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index-pack: kill union delta_base to save memory 

Once we know the number of objects in the input pack, we allocate an
array of nr_objects of struct delta_entry. On x86-64, this struct is
32 bytes long. The union delta_base, which is part of struct
delta_entry, provides enough space to store either ofs-delta (8 bytes)
or ref-delta (20 bytes).

Because ofs-delta encoding is more efficient space-wise and more
performant at runtime than ref-delta encoding, Git packers try to use
ofs-delta whenever possible, and it is expected that objects encoded
as ref-delta are minority.

In the best clone case where no ref-delta object is present, we waste
(20-8) * nr_objects bytes because of this union. That's about 38MB out
of 100MB for deltas[] with 3.4M objects, or 38%. deltas[] would be
around 62MB without the waste.

This patch attempts to eliminate that. deltas[] array is split into
two: one for ofs-delta and one for ref-delta. Many functions are also
duplicated because of this split. With this patch, ofs_deltas[] array
takes 51MB. ref_deltas[] should remain unallocated in clone case (0
bytes). This array grows as we see ref-delta. We save about half in
this case, or 25% of total bookkeeping.

The saving is more than the calculation above because some padding in
the old delta_entry struct is removed. ofs_delta_entry is 16 bytes,
including the 4 bytes padding. That's 13MB for padding, but packing
the struct could break platforms that do not support unaligned
access. If someone on 32-bit is really low on memory and only deals
with packs smaller than 2G, using 32-bit off_t would eliminate the
padding and save 27MB on top.

A note about ofs_deltas allocation. We could use ref_deltas memory
allocation strategy for ofs_deltas. But that probably just adds more
overhead on top. ofs-deltas are generally the majority (1/2 to 2/3) in
any pack. Incremental realloc may lead to too many memcpy. And if we
preallocate, say 1/2 or 2/3 of nr_objects initially, the growth rate
of ALLOC_GROW() could make this array larger than nr_objects, wasting
more memory.

