Disable threaded index writing by default

The interlocking on the write threads was not being done properly (index
entries were sometimes written out of order). With proper interlocking,
the threaded write is only marginally faster on big index files, and
slower on the smaller ones because of the overhead when creating
threads.

The threaded index writing has been temporarily disabled; after more
accurate benchmarks, if might be possible to enable it again only when
writing very large index files (> 1000 entries).

Signed-off-by: Vicent Marti <tanoku@gmail.com>

src/index.c

@@ -730,9 +730,9 @@ static void *create_disk_entry(size_t *disk_size, git_index_entry *entry)
 	return ondisk;
 }
 
-#ifdef GIT_THREADS
+#if defined(GIT_THREADS) && defined(GIT_INDEX_THREADED)
 
-#define THREAD_QUEUE_SIZE 8
+#define THREAD_QUEUE_SIZE 64
 
 typedef struct {
 	void *data;
@@ -745,7 +745,7 @@ typedef struct {
 	void (*process_entry)(void *extra_data, index_thread_entry *entry);
 
 	index_thread_entry *buffer[THREAD_QUEUE_SIZE];
-	int count, pos;
+	int count, read_pos, write_pos;
 
 	git_lck mutex;
 	git_cnd entry_available, space_available;
@@ -757,7 +757,7 @@ void index_thread_enqueue(index_thread_queue *queue, index_thread_entry *entry)
 	if (queue->count == THREAD_QUEUE_SIZE)
 		gitcnd_wait(&queue->space_available, &queue->mutex);
 
-	queue->buffer[queue->pos++ % THREAD_QUEUE_SIZE] = entry;
+	queue->buffer[queue->write_pos++ % THREAD_QUEUE_SIZE] = entry;
 	queue->count++;
 
 	gitcnd_signal(&queue->entry_available);
@@ -785,12 +785,15 @@ void *index_thread(void *attr)
 		if (queue->count == 0)
 			gitcnd_wait(&queue->entry_available, &queue->mutex);
 
-		entry = queue->buffer[(queue->pos - 1) % THREAD_QUEUE_SIZE];
+		entry = queue->buffer[queue->read_pos++ % THREAD_QUEUE_SIZE];
 		queue->count--;
 
 		gitcnd_signal(&queue->space_available);
 		gitlck_unlock(&queue->mutex);
 
+		if (entry == NULL)
+			break;
+
 		queue->process_entry(queue->extra_data, entry);
 
 		if (gitrc_dec(&entry->refcount)) {
@@ -827,14 +830,16 @@ static int write_entries(git_index *index, git_filelock *file, git_hash_ctx *dig
 		write_queue->process_entry = thread_write_entry;
 
 		write_queue->count = 0;
-		write_queue->pos = 0;
+		write_queue->read_pos = 0;
+		write_queue->write_pos = 0;
+
 		gitlck_init(&write_queue->mutex);
 		gitcnd_init(&write_queue->space_available, NULL);
 		gitcnd_init(&write_queue->entry_available, NULL);
 
 		if (git_thread_create(&write_thread, NULL, index_thread, (void *)write_queue) < 0) {
 			error = GIT_EOSERR;
-			goto cleanup;
+			goto thread_error;
 		}
 	}
 
@@ -850,14 +855,16 @@ static int write_entries(git_index *index, git_filelock *file, git_hash_ctx *dig
 		hash_queue->process_entry = thread_hash_entry;
 
 		hash_queue->count = 0;
-		hash_queue->pos = 0;
+		hash_queue->read_pos = 0;
+		hash_queue->write_pos = 0;
+
 		gitlck_init(&hash_queue->mutex);
 		gitcnd_init(&hash_queue->space_available, NULL);
 		gitcnd_init(&hash_queue->entry_available, NULL);
 
 		if (git_thread_create(&hash_thread, NULL, index_thread, (void *)hash_queue) < 0) {
 			error = GIT_EOSERR;
-			goto cleanup;
+			goto thread_error;
 		}
 	}
 
@@ -875,13 +882,13 @@ static int write_entries(git_index *index, git_filelock *file, git_hash_ctx *dig
 		thread_entry = git__malloc(sizeof(index_thread_entry));
 		if (thread_entry == NULL) {
 			error = GIT_ENOMEM;
-			goto cleanup;
+			goto thread_error;
 		}
 
 		thread_entry->data = create_disk_entry(&thread_entry->size, entry);
 		if (thread_entry->data == NULL) {
 			error = GIT_ENOMEM;
-			goto cleanup;
+			goto thread_error;
 		}
 
 		/* queue in both queues */
@@ -891,7 +898,22 @@ static int write_entries(git_index *index, git_filelock *file, git_hash_ctx *dig
 		index_thread_enqueue(hash_queue, thread_entry);
 	}
 
-cleanup:
+	/* kill the two threads by queuing a NULL item */
+	{
+		index_thread_enqueue(write_queue, NULL);
+		index_thread_enqueue(hash_queue, NULL);
+	}
+
+	/* wait for them to terminate */
+	git_thread_join(write_thread, NULL);
+	git_thread_join(hash_thread, NULL);
+
+	free(write_queue);
+	free(hash_queue);
+
+	return GIT_SUCCESS;
+
+thread_error:
 	git_thread_kill(write_thread);
 	git_thread_kill(hash_thread);