dca924b450
As we will explore in later changes, the default name-hash function used
in 'git pack-objects' has a tendency to cause collisions and cause poor
delta selection. This change creates an alternative that avoids some
collisions while preserving some amount of hash locality.
The pack_name_hash() method has not been materially changed since it was
introduced in ce0bd64 (pack-objects: improve path grouping
heuristics., 2006-06-05). The intention here is to group objects by path
name, but also attempt to group similar file types together by making
the most-significant digits of the hash be focused on the final
characters.
Here's the crux of the implementation:
/*
* 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);
}
As the comment mentions, this only cares about the last sixteen
non-whitespace characters. This cause some filenames to collide more than
others. This collision is somewhat by design in order to promote hash
locality for files that have similar types (.c, .h, .json) or could be the
same file across a directory rename (a/foo.txt to b/foo.txt). This leads to
decent cross-path deltas in cases like shallow clones or packing a
repository with very few historical versions of files that share common data
with other similarly-named files.
However, when the name-hash instead leads to a large number of name-hash
collisions for otherwise unrelated files, this can lead to confusing the
delta calculation to prefer cross-path deltas over previous versions of the
same file.
The new pack_name_hash_v2() function attempts to fix this issue by
taking more of the directory path into account through its hash
function. Its naming implies that we will later wire up details for
choosing a name-hash function by version.
The first change is to be more careful about paths using non-ASCII
characters. With these characters in mind, reverse the bits in the byte
as the least-significant bits have the highest entropy and we want to
maximize their influence. This is done with some bit manipulation that
swaps the two halves, then the quarters within those halves, and then
the bits within those quarters.
The second change is to perform hash composition operations at every
level of the path. This is done by storing a 'base' hash value that
contains the hash of the parent directory. When reaching a directory
boundary, we XOR the current level's name-hash value with a downshift of
the previous level's hash. This perturbation intends to create low-bit
distinctions for paths with the same final 16 bytes but distinct parent
directory structures.
The collision rate and effectiveness of this hash function will be
explored in later changes as the function is integrated with 'git
pack-objects' and 'git repack'.
Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
337 lines
9.4 KiB
C
337 lines
9.4 KiB
C
#ifndef PACK_OBJECTS_H
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#define PACK_OBJECTS_H
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#include "object-store-ll.h"
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#include "thread-utils.h"
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#include "pack.h"
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struct repository;
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#define DEFAULT_DELTA_CACHE_SIZE (256 * 1024 * 1024)
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#define OE_DFS_STATE_BITS 2
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#define OE_DEPTH_BITS 12
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#define OE_IN_PACK_BITS 10
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#define OE_Z_DELTA_BITS 20
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/*
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* Note that oe_set_size() becomes expensive when the given size is
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* above this limit. Don't lower it too much.
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*/
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#define OE_SIZE_BITS 31
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#define OE_DELTA_SIZE_BITS 23
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/*
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* State flags for depth-first search used for analyzing delta cycles.
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*
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* The depth is measured in delta-links to the base (so if A is a delta
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* against B, then A has a depth of 1, and B a depth of 0).
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*/
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enum dfs_state {
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DFS_NONE = 0,
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DFS_ACTIVE,
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DFS_DONE,
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DFS_NUM_STATES
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};
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/*
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* The size of struct nearly determines pack-objects's memory
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* consumption. This struct is packed tight for that reason. When you
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* add or reorder something in this struct, think a bit about this.
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*
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* basic object info
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* -----------------
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* idx.oid is filled up before delta searching starts. idx.crc32 is
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* only valid after the object is written out and will be used for
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* generating the index. idx.offset will be both gradually set and
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* used in writing phase (base objects get offset first, then deltas
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* refer to them)
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*
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* "size" is the uncompressed object size. Compressed size of the raw
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* data for an object in a pack is not stored anywhere but is computed
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* and made available when reverse .idx is made. Note that when a
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* delta is reused, "size" is the uncompressed _delta_ size, not the
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* canonical one after the delta has been applied.
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*
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* "hash" contains a path name hash which is used for sorting the
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* delta list and also during delta searching. Once prepare_pack()
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* returns it's no longer needed.
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*
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* source pack info
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* ----------------
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* The (in_pack, in_pack_offset) tuple contains the location of the
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* object in the source pack. in_pack_header_size allows quickly
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* skipping the header and going straight to the zlib stream.
