mirror/ git
1
0
Fork 0
mirror of https://github.com/git/git.git synced 2024-06-10 23:56:14 +02:00
Code
git/pack-objects.h
Jeff King f66e0401ab pack-objects: avoid pointless oe_map_new_pack() calls 
This patch fixes an extreme slowdown in pack-objects when you have more
than 1023 packs. See below for numbers.

Since 43fa44fa3b (pack-objects: move in_pack out of struct object_entry,
2018-04-14), we use a complicated system to save some per-object memory.

Each object_entry structs gets a 10-bit field to store the index of the
pack it's in. We map those indices into pointers using
packing_data->in_pack_by_idx, which we initialize at the start of the
program. If we have 2^10 or more packs, then we instead create an array
of pack pointers, one per object. This is packing_data->in_pack.

So far so good. But there's one other tricky case: if a new pack arrives
after we've initialized in_pack_by_idx, it won't have an index yet. We
solve that by calling oe_map_new_pack(), which just switches on the fly
to the less-optimal in_pack mechanism, allocating the array and
back-filling it for already-seen objects.

But that logic kicks in even when we've switched to it already (whether
because we really did see a new pack, or because we had too many packs
in the first place). The result doesn't produce a wrong outcome, but
it's very slow. What happens is this:

  - imagine you have a repo with 500k objects and 2000 packs that you
    want to repack.

  - before looking at any objects, we call prepare_in_pack_by_idx(). It
    starts allocating an index for each pack. On the 1024th pack, it
    sees there are too many, so it bails, leaving in_pack_by_idx as
    NULL.

  - while actually adding objects to the packing list, we call
    oe_set_in_pack(), which checks whether the pack already has an
    index. If it's one of the packs after the first 1023, then it
    doesn't have one, and we'll call oe_map_new_pack().

    But there's no useful work for that function to do. We're already
    using in_pack, so it just uselessly walks over the complete list of
    objects, trying to backfill in_pack.

    And we end up doing this for almost 1000 packs (each of which may be
    triggered by more than one object). And each time it triggers, we
    may iterate over up to 500k objects. So in the absolute worst case,
    this is quadratic in the number of objects.

The solution is simple: we don't need to bother checking whether the
pack has an index if we've already converted to using in_pack, since by
definition we're not going to use it. So we can just push the "does the
pack have a valid index" check down into that half of the conditional,
where we know we're going to use it.

The current test in p5303 sadly doesn't notice this problem, since it
maxes out at 1000 packs. If we add a new test to it at 2000 packs, it
does show the improvement:

  Test                      HEAD^               HEAD
  ----------------------------------------------------------------------
  5303.12: repack (2000)    26.72(39.68+0.67)   15.70(28.70+0.66) -41.2%

However, these many-pack test cases are rather expensive to run, so
adding larger and larger numbers isn't appealing. Instead, we can show
it off more easily by using GIT_TEST_FULL_IN_PACK_ARRAY, which forces us
into the absolute worst case: no pack has an index, so we'll trigger
oe_map_new_pack() pointlessly for every single object, making it truly
quadratic.

Here are the numbers (on git.git) with the included change to p5303:

  Test                      HEAD^               HEAD
  ----------------------------------------------------------------------
  5303.3: rev-list (1)      2.05(1.98+0.06)     2.06(1.99+0.06) +0.5%
  5303.4: repack (1)        33.45(33.46+0.19)   2.75(2.73+0.22) -91.8%
  5303.6: rev-list (50)     2.07(2.01+0.06)     2.06(2.01+0.05) -0.5%
  5303.7: repack (50)       34.21(35.18+0.16)   3.49(4.50+0.12) -89.8%
  5303.9: rev-list (1000)   2.87(2.78+0.08)     2.88(2.80+0.07) +0.3%
  5303.10: repack (1000)    41.26(51.30+0.47)   10.75(20.75+0.44) -73.9%

Again, those improvements aren't realistic for the 1-pack case (because
in the real world, the full-array solution doesn't kick in), but it's
more useful to be testing the more-complicated code path.

