@matheusbsbs / matheus-bagliano / commits / 736eb88fdc

for_each_packed_object: support iterating in pack-order

We currently iterate over objects within a pack in .idx order, which uses the object hashes. That means that it is effectively random with respect to the location of the object within the pack. If you're going to access the actual object data, there are two reasons to move linearly through the pack itself: 1. It improves the locality of access in the packfile. In the cold-cache case, this may mean fewer disk seeks, or better usage of disk cache. 2. We store related deltas together in the packfile. Which means that the delta base cache can operate much more efficiently if we visit all of those related deltas in sequence, as the earlier items are likely to still be in the cache. Whereas if we visit the objects in random order, our cache entries are much more likely to have been evicted by unrelated deltas in the meantime. So in general, if you're going to access the object contents pack order is generally going to end up more efficient. But if you're simply generating a list of object names, or if you're going to end up sorting the result anyway, you're better off just using the .idx order, as finding the pack order means generating the in-memory pack-revindex. According to the numbers in 8b8dfd5132 (pack-revindex: radix-sort the revindex, 2013-07-11), that takes about 200ms for linux.git, and 20ms for git.git (those numbers are a few years old but are still a good ballpark). That makes it a good optimization for some cases (we can save tens of seconds in git.git by having good locality of delta access, for a 20ms cost), but a bad one for others (e.g., right now "cat-file --batch-all-objects --batch-check="%(objectname)" is 170ms in git.git, so adding 20ms to that is noticeable). Hence this patch makes it an optional flag. You can't actually do any interesting timings yet, as it's not plumbed through to any user-facing tools like cat-file. That will come in a later patch. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Jeff King committed Aug 10, 2018 at 19:15 UTC 736eb88fdc8a2dea4302114d2f74b580d0f83cfe

4 files changed +27 -9

cache.h

@@ -1633,6 +1633,11 @@ enum for_each_object_flags {

1633

1634 /* Only iterate over packs obtained from the promisor remote. */

1635 FOR_EACH_OBJECT_PROMISOR_ONLY = (1<<1),

1636 +

1637 + /*

1638 + * Visit objects within a pack in packfile order rather than .idx order

1639 + */

1640 + FOR_EACH_OBJECT_PACK_ORDER = (1<<2),

1641 };

1642

1643 /*

commit-graph.c

+1 -1

@@ -730,7 +730,7 @@ void write_commit_graph(const char *obj_dir,

730 die("error adding pack %s", packname.buf);

731 if (open_pack_index(p))

732 die("error opening index for %s", packname.buf);

733 - for_each_object_in_pack(p, add_packed_commits, &oids);

733 + for_each_object_in_pack(p, add_packed_commits, &oids, 0);

734 close_pack(p);

735 }

736 strbuf_release(&packname);

packfile.c

+16 -5

@@ -1885,19 +1885,30 @@ int has_pack_index(const unsigned char *sha1)

1885 return 1;

1886 }

1887

1888 -int for_each_object_in_pack(struct packed_git *p, each_packed_object_fn cb, void *data)

1888 +int for_each_object_in_pack(struct packed_git *p,

1889 + each_packed_object_fn cb, void *data,

1890 + enum for_each_object_flags flags)

1891 {

1892 uint32_t i;

1893 int r = 0;

1894

1895 + if (flags & FOR_EACH_OBJECT_PACK_ORDER)

1896 + load_pack_revindex(p);

1897 +

1898 for (i = 0; i < p->num_objects; i++) {

1899 + uint32_t pos;

1900 struct object_id oid;

1901

1896 - if (!nth_packed_object_oid(&oid, p, i))

1902 + if (flags & FOR_EACH_OBJECT_PACK_ORDER)

1903 + pos = p->revindex[i].nr;

1904 + else

1905 + pos = i;

1906 +

1907 + if (!nth_packed_object_oid(&oid, p, pos))

1908 return error("unable to get sha1 of object %u in %s",

1898 - i, p->pack_name);

1909 + pos, p->pack_name);

1910

1900 - r = cb(&oid, p, i, data);

1911 + r = cb(&oid, p, pos, data);

1912 if (r)

1913 break;

1914 }

@@ -1922,7 +1933,7 @@ int for_each_packed_object(each_packed_object_fn cb, void *data,

1933 pack_errors = 1;

1934 continue;

1935 }

1925 - r = for_each_object_in_pack(p, cb, data);

1936 + r = for_each_object_in_pack(p, cb, data, flags);

1937 if (r)

1938 break;

1939 }

packfile.h

+5 -3

@@ -153,8 +153,8 @@ extern int has_pack_index(const unsigned char *sha1);

153 * By default, this includes both local and alternate packs.

154 *

155 * Note that some objects may appear twice if they are found in multiple packs.

156 - * Each pack is visited in an unspecified order. Objects within a pack are

157 - * visited in pack-idx order (i.e., sorted by oid).

156 + * Each pack is visited in an unspecified order. By default, objects within a

157 + * pack are visited in pack-idx order (i.e., sorted by oid).

158 *

159 * The list of flags can be found in cache.h.

160 */

@@ -162,7 +162,9 @@ typedef int each_packed_object_fn(const struct object_id *oid,

162 struct packed_git *pack,

163 uint32_t pos,

164 void *data);

165 -int for_each_object_in_pack(struct packed_git *p, each_packed_object_fn, void *data);

165 +int for_each_object_in_pack(struct packed_git *p,

166 + each_packed_object_fn, void *data,

167 + enum for_each_object_flags flags);

168 int for_each_packed_object(each_packed_object_fn, void *,

169 enum for_each_object_flags flags);

170