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*
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* "type" and "in_pack_type" both describe object type. in_pack_type
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* may contain a delta type, while type is always the canonical type.
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*
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* deltas
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* ------
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* Delta links (delta, delta_child and delta_sibling) are created to
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* reflect that delta graph from the source pack then updated or added
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* during delta searching phase when we find better deltas.
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*
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* delta_child and delta_sibling are last needed in
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* compute_write_order(). "delta" and "delta_size" must remain valid
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* at object writing phase in case the delta is not cached.
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*
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* If a delta is cached in memory and is compressed, delta_data points
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* to the data and z_delta_size contains the compressed size. If it's
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* uncompressed [1], z_delta_size must be zero. delta_size is always
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* the uncompressed size and must be valid even if the delta is not
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* cached.
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*
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* [1] during try_delta phase we don't bother with compressing because
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* the delta could be quickly replaced with a better one.
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*/
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struct object_entry {
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struct pack_idx_entry idx;
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void *delta_data; /* cached delta (uncompressed) */
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off_t in_pack_offset;
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uint32_t hash; /* name hint hash */
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unsigned size_:OE_SIZE_BITS;
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unsigned size_valid:1;
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uint32_t delta_idx; /* delta base object */
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uint32_t delta_child_idx; /* deltified objects who bases me */
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uint32_t delta_sibling_idx; /* other deltified objects who
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* uses the same base as me
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*/
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unsigned delta_size_:OE_DELTA_SIZE_BITS; /* delta data size (uncompressed) */
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unsigned delta_size_valid:1;
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unsigned char in_pack_header_size;
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unsigned in_pack_idx:OE_IN_PACK_BITS; /* already in pack */
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unsigned z_delta_size:OE_Z_DELTA_BITS;
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unsigned type_valid:1;
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unsigned no_try_delta:1;
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unsigned type_:TYPE_BITS;
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unsigned in_pack_type:TYPE_BITS; /* could be delta */
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unsigned preferred_base:1; /*
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* we do not pack this, but is available
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* to be used as the base object to delta
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* objects against.
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*/
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unsigned tagged:1; /* near the very tip of refs */
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unsigned filled:1; /* assigned write-order */
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unsigned dfs_state:OE_DFS_STATE_BITS;
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unsigned depth:OE_DEPTH_BITS;
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unsigned ext_base:1; /* delta_idx points outside packlist */
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};
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struct packing_data {
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struct repository *repo;
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struct object_entry *objects;
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uint32_t nr_objects, nr_alloc;
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int32_t *index;
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uint32_t index_size;
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unsigned int *in_pack_pos;
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unsigned long *delta_size;
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/*
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* Only one of these can be non-NULL and they have different
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* sizes. if in_pack_by_idx is allocated, oe_in_pack() returns
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* the pack of an object using in_pack_idx field. If not,
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* in_pack[] array is used the same way as in_pack_pos[]
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*/
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struct packed_git **in_pack_by_idx;
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struct packed_git **in_pack;
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/*
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* During packing with multiple threads, protect the in-core
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* object database from concurrent accesses.
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*/
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pthread_mutex_t odb_lock;
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/*
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* This list contains entries for bases which we know the other side
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* has (e.g., via reachability bitmaps), but which aren't in our
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* "objects" list.
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*/
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struct object_entry *ext_bases;
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uint32_t nr_ext, alloc_ext;
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uintmax_t oe_size_limit;
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uintmax_t oe_delta_size_limit;
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/* delta islands */
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unsigned int *tree_depth;
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unsigned char *layer;
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/*
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* Used when writing cruft packs.
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*
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* Object mtimes are stored in pack order when writing, but
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* written out in lexicographic (index) order.
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*/
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uint32_t *cruft_mtime;
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};
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void prepare_packing_data(struct repository *r, struct packing_data *pdata);
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void clear_packing_data(struct packing_data *pdata);
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/* Protect access to object database */
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static inline void packing_data_lock(struct packing_data *pdata)
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{
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pthread_mutex_lock(&pdata->odb_lock);
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}
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static inline void packing_data_unlock(struct packing_data *pdata)
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{
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pthread_mutex_unlock(&pdata->odb_lock);
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}
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struct object_entry *packlist_alloc(struct packing_data *pdata,
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const struct object_id *oid);
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struct object_entry *packlist_find(struct packing_data *pdata,
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const struct object_id *oid);
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static inline uint32_t pack_name_hash(const char *name)
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{
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uint32_t c, hash = 0;
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if (!name)
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return 0;
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/*
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* This effectively just creates a sortable number from the
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* last sixteen non-whitespace characters. Last characters
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* count "most", so things that end in ".c" sort together.
