From 2f4ba2a867f0390f139b622dbafcab766cb88e80 Mon Sep 17 00:00:00 2001 From: Taylor Blau Date: Mon, 25 Jan 2021 18:37:14 -0500 Subject: [PATCH] packfile: prepare for the existence of '*.rev' files MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Specify the format of the on-disk reverse index 'pack-*.rev' file, as well as prepare the code for the existence of such files. The reverse index maps from pack relative positions (i.e., an index into the array of object which is sorted by their offsets within the packfile) to their position within the 'pack-*.idx' file. Today, this is done by building up a list of (off_t, uint32_t) tuples for each object (the off_t corresponding to that object's offset, and the uint32_t corresponding to its position in the index). To convert between pack and index position quickly, this array of tuples is radix sorted based on its offset. This has two major drawbacks: First, the in-memory cost scales linearly with the number of objects in a pack. Each 'struct revindex_entry' is sizeof(off_t) + sizeof(uint32_t) + padding bytes for a total of 16. To observe this, force Git to load the reverse index by, for e.g., running 'git cat-file --batch-check="%(objectsize:disk)"'. When asking for a single object in a fresh clone of the kernel, Git needs to allocate 120+ MB of memory in order to hold the reverse index in memory. Second, the cost to sort also scales with the size of the pack. Luckily, this is a linear function since 'load_pack_revindex()' uses a radix sort, but this cost still must be paid once per pack per process. As an example, it takes ~60x longer to print the _size_ of an object as it does to print that entire object's _contents_: Benchmark #1: git.compile cat-file --batch /dev/null Time (mean ± σ): 22.6 ms ± 0.5 ms [User: 2.4 ms, System: 7.9 ms] Range (min … max): 21.4 ms … 23.5 ms 41 runs Benchmark #2: git.compile cat-file --batch /dev/null Time (mean ± σ): 17.2 ms ± 0.7 ms [User: 2.8 ms, System: 5.5 ms] Range (min … max): 15.6 ms … 18.2 ms 45 runs (Numbers taken in the kernel after cheating and using the next patch to generate a reverse index). There are a couple of approaches to improve cold cache performance not pursued here: - We could include the object offsets in the reverse index format. Predictably, this does result in fewer page faults, but it triples the size of the file, while simultaneously duplicating a ton of data already available in the .idx file. (This was the original way I implemented the format, and it did show `--batch-check='%(objectsize:disk)'` winning out against `--batch`.) On the other hand, this increase in size also results in a large block-cache footprint, which could potentially hurt other workloads. - We could store the mapping from pack to index position in more cache-friendly way, like constructing a binary search tree from the table and writing the values in breadth-first order. This would result in much better locality, but the price you pay is trading O(1) lookup in 'pack_pos_to_index()' for an O(log n) one (since you can no longer directly index the table). So, neither of these approaches are taken here. (Thankfully, the format is versioned, so we are free to pursue these in the future.) But, cold cache performance likely isn't interesting outside of one-off cases like asking for the size of an object directly. In real-world usage, Git is often performing many operations in the revindex (i.e., asking about many objects rather than a single one). The trade-off is worth it, since we will avoid the vast majority of the cost of generating the revindex that the extra pointer chase will look like noise in the following patch's benchmarks. This patch describes the format and prepares callers (like in pack-revindex.c) to be able to read *.rev files once they exist. An implementation of the writer will appear in the next patch, and callers will gradually begin to start using the writer in the patches that follow after that. Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano --- Documentation/technical/pack-format.txt | 20 ++++ builtin/repack.c | 1 + object-store.h | 3 + pack-revindex.c | 144 ++++++++++++++++++++++-- pack-revindex.h | 10 +- packfile.c | 13 ++- packfile.h | 1 + tmp-objdir.c | 6 +- 8 files changed, 184 insertions(+), 14 deletions(-) diff --git a/Documentation/technical/pack-format.txt b/Documentation/technical/pack-format.txt index 96d2fc589f..8833b71c8b 100644 --- a/Documentation/technical/pack-format.txt +++ b/Documentation/technical/pack-format.txt @@ -274,6 +274,26 @@ Pack file entry: <+ Index checksum of all of the above. +== pack-*.rev files have the format: + + - A 4-byte magic number '0x52494458' ('RIDX'). + + - A 4-byte version identifier (= 1). + + - A 4-byte hash function identifier (= 1 for SHA-1, 2 for SHA-256). + + - A table of index positions (one per packed object, num_objects in + total, each a 4-byte unsigned integer in network order), sorted by + their corresponding offsets in the packfile. + + - A trailer, containing a: + + checksum of the corresponding packfile, and + + a checksum of all of the above. + +All 4-byte numbers are in network order. + == multi-pack-index (MIDX) files have the following format: The multi-pack-index files refer to multiple pack-files and loose objects. diff --git a/builtin/repack.c b/builtin/repack.c index 2158b48f4c..01440de2d5 100644 --- a/builtin/repack.c +++ b/builtin/repack.c @@ -209,6 +209,7 @@ static struct { } exts[] = { {".pack"}, {".idx"}, + {".rev", 1}, {".bitmap", 1}, {".promisor", 1}, }; diff --git a/object-store.h b/object-store.h index c4fc9dd74e..541dab0858 100644 --- a/object-store.h +++ b/object-store.h @@ -85,6 +85,9 @@ struct packed_git { multi_pack_index:1; unsigned char hash[GIT_MAX_RAWSZ]; struct revindex_entry *revindex; + const uint32_t *revindex_data; + const uint32_t *revindex_map; + size_t revindex_size; /* something like ".git/objects/pack/xxxxx.pack" */ char pack_name[FLEX_ARRAY]; /* more */ }; diff --git a/pack-revindex.c b/pack-revindex.c index 5e69bc7372..a174fa5388 100644 --- a/pack-revindex.c +++ b/pack-revindex.c @@ -164,14 +164,128 @@ static void create_pack_revindex(struct packed_git *p) sort_revindex(p->revindex, num_ent, p->pack_size); } +static int create_pack_revindex_in_memory(struct packed_git *p) +{ + if (open_pack_index(p)) + return -1; + create_pack_revindex(p); + return 0; +} + +static char *pack_revindex_filename(struct packed_git *p) +{ + size_t len; + if (!strip_suffix(p->pack_name, ".pack", &len)) + BUG("pack_name does not end in .pack"); + return xstrfmt("%.*s.rev", (int)len, p->pack_name); +} + +#define RIDX_HEADER_SIZE (12) +#define RIDX_MIN_SIZE (RIDX_HEADER_SIZE + (2 * the_hash_algo->rawsz)) + +struct revindex_header { + uint32_t signature; + uint32_t version; + uint32_t hash_id; +}; + +static int load_revindex_from_disk(char *revindex_name, + uint32_t num_objects, + const uint32_t **data_p, size_t *len_p) +{ + int fd, ret = 0; + struct stat st; + void *data = NULL; + size_t revindex_size; + struct revindex_header *hdr; + + fd = git_open(revindex_name); + + if (fd < 0) { + ret = -1; + goto cleanup; + } + if (fstat(fd, &st)) { + ret = error_errno(_("failed to read %s"), revindex_name); + goto cleanup; + } + + revindex_size = xsize_t(st.st_size); + + if (revindex_size < RIDX_MIN_SIZE) { + ret = error(_("reverse-index file %s is too small"), revindex_name); + goto cleanup; + } + + if (revindex_size - RIDX_MIN_SIZE != st_mult(sizeof(uint32_t), num_objects)) { + ret = error(_("reverse-index file %s is corrupt"), revindex_name); + goto cleanup; + } + + data = xmmap(NULL, revindex_size, PROT_READ, MAP_PRIVATE, fd, 0); + hdr = data; + + if (ntohl(hdr->signature) != RIDX_SIGNATURE) { + ret = error(_("reverse-index file %s has unknown signature"), revindex_name); + goto cleanup; + } + if (ntohl(hdr->version) != 1) { + ret = error(_("reverse-index file %s has unsupported version %"PRIu32), + revindex_name, ntohl(hdr->version)); + goto cleanup; + } + if (!(ntohl(hdr->hash_id) == 1 || ntohl(hdr->hash_id) == 2)) { + ret = error(_("reverse-index file %s has unsupported hash id %"PRIu32), + revindex_name, ntohl(hdr->hash_id)); + goto cleanup; + } + +cleanup: + if (ret) { + if (data) + munmap(data, revindex_size); + } else { + *len_p = revindex_size; + *data_p = (const uint32_t *)data; + } + + close(fd); + return ret; +} + +static int load_pack_revindex_from_disk(struct packed_git *p) +{ + char *revindex_name; + int ret; + if (open_pack_index(p)) + return -1; + + revindex_name = pack_revindex_filename(p); + + ret = load_revindex_from_disk(revindex_name, + p->num_objects, + &p->revindex_map, + &p->revindex_size); + if (ret) + goto cleanup; + + p->revindex_data = (const uint32_t *)((const char *)p->revindex_map + RIDX_HEADER_SIZE); + +cleanup: + free(revindex_name); + return ret; +} + int load_pack_revindex(struct packed_git *p) { - if (!p->revindex) { - if (open_pack_index(p)) - return -1; - create_pack_revindex(p); - } - return 0; + if (p->revindex || p->revindex_data) + return 0; + + if (!load_pack_revindex_from_disk(p)) + return 0; + else if (!create_pack_revindex_in_memory(p)) + return 0; + return -1; } int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos) @@ -203,18 +317,28 @@ int offset_to_pack_pos(struct packed_git *p, off_t ofs, uint32_t *pos) uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos) { - if (!p->revindex) + if (!