While we're looking at this issue, we'll tweak one more thing: in
oe_map_new_pack(), we call REALLOC_ARRAY(pack->in_pack). But we'd never
expect to get here unless we're back-filling it for the first time, in
which case it would be NULL. So let's switch that to ALLOC_ARRAY() for
clarity, and add a BUG() to document the expectation. Unfortunately this
code isn't well-covered in the test suite because it's inherently racy
(it only kicks in if somebody else adds a new pack while we're in the
middle of repacking).

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2019-11-12 13:36:36 +09:00

454 lines
12 KiB
C
Raw Blame History

#ifndef PACK_OBJECTS_H
#define PACK_OBJECTS_H
#include "object-store.h"
#include "thread-utils.h"
#include "pack.h"
struct repository;
#define DEFAULT_DELTA_CACHE_SIZE (256 * 1024 * 1024)
#define OE_DFS_STATE_BITS 2
#define OE_DEPTH_BITS 12
#define OE_IN_PACK_BITS 10
#define OE_Z_DELTA_BITS 20
/*
* Note that oe_set_size() becomes expensive when the given size is
* above this limit. Don't lower it too much.
*/
#define OE_SIZE_BITS 31
#define OE_DELTA_SIZE_BITS 23
/*
* State flags for depth-first search used for analyzing delta cycles.
*
* The depth is measured in delta-links to the base (so if A is a delta
* against B, then A has a depth of 1, and B a depth of 0).
*/
enum dfs_state {
DFS_NONE = 0,
DFS_ACTIVE,
DFS_DONE,
DFS_NUM_STATES
};
/*
* 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
* only valid after the object is written out and will be used for
* generating the index. idx.offset will be both gradually set and
* used in writing phase (base objects get offset first, then deltas
* refer to them)
*
* "size" is the uncompressed object size. Compressed size of the raw
* data for an object in a pack is not stored anywhere but is computed
* and made available when reverse .idx is made. Note that when a
* delta is reused, "size" is the uncompressed _delta_ size, not the
* canonical one after the delta has been applied.
*
* "hash" contains a path name hash which is used for sorting the
* delta list and also during delta searching. Once prepare_pack()
* returns it's no longer needed.
*
* source pack info
* ----------------
* The (in_pack, in_pack_offset) tuple contains the location of the
* object in the source pack. in_pack_header_size allows quickly
* skipping the header and going straight to the zlib stream.
*
* "type" and "in_pack_type" both describe object type. in_pack_type
* may contain a delta type, while type is always the canonical type.
*
* deltas
* ------
* Delta links (delta, delta_child and delta_sibling) are created to
* reflect that delta graph from the source pack then updated or added
* during delta searching phase when we find better deltas.
*
* delta_child and delta_sibling are last needed in
* compute_write_order(). "delta" and "delta_size" must remain valid
* at object writing phase in case the delta is not cached.
*
* If a delta is cached in memory and is compressed, delta_data points
* to the data and z_delta_size contains the compressed size. If it's
* uncompressed [1], z_delta_size must be zero. delta_size is always
* the uncompressed size and must be valid even if the delta is not
* cached.
*
* [1] during try_delta phase we don't bother with compressing because
* the delta could be quickly replaced with a better one.
*/
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;
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
*/
unsigned delta_size_:OE_DELTA_SIZE_BITS; /* delta data size (uncompressed) */
unsigned delta_size_valid:1;
unsigned char in_pack_header_size;
unsigned in_pack_idx:OE_IN_PACK_BITS; /* already in pack */
unsigned z_delta_size:OE_Z_DELTA_BITS;
unsigned type_valid:1;
unsigned no_try_delta:1;
unsigned type_:TYPE_BITS;
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 tagged:1; /* near the very tip of refs */
unsigned filled:1; /* assigned write-order */
