[PATCH v2] checkout: eliminate unnecessary merge for trivial checkout
From: Ben Peart <hidden>
Date: 2016-09-09 19:25:57
Subsystem:
the rest · Maintainer:
Linus Torvalds
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 <redacted>
---
builtin/checkout.c | 99 +++++++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 95 insertions(+), 4 deletions(-)
diff --git a/builtin/checkout.c b/builtin/checkout.c
index 8672d07..4396cb3 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;
@@ -802,6 +806,87 @@ static void orphaned_commit_warning(struct commit *old, struct commit *new) free(refs.objects); } +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 switch_branches(const struct checkout_opts *opts, struct branch_info *new) {
@@ -827,10 +912,16 @@ 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 "git checkout foo" by skipping the call + * to merge_working_tree where possible. + */ + if (needs_working_tree_merge(opts, &old, new)) { + 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