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*/
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while ((c = *name++) != 0) {
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if (isspace(c))
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continue;
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hash = (hash >> 2) + (c << 24);
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}
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return hash;
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}
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static inline uint32_t pack_name_hash_v2(const unsigned char *name)
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{
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uint32_t hash = 0, base = 0, c;
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if (!name)
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return 0;
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while ((c = *name++)) {
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if (isspace(c))
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continue;
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if (c == '/') {
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base = (base >> 6) ^ hash;
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hash = 0;
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} else {
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/*
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* 'c' is only a single byte. Reverse it and move
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* it to the top of the hash, moving the rest to
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* less-significant bits.
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*/
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c = (c & 0xF0) >> 4 | (c & 0x0F) << 4;
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c = (c & 0xCC) >> 2 | (c & 0x33) << 2;
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c = (c & 0xAA) >> 1 | (c & 0x55) << 1;
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hash = (hash >> 2) + (c << 24);
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}
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}
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return (base >> 6) ^ hash;
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}
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static inline enum object_type oe_type(const struct object_entry *e)
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{
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return e->type_valid ? e->type_ : OBJ_BAD;
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}
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static inline void oe_set_type(struct object_entry *e,
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enum object_type type)
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{
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if (type >= OBJ_ANY)
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BUG("OBJ_ANY cannot be set in pack-objects code");
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e->type_valid = type >= OBJ_NONE;
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e->type_ = (unsigned)type;
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}
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static inline unsigned int oe_in_pack_pos(const struct packing_data *pack,
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const struct object_entry *e)
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{
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return pack->in_pack_pos[e - pack->objects];
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}
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static inline void oe_set_in_pack_pos(const struct packing_data *pack,
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const struct object_entry *e,
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unsigned int pos)
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{
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pack->in_pack_pos[e - pack->objects] = pos;
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}
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static inline struct packed_git *oe_in_pack(const struct packing_data *pack,
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const struct object_entry *e)
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{
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if (pack->in_pack_by_idx)
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return pack->in_pack_by_idx[e->in_pack_idx];
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else
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return pack->in_pack[e - pack->objects];
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}
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void oe_map_new_pack(struct packing_data *pack);
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static inline void oe_set_in_pack(struct packing_data *pack,
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struct object_entry *e,
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struct packed_git *p)
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{
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if (pack->in_pack_by_idx) {
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if (p->index) {
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e->in_pack_idx = p->index;
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return;
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}
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/*
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* We're accessing packs by index, but this pack doesn't have
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* an index (e.g., because it was added since we created the
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* in_pack_by_idx array). Bail to oe_map_new_pack(), which
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* will convert us to using the full in_pack array, and then
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* fall through to our in_pack handling.
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*/
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oe_map_new_pack(pack);
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}
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pack->in_pack[e - pack->objects] = p;
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}
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void oe_set_delta_ext(struct packing_data *pack,
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struct object_entry *e,
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const struct object_id *oid);
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static inline unsigned int oe_tree_depth(struct packing_data *pack,
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struct object_entry *e)
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{
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if (!pack->tree_depth)
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return 0;
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return pack->tree_depth[e - pack->objects];
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}
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static inline void oe_set_layer(struct packing_data *pack,
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struct object_entry *e,
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unsigned char layer)
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{
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if (!pack->layer)
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CALLOC_ARRAY(pack->layer, pack->nr_alloc);
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pack->layer[e - pack->objects] = layer;
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}
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static inline uint32_t oe_cruft_mtime(struct packing_data *pack,
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struct object_entry *e)
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{
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if (!pack->cruft_mtime)
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return 0;
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return pack->cruft_mtime[e - pack->objects];
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}
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static inline void oe_set_cruft_mtime(struct packing_data *pack,
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struct object_entry *e,
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uint32_t mtime)
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{
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if (!pack->cruft_mtime)
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CALLOC_ARRAY(pack->cruft_mtime, pack->nr_alloc);
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pack->cruft_mtime[e - pack->objects] = mtime;
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}
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#endif
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