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>
This commit is contained in:
Nguyễn Thái Ngọc Duy 2015-04-18 17:47:05 +07:00 committed by Junio C Hamano
parent 417305764a
commit c6458e60ed
1 changed files with 158 additions and 98 deletions
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256
builtin/index-pack.c
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@ -28,11 +28,6 @@ struct object_stat {
int base_object_no;
};
union delta_base {
unsigned char sha1[20];
off_t offset;
};
struct base_data {
struct base_data *base;
struct base_data *child;
@ -52,26 +47,28 @@ struct thread_local {
int pack_fd;
};
/*
* Even if sizeof(union delta_base) == 24 on 64-bit archs, we really want
* to memcmp() only the first 20 bytes.
*/
#define UNION_BASE_SZ 20
#define FLAG_LINK (1u<<20)
#define FLAG_CHECKED (1u<<21)
struct delta_entry {
union delta_base base;
struct ofs_delta_entry {
off_t offset;
int obj_no;
};
struct ref_delta_entry {
unsigned char sha1[20];
int obj_no;
};
static struct object_entry *objects;
static struct object_stat *obj_stat;
static struct delta_entry *deltas;
static struct ofs_delta_entry *ofs_deltas;
static struct ref_delta_entry *ref_deltas;
static struct thread_local nothread_data;
static int nr_objects;
static int nr_deltas;
static int nr_ofs_deltas;
static int nr_ref_deltas;
static int ref_deltas_alloc;
static int nr_resolved_deltas;
static int nr_threads;
@ -480,7 +477,8 @@ static void *unpack_entry_data(unsigned long offset, unsigned long size,
}
static void *unpack_raw_entry(struct object_entry *obj,
union delta_base *delta_base,
off_t *ofs_offset,
unsigned char *ref_sha1,
unsigned char *sha1)
{
unsigned char *p;
@ -509,11 +507,10 @@ static void *unpack_raw_entry(struct object_entry *obj,
switch (obj->type) {
case OBJ_REF_DELTA:
hashcpy(delta_base->sha1, fill(20));
hashcpy(ref_sha1, fill(20));
use(20);
break;
case OBJ_OFS_DELTA:
memset(delta_base, 0, sizeof(*delta_base));
p = fill(1);
c = *p;
use(1);
@ -527,8 +524,8 @@ static void *unpack_raw_entry(struct object_entry *obj,
use(1);
base_offset = (base_offset << 7) + (c & 127);
}
delta_base->offset = obj->idx.offset - base_offset;
if (delta_base->offset <= 0 || delta_base->offset >= obj->idx.offset)
*ofs_offset = obj->idx.offset - base_offset;
if (*ofs_offset <= 0 || *ofs_offset >= obj->idx.offset)
bad_object(obj->idx.offset, _("delta base offset is out of bound"));
break;
case OBJ_COMMIT:
@ -612,55 +609,108 @@ static void *get_data_from_pack(struct object_entry *obj)
return unpack_data(obj, NULL, NULL);
}
static int compare_delta_bases(const union delta_base *base1,
const union delta_base *base2,
enum object_type type1,
enum object_type type2)
static int compare_ofs_delta_bases(off_t offset1, off_t offset2,
enum object_type type1,
enum object_type type2)
{
int cmp = type1 - type2;
if (cmp)
return cmp;
return memcmp(base1, base2, UNION_BASE_SZ);
return offset1 - offset2;
}
static int find_delta(const union delta_base *base, enum object_type type)
static int find_ofs_delta(const off_t offset, enum object_type type)
{
int first = 0, last = nr_deltas;
int first = 0, last = nr_ofs_deltas;
while (first < last) {
int next = (first + last) / 2;
struct delta_entry *delta = &deltas[next];
int cmp;
while (first < last) {
int next = (first + last) / 2;
struct ofs_delta_entry *delta = &ofs_deltas[next];
int cmp;
cmp = compare_delta_bases(base, &delta->base,
type, objects[delta->obj_no].type);
if (!cmp)
return next;
if (cmp < 0) {
last = next;
continue;
}
first = next+1;
}
return -first-1;
cmp = compare_ofs_delta_bases(offset, delta->offset,
type, objects[delta->obj_no].type);
if (!cmp)
return next;
if (cmp < 0) {
last = next;
continue;
}
first = next+1;
}
return -first-1;
}
static void find_delta_children(const union delta_base *base,
int *first_index, int *last_index,
enum object_type type)
static void find_ofs_delta_children(off_t offset,
int *first_index, int *last_index,
enum object_type type)
{
int first = find_delta(base, type);
int first = find_ofs_delta(offset, type);
int last = first;
int end = nr_deltas - 1;
int end = nr_ofs_deltas - 1;
if (first < 0) {
*first_index = 0;
*last_index = -1;
return;
}
while (first > 0 && !memcmp(&deltas[first - 1].base, base, UNION_BASE_SZ))
while (first > 0 && ofs_deltas[first - 1].offset == offset)
--first;
while (last < end && !memcmp(&deltas[last + 1].base, base, UNION_BASE_SZ))
while (last < end && ofs_deltas[last + 1].offset == offset)
++last;
*first_index = first;
*last_index = last;
}
static int compare_ref_delta_bases(const unsigned char *sha1,
const unsigned char *sha2,
enum object_type type1,
