Teach git to avoid unnecessary merge during trivial checkout. When
running 'git checkout -b foo' git follows a common code path through
the expensive merge_working_tree even when it is unnecessary. As a
result, 95% of the time is spent in merge_working_tree doing the 2-way
merge between the new and old commit trees that is unneeded.
The time breakdown is as follows:
merge_working_tree <-- 95%
unpack_trees <-- 80%
traverse_trees <-- 50%
cache_tree_update <-- 17%
mark_new_skip_worktree <-- 10%
With a large repo, this cost is pronounced. Using "git checkout -b r"
to create and switch to a new branch costs 166 seconds (all times worst
case with a cold file system cache).
git.c:406 trace: built-in: git 'checkout' '-b' 'r'
read-cache.c:1667 performance: 17.442926555 s: read_index_from
name-hash.c:128 performance: 2.912145231 s: lazy_init_name_hash
read-cache.c:2208 performance: 4.387713335 s: write_locked_index
trace.c:420 performance: 166.458921289 s: git command:
'c:\Users\benpeart\bin\git.exe'
'checkout' '-b' 'r'
Switched to a new branch 'r'
By adding a test to skip the unnecessary call to merge_working_tree in
this case reduces the cost to 16 seconds.
git.c:406 trace: built-in: git 'checkout' '-b' 's'
read-cache.c:1667 performance: 16.100742476 s: read_index_from
trace.c:420 performance: 16.461547867 s: git command:
'c:\Users\benpeart\bin\git.exe' 'checkout' '-b' 's'
Switched to a new branch 's'
Signed-off-by: Ben Peart <benpe...@microsoft.com>
---
builtin/checkout.c | 23 +++++++++++++++++++----
1 file changed, 19 insertions(+), 4 deletions(-)
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 8672d07..595d64b 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -827,10 +827,25 @@ static int switch_branches(const struct checkout_opts
*opts,
parse_commit_or_die(new->commit);
}
- ret = merge_working_tree(opts, &old, new, &writeout_error);
- if (ret) {
- free(path_to_free);
- return ret;
+ /*
+ * Optimize the performance of checkout when the current and
+ * new branch have the same OID and avoid the trivial merge.
+ * For example, a "git checkout -b foo" just needs to create
+ * the new ref and report the stats.
+ */
+ if (!old.commit || !new->commit
+ || oidcmp(&old.commit->object.oid, &new->commit->object.oid)
+ || !opts->new_branch || opts->new_branch_force ||
opts->new_orphan_branch
+ || opts->patch_mode || opts->merge || opts->force ||
opts->force_detach
+ || opts->writeout_stage || !opts->overwrite_ignore
+ || opts->ignore_skipworktree || opts->ignore_other_worktrees
+ || opts->new_branch_log || opts->branch_exists || opts->prefix
+ || opts->source_tree) {
+ ret = merge_working_tree(opts, &old, new, &writeout_error);
+ if (ret) {
+ free(path_to_free);
+ return ret;
+ }
}
if (!opts->quiet && !old.path && old.commit && new->commit !=
old.commit)
--
2.10.0.windows.1
