From 521a4f4cf4bb2d59a51a5355bfb6b148c4aa31ed Mon Sep 17 00:00:00 2001 From: Linus Torvalds Date: Sun, 26 Jun 2005 13:43:41 -0700 Subject: [PATCH] git-pack-objects: do the delta search in reverse size order Starting from big objects and going backwards means that we end up picking a delta that goes from a bigger object to a smaller one. That's advantageous for two reasons: the bigger object is likely the newer one (since things tend to grow, rather than shrink), and doing a delete tends to be smaller than doing an add. So the deltas don't tend to be top-of-tree, and the packed end result is just slightly smaller. --- pack-objects.c | 28 ++++++++++++++++------------ 1 file changed, 16 insertions(+), 12 deletions(-) diff --git a/pack-objects.c b/pack-objects.c index aa6cb6d35..286e4501c 100644 --- a/pack-objects.c +++ b/pack-objects.c @@ -280,18 +280,18 @@ struct unpacked { }; /* - * We search for deltas in a list sorted by type and by size, and - * walk the "old" chain backwards in the list, so if the type - * has changed or the size difference is too big, there's no point - * in even continuing the walk, since the other old objects are - * going to be even smaller or of a different type. So return -1 - * once we determine that there's no point even trying. + * We search for deltas _backwards_ in a list sorted by type and + * by size, so that we see progressively smaller and smaller files. + * That's because we prefer deltas to be from the bigger file + * to the smaller - deletes are potentially cheaper, but perhaps + * more importantly, the bigger file is likely the more recent + * one. */ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_depth) { struct object_entry *cur_entry = cur->entry; struct object_entry *old_entry = old->entry; - unsigned long size, oldsize, delta_size; + unsigned long size, oldsize, delta_size, sizediff; long max_size; void *delta_buf; @@ -299,12 +299,12 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de if (cur_entry->type != old_entry->type) return -1; - /* Size is guaranteed to be larger than or equal to oldsize */ size = cur_entry->size; if (size < 50) return -1; oldsize = old_entry->size; - if (size - oldsize > oldsize / 4) + sizediff = oldsize > size ? oldsize - size : size - oldsize; + if (sizediff > size / 8) return -1; if (old_entry->depth >= max_depth) return 0; @@ -332,13 +332,14 @@ static int try_delta(struct unpacked *cur, struct unpacked *old, unsigned max_de static void find_deltas(struct object_entry **list, int window, int depth) { - unsigned int i; + int i, idx; unsigned int array_size = window * sizeof(struct unpacked); struct unpacked *array = xmalloc(array_size); memset(array, 0, array_size); - for (i = 0; i < nr_objects; i++) { - unsigned int idx = i % window; + i = nr_objects; + idx = 0; + while (--i >= 0) { struct object_entry *entry = list[i]; struct unpacked *n = array + idx; unsigned long size; @@ -362,6 +363,9 @@ static void find_deltas(struct object_entry **list, int window, int depth) if (try_delta(n, m, depth) < 0) break; } + idx++; + if (idx >= window) + idx = 0; } } -- 2.26.2