index-pack: reduce object_entry size to save memory

For each object in the input pack, we need one struct object_entry. On
x86-64, this struct is 64 bytes long. Although:

 - The 8 bytes for delta_depth and base_object_no are only useful when
   show_stat is set. And it's never set unless someone is debugging.

 - The three fields hdr_size, type and real_type take 4 bytes each
   even though they never use more than 4 bits.

By moving delta_depth and base_object_no out of struct object_entry
and make the other 3 fields one byte long instead of 4, we shrink 25%
of this struct.

On a 3.4M object repo (*) that's about 53MB. The saving is less
impressive compared to index-pack memory use for basic bookkeeping (**),
about 16%.

(*) linux-2.6.git already has 4M objects as of v3.19-rc7 so this is
not an unrealistic number of objects that we have to deal with.

(**)  3.4M * (sizeof(object_entry) + sizeof(delta_entry)) = 311MB

Brought-up-by: Matthew Sporleder <msporleder@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/builtin/index-pack.c b/builtin/index-pack.c
index 46321176719808dd53a38675dd0d9c558ae336dd..9d854fb751931840449b2d5dc0b30c9edcb6164c 100644 (file)
--- a/builtin/index-pack.c
+++ b/builtin/index-pack.c
@@ -18,9 +18,12 @@ static const char index_pack_usage[] =
 struct object_entry {
        struct pack_idx_entry idx;
        unsigned long size;
-       unsigned int hdr_size;
-       enum object_type type;
-       enum object_type real_type;
+       unsigned char hdr_size;
+       signed char type;
+       signed char real_type;
+};
+
+struct object_stat {
        unsigned delta_depth;
        int base_object_no;
 };
@@ -64,6 +67,7 @@ struct delta_entry {
 };
 
 static struct object_entry *objects;
+static struct object_stat *obj_stat;
 static struct delta_entry *deltas;
 static struct thread_local nothread_data;
 static int nr_objects;
@@ -873,13 +877,15 @@ static void resolve_delta(struct object_entry *delta_obj,
        void *base_data, *delta_data;
 
        if (show_stat) {
-               delta_obj->delta_depth = base->obj->delta_depth + 1;
+               int i = delta_obj - objects;
+               int j = base->obj - objects;
+               obj_stat[i].delta_depth = obj_stat[j].delta_depth + 1;
                deepest_delta_lock();
-               if (deepest_delta < delta_obj->delta_depth)
-                       deepest_delta = delta_obj->delta_depth;
+               if (deepest_delta < obj_stat[i].delta_depth)
+                       deepest_delta = obj_stat[i].delta_depth;
                deepest_delta_unlock();
+               obj_stat[i].base_object_no = j;
        }
-       delta_obj->base_object_no = base->obj - objects;
        delta_data = get_data_from_pack(delta_obj);
        base_data = get_base_data(base);
        result->obj = delta_obj;
@@ -902,7 +908,7 @@ static void resolve_delta(struct object_entry *delta_obj,
  * "want"; if so, swap in "set" and return true. Otherwise, leave it untouched
  * and return false.
  */
-static int compare_and_swap_type(enum object_type *type,
+static int compare_and_swap_type(signed char *type,
                                 enum object_type want,
                                 enum object_type set)
 {
@@ -1499,7 +1505,7 @@ static void show_pack_info(int stat_only)
                struct object_entry *obj = &objects[i];
 
                if (is_delta_type(obj->type))
-                       chain_histogram[obj->delta_depth - 1]++;
+                       chain_histogram[obj_stat[i].delta_depth - 1]++;
                if (stat_only)
                        continue;
                printf("%s %-6s %lu %lu %"PRIuMAX,
@@ -1508,8 +1514,8 @@ static void show_pack_info(int stat_only)
                       (unsigned long)(obj[1].idx.offset - obj->idx.offset),
                       (uintmax_t)obj->idx.offset);
                if (is_delta_type(obj->type)) {
-                       struct object_entry *bobj = &objects[obj->base_object_no];
-                       printf(" %u %s", obj->delta_depth, sha1_to_hex(bobj->idx.sha1));
+                       struct object_entry *bobj = &objects[obj_stat[i].base_object_no];
+                       printf(" %u %s", obj_stat[i].delta_depth, sha1_to_hex(bobj->idx.sha1));
                }
                putchar('\n');
        }
@@ -1672,6 +1678,8 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
        curr_pack = open_pack_file(pack_name);
        parse_pack_header();
        objects = xcalloc(nr_objects + 1, sizeof(struct object_entry));
+       if (show_stat)
+               obj_stat = xcalloc(nr_objects + 1, sizeof(struct object_stat));
        deltas = xcalloc(nr_objects, sizeof(struct delta_entry));
        parse_pack_objects(pack_sha1);
        resolve_deltas();