@matheusbsbs / matheus-bagliano / commits / 65c1a28bb6

Fork 0

bloom: de-duplicate directory entries

When computing a changed-path Bloom filter, we need to take the files that changed from the diff computation and extract the parent directories. That way, a directory pathspec such as "Documentation" could match commits that change "Documentation/git.txt". However, the current code does a poor job of this process. The paths are added to a hashmap, but we do not check if an entry already exists with that path. This can create many duplicate entries and cause the filter to have a much larger length than it should. This means that the filter is more sparse than intended, which helps the false positive rate, but wastes a lot of space. Properly use hashmap_get() before hashmap_add(). Also be sure to include a comparison function so these can be matched correctly. This has an effect on a test in t0095-bloom.sh. This makes sense, there are ten changes inside "smallDir" so the total number of paths in the filter should be 11. This would result in 11 * 10 bits required, and with 8 bits per byte, this results in 14 bytes. Signed-off-by: Derrick Stolee <dstolee@microsoft.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Derrick Stolee committed May 11, 2020 at 11:56 UTC 65c1a28bb60d1c17a672306c651bb402378f81b0

2 files changed +28 -11

bloom.c

+26 -9

@@ -158,6 +158,19 @@ void init_bloom_filters(void)

158 init_bloom_filter_slab(&bloom_filters);

159 }

160

161 +static int pathmap_cmp(const void *hashmap_cmp_fn_data,

162 + const struct hashmap_entry *eptr,

163 + const struct hashmap_entry *entry_or_key,

164 + const void *keydata)

165 +{

166 + const struct pathmap_hash_entry *e1, *e2;

167 +

168 + e1 = container_of(eptr, const struct pathmap_hash_entry, entry);

169 + e2 = container_of(entry_or_key, const struct pathmap_hash_entry, entry);

170 +

171 + return strcmp(e1->path, e2->path);

172 +}

173 +

174 struct bloom_filter *get_bloom_filter(struct repository *r,

175 struct commit *c,

176 int compute_if_not_present)

@@ -206,25 +219,29 @@ struct bloom_filter *get_bloom_filter(struct repository *r,

219 struct hashmap pathmap;

220 struct pathmap_hash_entry *e;

221 struct hashmap_iter iter;

209 - hashmap_init(&pathmap, NULL, NULL, 0);

222 + hashmap_init(&pathmap, pathmap_cmp, NULL, 0);

223

224 for (i = 0; i < diff_queued_diff.nr; i++) {

225 const char *path = diff_queued_diff.queue[i]->two->path;

226

227 /*

215 - * Add each leading directory of the changed file, i.e. for

216 - * 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so

217 - * the Bloom filter could be used to speed up commands like

218 - * 'git log dir/subdir', too.

219 - *

220 - * Note that directories are added without the trailing '/'.

221 - */

228 + * Add each leading directory of the changed file, i.e. for

229 + * 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so

230 + * the Bloom filter could be used to speed up commands like

231 + * 'git log dir/subdir', too.

232 + *

233 + * Note that directories are added without the trailing '/'.

234 + */

235 do {

236 char *last_slash = strrchr(path, '/');

237

238 FLEX_ALLOC_STR(e, path, path);

239 hashmap_entry_init(&e->entry, strhash(path));

227 - hashmap_add(&pathmap, &e->entry);

240 +

241 + if (!hashmap_get(&pathmap, &e->entry, NULL))

242 + hashmap_add(&pathmap, &e->entry);

243 + else

244 + free(e);

245

246 if (!last_slash)

247 last_slash = (char*)path;

t/t0095-bloom.sh

+2 -2

@@ -89,8 +89,8 @@ test_expect_success 'get bloom filter for commit with 10 changes' '

89 git add smallDir &&

90 git commit -m "commit with 10 changes" &&

91 cat >expect <<-\EOF &&

92 - Filter_Length:25

93 - Filter_Data:82|a0|65|47|0c|92|90|c0|a1|40|02|a0|e2|40|e0|04|0a|9a|66|cf|80|19|85|42|23|

92 + Filter_Length:14

93 + Filter_Data:02|b3|c4|a0|34|e7|fe|eb|cb|47|fe|a0|e8|72|

94 EOF

95 test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&

96 test_cmp expect actual