(p->revindex || p->revindex_data)) BUG("pack_pos_to_index: reverse index not yet loaded"); if (p->num_objects <= pos) BUG("pack_pos_to_index: out-of-bounds object at %"PRIu32, pos); - return p->revindex[pos].nr; + + if (p->revindex) + return p->revindex[pos].nr; + else + return get_be32(p->revindex_data + pos); } off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos) { - if (!p->revindex) + if (!(p->revindex || p->revindex_data)) BUG("pack_pos_to_index: reverse index not yet loaded"); if (p->num_objects < pos) BUG("pack_pos_to_offset: out-of-bounds object at %"PRIu32, pos); - return p->revindex[pos].offset; + + if (p->revindex) + return p->revindex[pos].offset; + else if (pos == p->num_objects) + return p->pack_size - the_hash_algo->rawsz; + else + return nth_packed_object_offset(p, pack_pos_to_index(p, pos)); } diff --git a/pack-revindex.h b/pack-revindex.h index 6e0320b08b..61b2f3ab75 100644 --- a/pack-revindex.h +++ b/pack-revindex.h @@ -16,11 +16,17 @@ * can be found */ +#define RIDX_SIGNATURE 0x52494458 /* "RIDX" */ +#define RIDX_VERSION 1 + struct packed_git; /* * load_pack_revindex populates the revindex's internal data-structures for the * given pack, returning zero on success and a negative value otherwise. + * + * If a '.rev' file is present it is mmap'd, and pointers are assigned into it + * (instead of using the in-memory variant). */ int load_pack_revindex(struct packed_git *p); @@ -55,7 +61,9 @@ uint32_t pack_pos_to_index(struct packed_git *p, uint32_t pos); * If the reverse index has not yet been loaded, or the position is out of * bounds, this function aborts. * - * This function runs in constant time. + * This function runs in constant time under both in-memory and on-disk reverse + * indexes, but an additional step is taken to consult the corresponding .idx + * file when using the on-disk format. */ off_t pack_pos_to_offset(struct packed_git *p, uint32_t pos); diff --git a/packfile.c b/packfile.c index 4b938b4372..1fec12ac5f 100644 --- a/packfile.c +++ b/packfile.c @@ -324,11 +324,21 @@ void close_pack_index(struct packed_git *p) } } +void close_pack_revindex(struct packed_git *p) { + if (!p->revindex_map) + return; + + munmap((void *)p->revindex_map, p->revindex_size); + p->revindex_map = NULL; + p->revindex_data = NULL; +} + void close_pack(struct packed_git *p) { close_pack_windows(p); close_pack_fd(p); close_pack_index(p); + close_pack_revindex(p); } void close_object_store(struct raw_object_store *o) @@ -351,7 +361,7 @@ void close_object_store(struct raw_object_store *o) void unlink_pack_path(const char *pack_name, int force_delete) { - static const char *exts[] = {".pack", ".idx", ".keep", ".bitmap", ".promisor"}; + static const char *exts[] = {".pack", ".idx", ".rev", ".keep", ".bitmap", ".promisor"}; int i; struct strbuf buf = STRBUF_INIT; size_t plen; @@ -853,6 +863,7 @@ static void prepare_pack(const char *full_name, size_t full_name_len, if (!strcmp(file_name, "multi-pack-index")) return; if (ends_with(file_name, ".idx") || + ends_with(file_name, ".rev") || ends_with(file_name, ".pack") || ends_with(file_name, ".bitmap") || ends_with(file_name, ".keep") || diff --git a/packfile.h b/packfile.h index a58fc738e0..4cfec9e8d3 100644 --- a/packfile.h +++ b/packfile.h @@ -90,6 +90,7 @@ uint32_t get_pack_fanout(struct packed_git *p, uint32_t value); unsigned char *use_pack(struct packed_git *, struct pack_window **, off_t, unsigned long *); void close_pack_windows(struct packed_git *); +void close_pack_revindex(struct packed_git *); void close_pack(struct packed_git *); void close_object_store(struct raw_object_store *o); void unuse_pack(struct pack_window **); diff --git a/tmp-objdir.c b/tmp-objdir.c index 42ed4db5d3..b8d880e362 100644 --- a/tmp-objdir.c +++ b/tmp-objdir.c @@ -185,9 +185,11 @@ static int pack_copy_priority(const char *name) return 1; if (ends_with(name, ".pack")) return 2; - if (ends_with(name, ".idx")) + if (ends_with(name, ".rev")) return 3; - return 4; + if (ends_with(name, ".idx")) + return 4; + return 5; } static int pack_copy_cmp(const char *a, const char *b)