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unpack-trees: optimize walking same trees with cache-tree
In order to merge one or many trees with the index, unpack-trees code walks multiple trees in parallel with the index and performs n-way merge. If we find out at start of a directory that all trees are the same (by comparing OID) and cache-tree happens to be available for that directory as well, we could avoid walking the trees because we already know what these trees contain: it's flattened in what's called "the index". The upside is of course a lot less I/O since we can potentially skip lots of trees (think subtrees). We also save CPU because we don't have to inflate and apply the deltas. The downside is of course more fragile code since the logic in some functions are now duplicated elsewhere. "checkout -" with this patch on webkit.git (275k files): baseline new -------------------------------------------------------------------- 0.056651714 0.080394752 s: read cache .git/index 0.183101080 0.216010838 s: preload index 0.008584433 0.008534301 s: refresh index 0.633767589 0.251992198 s: traverse_trees 0.340265448 0.377031383 s: check_updates 0.381884638 0.372768105 s: cache_tree_update 1.401562947 1.045887251 s: unpack_trees 0.338687914 0.314983512 s: write index, changed mask = 2e 0.411927922 0.062572653 s: traverse_trees 0.000023335 0.000022544 s: check_updates 0.423697246 0.073795585 s: unpack_trees 0.423708360 0.073807557 s: diff-index 2.559524127 1.938191592 s: git command: git checkout - Another measurement from Ben's running "git checkout" with over 500k trees (on the whole series): baseline new ---------------------------------------------------------------------- 0.535510167 0.556558733 s: read cache .git/index 0.3057373 0.3147105 s: initialize name hash 0.0184082 0.023558433 s: preload index 0.086910967 0.089085967 s: refresh index 7.889590767 2.191554433 s: unpack trees 0.120760833 0.131941267 s: update worktree after a merge 2.2583504 2.572663167 s: repair cache-tree 0.8916137 0.959495233 s: write index, changed mask = 28 3.405199233 0.2710663 s: unpack trees 0.000999667 0.0021554 s: update worktree after a merge 3.4063306 0.273318333 s: diff-index 16.9524923 9.462943133 s: git command: git.exe checkout This command calls unpack_trees() twice, the first time on 2way merge and the second 1way merge. In both times, "unpack trees" time is reduced to one third. Overall time reduction is not that impressive of course because index operations take a big chunk. And there's that repair cache-tree line. PS. A note about cache-tree invalidation and the use of it in this code. We do invalidate cache-tree in _source_ index when we add new entries to the (temporary) "result" index. But we also use the cache-tree from source index in this optimization. Does this mean we end up having no cache-tree in the source index to activate this optimization? The answer is twisted: the order of finding a good cache-tree and invalidating it matters. In this case we check for a good cache-tree first in all_trees_same_as_cache_tree(), then we start to merge things and potentially invalidate that same cache-tree in the process. Since cache-tree invalidation happens after the optimization kicks in, we're still good. But we may lose that cache-tree at the very first call_unpack_fn() call in traverse_by_cache_tree(). Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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0d1ed5963d
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127
unpack-trees.c
127
unpack-trees.c
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@ -635,6 +635,102 @@ static inline int are_same_oid(struct name_entry *name_j, struct name_entry *nam
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return name_j->oid && name_k->oid && !oidcmp(name_j->oid, name_k->oid);
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}
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static int all_trees_same_as_cache_tree(int n, unsigned long dirmask,
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struct name_entry *names,
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struct traverse_info *info)
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{
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struct unpack_trees_options *o = info->data;
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int i;
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if (!o->merge || dirmask != ((1 << n) - 1))
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return 0;
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for (i = 1; i < n; i++)
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if (!are_same_oid(names, names + i))
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return 0;
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return cache_tree_matches_traversal(o->src_index->cache_tree, names, info);
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}
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static int index_pos_by_traverse_info(struct name_entry *names,
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struct traverse_info *info)
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{
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struct unpack_trees_options *o = info->data;
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int len = traverse_path_len(info, names);
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char *name = xmalloc(len + 1 /* slash */ + 1 /* NUL */);
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int pos;
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make_traverse_path(name, info, names);
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name[len++] = '/';
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name[len] = '\0';
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pos = index_name_pos(o->src_index, name, len);
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if (pos >= 0)
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BUG("This is a directory and should not exist in index");
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pos = -pos - 1;
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if (!starts_with(o->src_index->cache[pos]->name, name) ||
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(pos > 0 && starts_with(o->src_index->cache[pos-1]->name, name)))
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BUG("pos must point at the first entry in this directory");
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free(name);
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return pos;
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}
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/*
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* Fast path if we detect that all trees are the same as cache-tree at this
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* path. We'll walk these trees recursively using cache-tree/index instead of
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* ODB since already know what these trees contain.
