Re: [PATCH] pack-objects: fix performance issues on packing large deltas

[Duy Nguyen <pclouds@gmail.com>]

On Fri, Jul 20, 2018 at 10:43:25AM -0700, Elijah Newren wrote:
> > This patch provides a better fallback that is
> >
> > - cheaper in terms of cpu and io because we won't have to read
> >   existing pack files as much
> >
> > - better in terms of pack size because the pack heuristics is back to
> >   2.17.0 time, we do not drop large deltas at all
> >
> > If we encounter any delta (on-disk or created during try_delta phase)
> > that is larger than the 2MB limit, we stop using delta_size_ field for
> > this because it can't contain such size anyway. A new array of delta
> > size is dynamically allocated and can hold all the deltas that 2.17.0
> > can [1]. All current delta sizes are migrated over.
> >
> > With this, we do not have to drop deltas in try_delta() anymore. Of
> > course the downside is we use slightly more memory, even compared to
> > 2.17.0. But since this is considered an uncommon case, a bit more
> > memory consumption should not be a problem.
> 
> Out of curiosity, would it be possible to use the delta_size_ field
> for deltas that are small enough, and only use an external data
> structure (perhaps a hash rather than an array) for the few deltas
> that are large?

We could. And because repack time is still a bit higher in your
linux.git case. Let's try this. No locking in common case and very
small locked region when we hit large deltas

-- 8< --
diff --git a/pack-objects.c b/pack-objects.c
index eef344b7c1..e3c32bbfc2 100644
--- a/pack-objects.c
+++ b/pack-objects.c
@@ -88,23 +88,6 @@ struct object_entry *packlist_find(struct packing_data *pdata,
 	return &pdata->objects[pdata->index[i] - 1];
 }
 
-uint32_t *new_delta_size_array(struct packing_data *pack)
-{
-	uint32_t *delta_size;
-	uint32_t i;
-
-	/*
-	 * nr_alloc, not nr_objects to align with realloc() strategy in
-	 * packlist_alloc()
-	 */
-	ALLOC_ARRAY(delta_size, pack->nr_alloc);
-
-	for (i = 0; i < pack->nr_objects; i++)
-		delta_size[i] = pack->objects[i].delta_size_;
-
-	return delta_size;
-}
-
 static void prepare_in_pack_by_idx(struct packing_data *pdata)
 {
 	struct packed_git **mapping, *p;
diff --git a/pack-objects.h b/pack-objects.h
index 9f977ae800..11890e7217 100644
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -15,7 +15,7 @@
  * above this limit. Don't lower it too much.
  */
 #define OE_SIZE_BITS		31
-#define OE_DELTA_SIZE_BITS	24
+#define OE_DELTA_SIZE_BITS	23
 
 /*
  * State flags for depth-first search used for analyzing delta cycles.
@@ -94,6 +94,7 @@ struct object_entry {
 				     * uses the same base as me
 				     */
 	unsigned delta_size_:OE_DELTA_SIZE_BITS; /* delta data size (uncompressed) */
+	unsigned delta_size_valid:1;
 	unsigned char in_pack_header_size;
 	unsigned in_pack_idx:OE_IN_PACK_BITS;	/* already in pack */
 	unsigned z_delta_size:OE_Z_DELTA_BITS;
@@ -353,37 +354,26 @@ static inline void oe_set_size(struct packing_data *pack,
 static inline unsigned long oe_delta_size(struct packing_data *pack,
 					  const struct object_entry *e)
 {
-	unsigned long size;
-
-	packing_data_lock(pack);
-	if (pack->delta_size)
-		size = pack->delta_size[e - pack->objects];
+	if (e->delta_size_valid)
+		return e->delta_size_;
 	else
-		size = e->delta_size_;
-	packing_data_unlock(pack);
-	return size;
+		return pack->delta_size[e - pack->objects];
 }
 
-uint32_t *new_delta_size_array(struct packing_data *pdata);
 static inline void oe_set_delta_size(struct packing_data *pack,
 				     struct object_entry *e,
 				     unsigned long size)
 {
-	packing_data_lock(pack);
-	if (!pack->delta_size && size < pack->oe_delta_size_limit) {
-		packing_data_unlock(pack);
+	if (size < pack->oe_delta_size_limit) {
 		e->delta_size_ = size;
-		return;
+		e->delta_size_valid = 1;
+	} else {
+		packing_data_lock(pack);
+		if (!pack->delta_size)
+			ALLOC_ARRAY(pack->delta_size, pack->nr_alloc);
+		packing_data_unlock(pack);
+		pack->delta_size[e - pack->objects] = size;
 	}
-	/*
-	 * We have had at least one delta size exceeding OE_DELTA_SIZE_BITS
-	 * limit. delta_size_ will not be used anymore. All delta sizes are
-	 * now from the delta_size[] array.
-	 */
-	if (!pack->delta_size)
-		pack->delta_size = new_delta_size_array(pack);
-	pack->delta_size[e - pack->objects] = size;
-	packing_data_unlock(pack);
 }
 
 #endif
-- 8< --

> > --
> > 2.18.0.rc2.476.g39500d3211
> 
> Missing the 2.18.0 tag?  ;-)

Hehe I was a bit busy lately and have not rebased my branch on the
latest and greatest version.
--
Duy