[PATCH (experimental)] preload-index: make parallel IO configurable

[Karsten Blees <karsten.blees@gmail.com>]

Make the amount of IO parallelism configurable (i.e. number of separate
index sections to be processed in parallel).

Defining PARALLEL_IO = 20 restores approximately the IO pattern of the
original preload_index() implementation. Best performance is achieved with
PARALLEL_IO = 1 (i.e. sequential IO).

Signed-off-by: Karsten Blees <blees@dcon.de>
---

Applies on top of "preload-index: optimize for sequential IO".

 preload-index.c | 22 +++++++++++++++++-----
 1 file changed, 17 insertions(+), 5 deletions(-)

diff --git a/preload-index.c b/preload-index.c
index 6ac368d..5fe5521 100644
--- a/preload-index.c
+++ b/preload-index.c
@@ -23,6 +23,7 @@ static void preload_index(struct index_state *index,
  */
 #define MAX_PARALLEL (20)
 #define THREAD_COST (500)
+#define PARALLEL_IO (1)
 
 struct thread_data {
 	pthread_t pthread;
@@ -38,23 +39,34 @@ static void *preload_thread(void *_data)
 	struct thread_data *p = _data;
 	struct index_state *index = p->index;
 	struct cache_def cache;
+	int blocks = DIV_ROUND_UP(index->cache_nr, THREAD_COST);
+	int step = DIV_ROUND_UP(blocks, PARALLEL_IO);
 
 	memset(&cache, 0, sizeof(cache));
 	for (;;) {
 		/* get next batch of entries to check */
 		pthread_mutex_lock(p->pmutex);
 		nr = *p->pnr;
-		*p->pnr += THREAD_COST;
+		(*p->pnr)++;
 		pthread_mutex_unlock(p->pmutex);
 
+		/* break loop if no more work to do */
+		if (nr >= blocks + PARALLEL_IO - 1)
+			break;
+
+		/*
+		 * rearrange iteration order, i.e. with PARALLEL_IO = 2,
+		 * [0, 1, 2, 3, 4, 5, 6, 7] becomes [0, 4, 1, 5, 2, 6, 3, 7]
+		 */
+		nr = (nr / PARALLEL_IO) + (nr % PARALLEL_IO) * step;
+		if (nr >= blocks)
+			continue;
+		nr *= THREAD_COST;
+
 		max_nr = nr + THREAD_COST;
 		if (max_nr > index->cache_nr)
 			max_nr = index->cache_nr;
 
-		/* break loop if no more work to do */
-		if (nr >= max_nr)
-			break;
-
 		for (; nr < max_nr; nr++) {
 			struct cache_entry *ce = index->cache[nr];
 			struct stat st;
-- 
1.9.4.msysgit.0.1.gc8a51b4