pack-objects: reorder members to shrink struct object_entry

Previous patches leave lots of holes and padding in this struct. This
patch reorders the members and shrinks the struct down to 80 bytes
(from 136 bytes on 64-bit systems, before any field shrinking is done)
with 16 bits to spare (and a couple more in in_pack_header_size when
we really run out of bits).

This is the last in a series of memory reduction patches (see
"pack-objects: a bit of document about struct object_entry" for the
first one).

Overall they've reduced repack memory size on linux-2.6.git from
3.747G to 3.424G, or by around 320M, a decrease of 8.5%. The runtime
of repack has stayed the same throughout this series. Ævar's testing
on a big monorepo he has access to (bigger than linux-2.6.git) has
shown a 7.9% reduction, so the overall expected improvement should be
somewhere around 8%.

See 87po42cwql.fsf@evledraar.gmail.com on-list
(https://public-inbox.org/git/87po42cwql.fsf@evledraar.gmail.com/) for
more detailed numbers and a test script used to produce the numbers
cited above.

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

diff --git a/pack-objects.h b/pack-objects.h
index 1c588184b28658f9fdaee458019b9c81e5e8f252..e5456c6c89173b13090a58e95d3075a7c926ab39 100644 (file)
--- a/pack-objects.h
+++ b/pack-objects.h
@@ -28,6 +28,10 @@ enum dfs_state {
 };
 
 /*
+ * The size of struct nearly determines pack-objects's memory
+ * consumption. This struct is packed tight for that reason. When you
+ * add or reorder something in this struct, think a bit about this.
+ *
  * basic object info
  * -----------------
  * idx.oid is filled up before delta searching starts. idx.crc32 is
@@ -76,34 +80,44 @@ enum dfs_state {
  */
 struct object_entry {
        struct pack_idx_entry idx;
+       void *delta_data;       /* cached delta (uncompressed) */
+       off_t in_pack_offset;
+       uint32_t hash;                  /* name hint hash */
        unsigned size_:OE_SIZE_BITS;
        unsigned size_valid:1;
-       unsigned in_pack_idx:OE_IN_PACK_BITS;   /* already in pack */
-       off_t in_pack_offset;
        uint32_t delta_idx;     /* delta base object */
        uint32_t delta_child_idx; /* deltified objects who bases me */
        uint32_t delta_sibling_idx; /* other deltified objects who
                                     * uses the same base as me
                                     */
-       void *delta_data;       /* cached delta (uncompressed) */
        unsigned delta_size_:OE_DELTA_SIZE_BITS; /* delta data size (uncompressed) */
        unsigned delta_size_valid:1;
+       unsigned in_pack_idx:OE_IN_PACK_BITS;   /* already in pack */
        unsigned z_delta_size:OE_Z_DELTA_BITS;
+       unsigned type_valid:1;
        unsigned type_:TYPE_BITS;
+       unsigned no_try_delta:1;
        unsigned in_pack_type:TYPE_BITS; /* could be delta */
-       unsigned type_valid:1;
-       uint32_t hash;                  /* name hint hash */
-       unsigned char in_pack_header_size;
        unsigned preferred_base:1; /*
                                    * we do not pack this, but is available
                                    * to be used as the base object to delta
                                    * objects against.
                                    */
-       unsigned no_try_delta:1;
        unsigned tagged:1; /* near the very tip of refs */
        unsigned filled:1; /* assigned write-order */
        unsigned dfs_state:OE_DFS_STATE_BITS;
+       unsigned char in_pack_header_size;
        unsigned depth:OE_DEPTH_BITS;
+
+       /*
+        * pahole results on 64-bit linux (gcc and clang)
+        *
+        *   size: 80, bit_padding: 20 bits, holes: 8 bits
+        *
+        * and on 32-bit (gcc)
+        *
+        *   size: 76, bit_padding: 20 bits, holes: 8 bits
+        */
 };
 
 struct packing_data {