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pack-objects: reorder members to shrink struct object_entry

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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>
This commit is contained in:
Nguyễn Thái Ngọc Duy 2018-04-14 17:35:12 +02:00 committed by Junio C Hamano
parent 0aca34e826
commit 3b13a5f263
1 changed files with 22 additions and 8 deletions
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30
pack-objects.h
View File

@ -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_:TYPE_BITS;
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 type_:TYPE_BITS;
unsigned no_try_delta:1;
unsigned in_pack_type:TYPE_BITS; /* could be delta */
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 {