We do n-way merge by walking the source index and n trees at the same
time and add merge results to a new temporary index called o->result.
The merge result for any given path could be either
- keep_entry(): same old index entry in o->src_index is reused
- merged_entry(): either a new entry is added, or an existing one updated
- deleted_entry(): one entry from o->src_index is removed
For some reason [1] we keep making sure that the source index's
cache-tree is still valid if used by o->result: for all those
merged/deleted entries, we invalidate the same path in o->src_index,
so only cache-trees covering the "keep_entry" parts remain good.
Because of this, the cache-tree from o->src_index can be perfectly
reused in o->result. And in fact we already rely on this logic to
reuse untracked cache in edf3b90553 (unpack-trees: preserve index
extensions - 2017-05-08). Move the cache-tree to o->result before
doing cache_tree_update() to reduce hashing cost.
Since cache_tree_update() has risen up as one of the most expensive
parts in unpack_trees() after the last few patches. This does help
reduce unpack_trees() time significantly (on webkit.git):
before after
--------------------------------------------------------------------
0.080394752 0.051258167 s: read cache .git/index
0.216010838 0.212106298 s: preload index
0.008534301 0.280521764 s: refresh index
0.251992198 0.218160442 s: traverse_trees
0.377031383 0.374948191 s: check_updates
0.372768105 0.037040114 s: cache_tree_update
1.045887251 0.672031609 s: unpack_trees
0.314983512 0.317456290 s: write index, changed mask = 2e
0.062572653 0.038382654 s: traverse_trees
0.000022544 0.000042731 s: check_updates
0.073795585 0.050930053 s: unpack_trees
0.073807557 0.051099735 s: diff-index
1.938191592 1.614241153 s: git command: git checkout -
[1] I'm pretty sure the reason is an oversight in 34110cd4e3 (Make
'unpack_trees()' have a separate source and destination index -
2008-03-06). That patch aims to _not_ update the source index at
all. The invalidation should have been done on o->result in that
patch. But then there was no cache-tree on o->result even then so
it's pointless to do so.
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@xxxxxxxxx>
---
read-cache.c | 2 ++
unpack-trees.c | 2 +-
2 files changed, 3 insertions(+), 1 deletion(-)
diff --git a/read-cache.c b/read-cache.c
index 4fd35f4f37..2b5646ef26 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -2763,4 +2763,6 @@ void move_index_extensions(struct index_state *dst, struct index_state *src)
{
dst->untracked = src->untracked;
src->untracked = NULL;
+ dst->cache_tree = src->cache_tree;
+ src->cache_tree = NULL;
}
diff --git a/unpack-trees.c b/unpack-trees.c
index 6deb04c163..d822662c75 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -1570,6 +1570,7 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
ret = check_updates(o) ? (-2) : 0;
if (o->dst_index) {
+ move_index_extensions(&o->result, o->src_index);
if (!ret) {
if (!o->result.cache_tree)
o->result.cache_tree = cache_tree();
@@ -1578,7 +1579,6 @@ int unpack_trees(unsigned len, struct tree_desc *t, struct unpack_trees_options
WRITE_TREE_SILENT |
WRITE_TREE_REPAIR);
}
- move_index_extensions(&o->result, o->src_index);
discard_index(o->dst_index);
*o->dst_index = o->result;
} else {
--
2.18.0.1004.g6639190530