enum object_type type2)
{
int cmp = type1 - type2;
if (cmp)
return cmp;
return hashcmp(sha1, sha2);
}
static int find_ref_delta(const unsigned char *sha1, enum object_type type)
{
int first = 0, last = nr_ref_deltas;
while (first < last) {
int next = (first + last) / 2;
struct ref_delta_entry *delta = &ref_deltas[next];
int cmp;
cmp = compare_ref_delta_bases(sha1, delta->sha1,
type, objects[delta->obj_no].type);
if (!cmp)
return next;
if (cmp < 0) {
last = next;
continue;
}
first = next+1;
}
return -first-1;
}
static void find_ref_delta_children(const unsigned char *sha1,
int *first_index, int *last_index,
enum object_type type)
{
int first = find_ref_delta(sha1, type);
int last = first;
int end = nr_ref_deltas - 1;
if (first < 0) {
*first_index = 0;
*last_index = -1;
return;
}
while (first > 0 && !hashcmp(ref_deltas[first - 1].sha1, sha1))
--first;
while (last < end && !hashcmp(ref_deltas[last + 1].sha1, sha1))
++last;
*first_index = first;
*last_index = last;
@ -927,16 +977,13 @@ static struct base_data *find_unresolved_deltas_1(struct base_data *base,
struct base_data *prev_base)
{
if (base->ref_last == -1 && base->ofs_last == -1) {
union delta_base base_spec;
find_ref_delta_children(base->obj->idx.sha1,
&base->ref_first, &base->ref_last,
OBJ_REF_DELTA);
hashcpy(base_spec.sha1, base->obj->idx.sha1);
find_delta_children(&base_spec,
&base->ref_first, &base->ref_last, OBJ_REF_DELTA);
memset(&base_spec, 0, sizeof(base_spec));
base_spec.offset = base->obj->idx.offset;
find_delta_children(&base_spec,
&base->ofs_first, &base->ofs_last, OBJ_OFS_DELTA);
find_ofs_delta_children(base->obj->idx.offset,
&base->ofs_first, &base->ofs_last,
OBJ_OFS_DELTA);
if (base->ref_last == -1 && base->ofs_last == -1) {
free(base->data);
@ -947,7 +994,7 @@ static struct base_data *find_unresolved_deltas_1(struct base_data *base,
}
if (base->ref_first <= base->ref_last) {
struct object_entry *child = objects + deltas[base->ref_first].obj_no;
struct object_entry *child = objects + ref_deltas[base->ref_first].obj_no;
struct base_data *result = alloc_base_data();
if (!compare_and_swap_type(&child->real_type, OBJ_REF_DELTA,
@ -963,7 +1010,7 @@ static struct base_data *find_unresolved_deltas_1(struct base_data *base,
}
if (base->ofs_first <= base->ofs_last) {
struct object_entry *child = objects + deltas[base->ofs_first].obj_no;
struct object_entry *child = objects + ofs_deltas[base->ofs_first].obj_no;
struct base_data *result = alloc_base_data();
assert(child->real_type == OBJ_OFS_DELTA);
@ -999,15 +1046,20 @@ static void find_unresolved_deltas(struct base_data *base)
}
}
static int compare_delta_entry(const void *a, const void *b)
static int compare_ofs_delta_entry(const void *a, const void *b)
{
const struct delta_entry *delta_a = a;
const struct delta_entry *delta_b = b;
const struct ofs_delta_entry *delta_a = a;
const struct ofs_delta_entry *delta_b = b;
/* group by type (ref vs ofs) and then by value (sha-1 or offset) */
return compare_delta_bases(&delta_a->base, &delta_b->base,
objects[delta_a->obj_no].type,
objects[delta_b->obj_no].type);
return delta_a->offset - delta_b->offset;
}
static int compare_ref_delta_entry(const void *a, const void *b)
{
const struct ref_delta_entry *delta_a = a;
const struct ref_delta_entry *delta_b = b;
return hashcmp(delta_a->sha1, delta_b->sha1);
}
static void resolve_base(struct object_entry *obj)
@ -1053,7 +1105,8 @@ static void *threaded_second_pass(void *data)
static void parse_pack_objects(unsigned char *sha1)
{
int i, nr_delays = 0;
struct delta_entry *delta = deltas;
struct ofs_delta_entry *ofs_delta = ofs_deltas;
unsigned char ref_delta_sha1[20];
struct stat st;
if (verbose)
@ -1062,12 +1115,18 @@ static void parse_pack_objects(unsigned char *sha1)
nr_objects);
for (i = 0; i < nr_objects; i++) {
struct object_entry *obj = &objects[i];
void *data = unpack_raw_entry(obj, &delta->base, obj->idx.sha1);
void *data = unpack_raw_entry(obj, &ofs_delta->offset,
ref_delta_sha1, obj->idx.sha1);
obj->real_type = obj->type;
if (is_delta_type(obj->type)) {
nr_deltas++;
delta->obj_no = i;
delta++;
if (obj->type == OBJ_OFS_DELTA) {
nr_ofs_deltas++;
ofs_delta->obj_no = i;
ofs_delta++;
} else if (obj->type == OBJ_REF_DELTA) {
ALLOC_GROW(ref_deltas, nr_ref_deltas + 1, ref_deltas_alloc);
hashcpy(ref_deltas[nr_ref_deltas].sha1, ref_delta_sha1);
ref_deltas[nr_ref_deltas].obj_no = i;
nr_ref_deltas++;
} else if (!data) {
/* large blobs, check later */
obj->real_type = OBJ_BAD;
@ -1118,15 +1177,18 @@ static void resolve_deltas(void)
{
int i;
if (!nr_deltas)
if (!nr_ofs_deltas && !nr_ref_deltas)
return;
/* Sort deltas by base SHA1/offset for fast searching */