unsigned dfs_state:OE_DFS_STATE_BITS;
unsigned depth:OE_DEPTH_BITS;
unsigned ext_base:1; /* delta_idx points outside packlist */
/*
* pahole results on 64-bit linux (gcc and clang)
*
* size: 80, bit_padding: 9 bits
*
* and on 32-bit (gcc)
*
* size: 76, bit_padding: 9 bits
*/
};
struct packing_data {
struct repository *repo;
struct object_entry *objects;
uint32_t nr_objects, nr_alloc;
int32_t *index;
uint32_t index_size;
unsigned int *in_pack_pos;
unsigned long *delta_size;
/*
* Only one of these can be non-NULL and they have different
* sizes. if in_pack_by_idx is allocated, oe_in_pack() returns
* the pack of an object using in_pack_idx field. If not,
* in_pack[] array is used the same way as in_pack_pos[]
*/
struct packed_git **in_pack_by_idx;
struct packed_git **in_pack;
/*
* During packing with multiple threads, protect the in-core
* object database from concurrent accesses.
*/
pthread_mutex_t odb_lock;
/*
* This list contains entries for bases which we know the other side
* has (e.g., via reachability bitmaps), but which aren't in our
* "objects" list.
*/
struct object_entry *ext_bases;
uint32_t nr_ext, alloc_ext;
uintmax_t oe_size_limit;
uintmax_t oe_delta_size_limit;
/* delta islands */
unsigned int *tree_depth;
unsigned char *layer;
};
void prepare_packing_data(struct repository *r, struct packing_data *pdata);
/* Protect access to object database */
static inline void packing_data_lock(struct packing_data *pdata)
{
pthread_mutex_lock(&pdata->odb_lock);
}
static inline void packing_data_unlock(struct packing_data *pdata)
{
pthread_mutex_unlock(&pdata->odb_lock);
}
struct object_entry *packlist_alloc(struct packing_data *pdata,
const unsigned char *sha1,
uint32_t index_pos);
struct object_entry *packlist_find(struct packing_data *pdata,
const struct object_id *oid,
uint32_t *index_pos);
static inline uint32_t pack_name_hash(const char *name)
{
uint32_t c, hash = 0;
if (!name)
return 0;
/*
* This effectively just creates a sortable number from the
* last sixteen non-whitespace characters. Last characters
* count "most", so things that end in ".c" sort together.
*/
while ((c = *name++) != 0) {
if (isspace(c))
continue;
hash = (hash >> 2) + (c << 24);
}
return hash;
}
static inline enum object_type oe_type(const struct object_entry *e)
{
return e->type_valid ? e->type_ : OBJ_BAD;
}
static inline void oe_set_type(struct object_entry *e,
enum object_type type)
{
if (type >= OBJ_ANY)
BUG("OBJ_ANY cannot be set in pack-objects code");
e->type_valid = type >= OBJ_NONE;
e->type_ = (unsigned)type;
}
static inline unsigned int oe_in_pack_pos(const struct packing_data *pack,
const struct object_entry *e)
{
return pack->in_pack_pos[e - pack->objects];
}
static inline void oe_set_in_pack_pos(const struct packing_data *pack,
const struct object_entry *e,
unsigned int pos)
{
pack->in_pack_pos[e - pack->objects] = pos;
}
static inline struct packed_git *oe_in_pack(const struct packing_data *pack,
const struct object_entry *e)
{
if (pack->in_pack_by_idx)
return pack->in_pack_by_idx[e->in_pack_idx];
else
return pack->in_pack[e - pack->objects];
}
void oe_map_new_pack(struct packing_data *pack);
static inline void oe_set_in_pack(struct packing_data *pack,
struct object_entry *e,
struct packed_git *p)
{
if (pack->in_pack_by_idx) {
if (p->index) {
e->in_pack_idx = p->index;
return;
}
/*
* We're accessing packs by index, but this pack doesn't have
* an index (e.g., because it was added since we created the
* in_pack_by_idx array). Bail to oe_map_new_pack(), which
* will convert us to using the full in_pack array, and then
* fall through to our in_pack handling.
*/
oe_map_new_pack(pack);
}
pack->in_pack[e - pack->objects] = p;
}
static inline struct object_entry *oe_delta(
const struct packing_data *pack,
const struct object_entry *e)
{
if (!e->delta_idx)
return NULL;
if (e->ext_base)
return &pack->ext_bases[e->delta_idx - 1];
else
return &pack->objects[e->delta_idx - 1];
}