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*/
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static int traverse_by_cache_tree(int pos, int nr_entries, int nr_names,
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struct name_entry *names,
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struct traverse_info *info)
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{
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struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
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struct unpack_trees_options *o = info->data;
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int i, d;
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if (!o->merge)
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BUG("We need cache-tree to do this optimization");
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/*
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* Do what unpack_callback() and unpack_nondirectories() normally
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* do. But we walk all paths in an iterative loop instead.
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*
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* D/F conflicts and higher stage entries are not a concern
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* because cache-tree would be invalidated and we would never
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* get here in the first place.
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*/
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for (i = 0; i < nr_entries; i++) {
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struct cache_entry *tree_ce;
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int len, rc;
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src[0] = o->src_index->cache[pos + i];
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len = ce_namelen(src[0]);
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tree_ce = xcalloc(1, cache_entry_size(len));
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tree_ce->ce_mode = src[0]->ce_mode;
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tree_ce->ce_flags = create_ce_flags(0);
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tree_ce->ce_namelen = len;
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oidcpy(&tree_ce->oid, &src[0]->oid);
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memcpy(tree_ce->name, src[0]->name, len + 1);
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for (d = 1; d <= nr_names; d++)
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src[d] = tree_ce;
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rc = call_unpack_fn((const struct cache_entry * const *)src, o);
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free(tree_ce);
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if (rc < 0)
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return rc;
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mark_ce_used(src[0], o);
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}
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if (o->debug_unpack)
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printf("Unpacked %d entries from %s to %s using cache-tree\n",
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nr_entries,
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o->src_index->cache[pos]->name,
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o->src_index->cache[pos + nr_entries - 1]->name);
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return 0;
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}
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static int traverse_trees_recursive(int n, unsigned long dirmask,
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unsigned long df_conflicts,
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struct name_entry *names,
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@ -646,6 +742,27 @@ static int traverse_trees_recursive(int n, unsigned long dirmask,
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void *buf[MAX_UNPACK_TREES];
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struct traverse_info newinfo;
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struct name_entry *p;
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int nr_entries;
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nr_entries = all_trees_same_as_cache_tree(n, dirmask, names, info);
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if (nr_entries > 0) {
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struct unpack_trees_options *o = info->data;
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int pos = index_pos_by_traverse_info(names, info);
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if (!o->merge || df_conflicts)
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BUG("Wrong condition to get here buddy");
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/*
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* All entries up to 'pos' must have been processed
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* (i.e. marked CE_UNPACKED) at this point. But to be safe,
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* save and restore cache_bottom anyway to not miss
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* unprocessed entries before 'pos'.
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*/
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bottom = o->cache_bottom;
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ret = traverse_by_cache_tree(pos, nr_entries, n, names, info);
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o->cache_bottom = bottom;
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return ret;
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}
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p = names;
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while (!p->mode)
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@ -812,6 +929,11 @@ static struct cache_entry *create_ce_entry(const struct traverse_info *info,
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return ce;
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}
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/*
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* Note that traverse_by_cache_tree() duplicates some logic in this function
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* without actually calling it. If you change the logic here you may need to
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* check and change there as well.
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*/
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static int unpack_nondirectories(int n, unsigned long mask,
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unsigned long dirmask,
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struct cache_entry **src,
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@ -1004,6 +1126,11 @@ static void debug_unpack_callback(int n,
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debug_name_entry(i, names + i);
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}
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/*
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* Note that traverse_by_cache_tree() duplicates some logic in this function
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* without actually calling it. If you change the logic here you may need to
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* check and change there as well.
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*/
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static int unpack_callback(int n, unsigned long mask, unsigned long dirmask, struct name_entry *names, struct traverse_info *info)
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{
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struct cache_entry *src[MAX_UNPACK_TREES + 1] = { NULL, };
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