qsort(deltas, nr_deltas, sizeof(struct delta_entry),
compare_delta_entry);
qsort(ofs_deltas, nr_ofs_deltas, sizeof(struct ofs_delta_entry),
compare_ofs_delta_entry);
qsort(ref_deltas, nr_ref_deltas, sizeof(struct ref_delta_entry),
compare_ref_delta_entry);
if (verbose)
progress = start_progress(_("Resolving deltas"), nr_deltas);
progress = start_progress(_("Resolving deltas"),
nr_ref_deltas + nr_ofs_deltas);
#ifndef NO_PTHREADS
nr_dispatched = 0;
@ -1164,7 +1226,7 @@ static void resolve_deltas(void)
static void fix_unresolved_deltas(struct sha1file *f, int nr_unresolved);
static void conclude_pack(int fix_thin_pack, const char *curr_pack, unsigned char *pack_sha1)
{
if (nr_deltas == nr_resolved_deltas) {
if (nr_ref_deltas + nr_ofs_deltas == nr_resolved_deltas) {
stop_progress(&progress);
/* Flush remaining pack final 20-byte SHA1. */
flush();
@ -1175,7 +1237,7 @@ static void conclude_pack(int fix_thin_pack, const char *curr_pack, unsigned cha
struct sha1file *f;
unsigned char read_sha1[20], tail_sha1[20];
struct strbuf msg = STRBUF_INIT;
int nr_unresolved = nr_deltas - nr_resolved_deltas;
int nr_unresolved = nr_ofs_deltas + nr_ref_deltas - nr_resolved_deltas;
int nr_objects_initial = nr_objects;
if (nr_unresolved <= 0)
die(_("confusion beyond insanity"));
@ -1197,11 +1259,11 @@ static void conclude_pack(int fix_thin_pack, const char *curr_pack, unsigned cha
die(_("Unexpected tail checksum for %s "
"(disk corruption?)"), curr_pack);
}
if (nr_deltas != nr_resolved_deltas)
if (nr_ofs_deltas + nr_ref_deltas != nr_resolved_deltas)
die(Q_("pack has %d unresolved delta",
"pack has %d unresolved deltas",
nr_deltas - nr_resolved_deltas),
nr_deltas - nr_resolved_deltas);
nr_ofs_deltas + nr_ref_deltas - nr_resolved_deltas),
nr_ofs_deltas + nr_ref_deltas - nr_resolved_deltas);
}
static int write_compressed(struct sha1file *f, void *in, unsigned int size)
@ -1261,14 +1323,14 @@ static struct object_entry *append_obj_to_pack(struct sha1file *f,
static int delta_pos_compare(const void *_a, const void *_b)
{
struct delta_entry *a = *(struct delta_entry **)_a;
struct delta_entry *b = *(struct delta_entry **)_b;
struct ref_delta_entry *a = *(struct ref_delta_entry **)_a;
struct ref_delta_entry *b = *(struct ref_delta_entry **)_b;
return a->obj_no - b->obj_no;
}
static void fix_unresolved_deltas(struct sha1file *f, int nr_unresolved)
{
struct delta_entry **sorted_by_pos;
struct ref_delta_entry **sorted_by_pos;
int i, n = 0;
/*
@ -1282,28 +1344,25 @@ static void fix_unresolved_deltas(struct sha1file *f, int nr_unresolved)
* resolving deltas in the same order as their position in the pack.
*/
sorted_by_pos = xmalloc(nr_unresolved * sizeof(*sorted_by_pos));
for (i = 0; i < nr_deltas; i++) {
if (objects[deltas[i].obj_no].real_type != OBJ_REF_DELTA)
continue;
sorted_by_pos[n++] = &deltas[i];
}
for (i = 0; i < nr_ref_deltas; i++)
sorted_by_pos[n++] = &ref_deltas[i];
qsort(sorted_by_pos, n, sizeof(*sorted_by_pos), delta_pos_compare);
for (i = 0; i < n; i++) {
struct delta_entry *d = sorted_by_pos[i];
struct ref_delta_entry *d = sorted_by_pos[i];
enum object_type type;
struct base_data *base_obj = alloc_base_data();
if (objects[d->obj_no].real_type != OBJ_REF_DELTA)
continue;
base_obj->data = read_sha1_file(d->base.sha1, &type, &base_obj->size);
base_obj->data = read_sha1_file(d->sha1, &type, &base_obj->size);
if (!base_obj->data)
continue;
if (check_sha1_signature(d->base.sha1, base_obj->data,
if (check_sha1_signature(d->sha1, base_obj->data,
base_obj->size, typename(type)))
die(_("local object %s is corrupt"), sha1_to_hex(d->base.sha1));
base_obj->obj = append_obj_to_pack(f, d->base.sha1,
die(_("local object %s is corrupt"), sha1_to_hex(d->sha1));
base_obj->obj = append_obj_to_pack(f, d->sha1,
base_obj->data, base_obj->size, type);
find_unresolved_deltas(base_obj);
display_progress(progress, nr_resolved_deltas);
@ -1495,7 +1554,7 @@ static void read_idx_option(struct pack_idx_option *opts, const char *pack_name)
static void show_pack_info(int stat_only)
{
int i, baseobjects = nr_objects - nr_deltas;
int i, baseobjects = nr_objects - nr_ref_deltas - nr_ofs_deltas;
unsigned long *chain_histogram = NULL;
if (deepest_delta)
@ -1680,11 +1739,12 @@ int cmd_index_pack(int argc, const char **argv, const char *prefix)
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));
ofs_deltas = xcalloc(nr_objects, sizeof(struct ofs_delta_entry));
parse_pack_objects(pack_sha1);
resolve_deltas();
conclude_pack(fix_thin_pack, curr_pack, pack_sha1);
free(deltas);
free(ofs_deltas);
free(ref_deltas);
if (strict)
foreign_nr = check_objects();