static inline void oe_set_delta(struct packing_data *pack,
struct object_entry *e,
struct object_entry *delta)
{
if (delta)
e->delta_idx = (delta - pack->objects) + 1;
else
e->delta_idx = 0;
}
void oe_set_delta_ext(struct packing_data *pack,
struct object_entry *e,
const unsigned char *sha1);
static inline struct object_entry *oe_delta_child(
const struct packing_data *pack,
const struct object_entry *e)
{
if (e->delta_child_idx)
return &pack->objects[e->delta_child_idx - 1];
return NULL;
}
static inline void oe_set_delta_child(struct packing_data *pack,
struct object_entry *e,
struct object_entry *delta)
{
if (delta)
e->delta_child_idx = (delta - pack->objects) + 1;
else
e->delta_child_idx = 0;
}
static inline struct object_entry *oe_delta_sibling(
const struct packing_data *pack,
const struct object_entry *e)
{
if (e->delta_sibling_idx)
return &pack->objects[e->delta_sibling_idx - 1];
return NULL;
}
static inline void oe_set_delta_sibling(struct packing_data *pack,
struct object_entry *e,
struct object_entry *delta)
{
if (delta)
e->delta_sibling_idx = (delta - pack->objects) + 1;
else
e->delta_sibling_idx = 0;
}
unsigned long oe_get_size_slow(struct packing_data *pack,
const struct object_entry *e);
static inline unsigned long oe_size(struct packing_data *pack,
const struct object_entry *e)
{
if (e->size_valid)
return e->size_;
return oe_get_size_slow(pack, e);
}
static inline int oe_size_less_than(struct packing_data *pack,
const struct object_entry *lhs,
unsigned long rhs)
{
if (lhs->size_valid)
return lhs->size_ < rhs;
if (rhs < pack->oe_size_limit) /* rhs < 2^x <= lhs ? */
return 0;
return oe_get_size_slow(pack, lhs) < rhs;
}
static inline int oe_size_greater_than(struct packing_data *pack,
const struct object_entry *lhs,
unsigned long rhs)
{
if (lhs->size_valid)
return lhs->size_ > rhs;
if (rhs < pack->oe_size_limit) /* rhs < 2^x <= lhs ? */
return 1;
return oe_get_size_slow(pack, lhs) > rhs;
}
static inline void oe_set_size(struct packing_data *pack,
struct object_entry *e,
unsigned long size)
{
if (size < pack->oe_size_limit) {
e->size_ = size;
e->size_valid = 1;
} else {
e->size_valid = 0;
if (oe_get_size_slow(pack, e) != size)
BUG("'size' is supposed to be the object size!");
}
}
static inline unsigned long oe_delta_size(struct packing_data *pack,
const struct object_entry *e)
{
if (e->delta_size_valid)
return e->delta_size_;
/*
* pack->delta_size[] can't be NULL because oe_set_delta_size()
* must have been called when a new delta is saved with
* oe_set_delta().
* If oe_delta() returns NULL (i.e. default state, which means
* delta_size_valid is also false), then the caller must never
* call oe_delta_size().
*/
return pack->delta_size[e - pack->objects];
}
static inline void oe_set_delta_size(struct packing_data *pack,
struct object_entry *e,
unsigned long size)
{
if (size < pack->oe_delta_size_limit) {
e->delta_size_ = size;
e->delta_size_valid = 1;
} else {
packing_data_lock(pack);
if (!pack->delta_size)
ALLOC_ARRAY(pack->delta_size, pack->nr_alloc);
packing_data_unlock(pack);
pack->delta_size[e - pack->objects] = size;
e->delta_size_valid = 0;
}
}
static inline unsigned int oe_tree_depth(struct packing_data *pack,
struct object_entry *e)
{
if (!pack->tree_depth)
return 0;
return pack->tree_depth[e - pack->objects];
}
static inline void oe_set_tree_depth(struct packing_data *pack,
struct object_entry *e,
unsigned int tree_depth)
{
if (!pack->tree_depth)
CALLOC_ARRAY(pack->tree_depth, pack->nr_alloc);
pack->tree_depth[e - pack->objects] = tree_depth;
}
static inline unsigned char oe_layer(struct packing_data *pack,
struct object_entry *e)
{
if (!pack->layer)
return 0;
return pack->layer[e - pack->objects];
}
static inline void oe_set_layer(struct packing_data *pack,
struct object_entry *e,
unsigned char layer)
{
if (!pack->layer)
CALLOC_ARRAY(pack->layer, pack->nr_alloc);
pack->layer[e - pack->objects] = layer;
}
#endif
View Git Blame Copy Permalink