!analyze_step)
+			analyze_step = analyze;
+		if (single || multi)
+			die("cannot use single or multi with analyze");
+		analyze_run();
+		return 0;
+	}
+
+	if (!single && !multi)
+		die("require either -s or -m or both");
+
+	if (single)
+		time_runs(0);
+	if (multi)
+		time_runs(1);
+
+	return 0;
+}
diff --git a/t/perf/p0004-lazy-init-name-hash.sh b/t/perf/p0004-lazy-init-name-hash.sh
new file mode 100644
index 0000000000..5afa8c8df3
--- /dev/null
+++ b/t/perf/p0004-lazy-init-name-hash.sh
@@ -0,0 +1,19 @@
+#!/bin/sh
+
+test_description='Tests multi-threaded lazy_init_name_hash'
+. ./perf-lib.sh
+
+test_perf_large_repo
+test_checkout_worktree
+
+test_expect_success 'verify both methods build the same hashmaps' '
+	$GIT_BUILD_DIR/t/helper/test-lazy-init-name-hash$X --dump --single | sort >out.single &&
+	$GIT_BUILD_DIR/t/helper/test-lazy-init-name-hash$X --dump --multi  | sort >out.multi  &&
+	test_cmp out.single out.multi
+'
+
+test_expect_success 'multithreaded should be faster' '
+	$GIT_BUILD_DIR/t/helper/test-lazy-init-name-hash$X --perf >out.perf
+'
+
+test_done