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 | 92 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 92 insertions(+)
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 8672d07..8b2f428 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -38,6 +38,10 @@ struct checkout_opts {
int ignore_skipworktree;
int ignore_other_worktrees;
int show_progress;
+ /*
+ * If new checkout options are added, needs_working_tree_merge
+ * should be updated accordingly.
+ */
const char *new_branch;
const char *new_branch_force;
@@ -460,11 +464,99 @@ static void setup_branch_path(struct branch_info *branch)
branch->path = strbuf_detach(&buf, NULL);
}
+static int needs_working_tree_merge(const struct checkout_opts *opts,
+ const struct branch_info *old,
+ const struct branch_info *new)
+{
+ /*
+ * We must do the merge if we are actually moving to a new
+ * commit tree.
+ */
+ if (!old->commit || !new->commit ||
+ oidcmp(&old->commit->tree->object.oid,
&new->commit->tree->object.oid))
+ return 1;
+
+ /*
+ * opts->patch_mode cannot be used with switching branches so is
+ * not tested here
+ */
+
+ /*
+ * opts->quiet only impacts output so doesn't require a merge
+ */
+
+ /*
+ * Honor the explicit request for a three-way merge or to throw away
+ * local changes
+ */
+ if (opts->merge || opts->force)
+ return 1;
+
+ /*
+ * Checking out the requested commit may require updating the working
+ * directory and index, let the merge handle it.
+ */
+ if (opts->force_detach)
+ return 1;
+
+ /*
+ * opts->writeout_stage cannot be used with switching branches so is
+ * not tested here
+ */
+
+ /*
+ * Honor the explicit ignore requests
+ */
+ if (!opts->overwrite_ignore || opts->ignore_skipworktree ||
+ opts->ignore_other_worktrees)
+ return 1;
+
+ /*
+ * opts->show_progress only impacts output so doesn't require a merge
+ */
+
+ /*
+ * If we're not creating a new branch, by definition we're changing
+ * the existing one so need to do the merge
+ */
+ if (!opts->new_branch)
+ return 1;
+
+ /*
+ * new_branch_force is defined to "create/reset and checkout a branch"
+ * so needs to go through the merge to do the reset
+ */
+ if (opts->new_branch_force)
+ return 1;
+
+ /*
+ * A new orphaned branch requrires the index and the working tree to be
+ * adjusted to <start_point>
+ */
+ if (opts->new_orphan_branch)
+ return 1;
+
+ /*
+ * Remaining variables are not checkout options but used to track state
+ * that doesn't trigger the need for a merge.
+ */
+
+ return 0;
+}
+
static int merge_working_tree(const struct checkout_opts *opts,
struct branch_info *old,
struct branch_info *new,
int *writeout_error)
{
+ /*
+ * Optimize the performance of "git checkout -b foo" by avoiding
+ * the expensive merge, index and working directory updates if they
+ * are not needed.
+ */
+ if (!needs_working_tree_merge(opts, old, new))
+ return 0;
+
int ret;
struct lock_file *lock_file = xcalloc(1, sizeof(struct lock_file));
--
2.10.0.windows.1
