From a2bb801f6a430f6049e5c9729a8f3bf9097d9b34 Mon Sep 17 00:00:00 2001 From: =?UTF-8?q?SZEDER=20G=C3=A1bor?= Date: Wed, 21 Aug 2019 13:04:24 +0200 Subject: line-log: avoid unnecessary full tree diffs MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit With rename detection enabled the line-level log is able to trace the evolution of line ranges across whole-file renames [1]. Alas, to achieve that it uses the diff machinery very inefficiently, making the operation very slow [2]. And since rename detection is enabled by default, the line-level log is very slow by default. When the line-level log processes a commit with rename detection enabled, it currently does the following (see queue_diffs()): 1. Computes a full tree diff between the commit and (one of) its parent(s), i.e. invokes diff_tree_oid() with an empty 'diffopt->pathspec'. 2. Checks whether any paths in the line ranges were modified. 3. Checks whether any modified paths in the line ranges are missing in the parent commit's tree. 4. If there is such a missing path, then calls diffcore_std() to figure out whether the path was indeed renamed based on the previously computed full tree diff. 5. Continues doing stuff that are unrelated to the slowness. So basically the line-level log computes a full tree diff for each commit-parent pair in step (1) to be used for rename detection in step (4) in the off chance that an interesting path is missing from the parent. Avoid these expensive and mostly unnecessary full tree diffs by limiting the diffs to paths in the line ranges. This is much cheaper, and makes step (2) unnecessary. If it turns out that an interesting path is missing from the parent, then fall back and compute a full tree diff, so the rename detection will still work. Care must be taken when to update the pathspec used to limit the diff in case of renames. A path might be renamed on one branch and modified on several parallel running branches, and while processing commits on these branches the line-level log might have to alternate between looking at a path's new and old name. However, at any one time there is only a single 'diffopt->pathspec'. So add a step (0) to the above to ensure that the paths in the pathspec match the paths in the line ranges associated with the currently processed commit, and re-parse the pathspec from the paths in the line ranges if they differ. The new test cases include a specially crafted piece of history with two merged branches and two files, where each branch modifies both files, renames on of them, and then modifies both again. Then two separate 'git log -L' invocations check the line-level log of each of those two files, which ensures that at least one of those invocations have to do that back-and-forth between the file's old and new name (no matter which branch is traversed first). 't/t4211-line-log.sh' already contains two tests involving renames, they don't don't trigger this back-and-forth. Avoiding these unnecessary full tree diffs can have huge impact on performance, especially in big repositories with big trees and mergy history. Tracing the evolution of a function through the whole history: # git.git $ time git --no-pager log -L:read_alternate_refs:sha1-file.c v2.23.0 Before: real 0m8.874s user 0m8.816s sys 0m0.057s After: real 0m2.516s user 0m2.456s sys 0m0.060s # linux.git $ time ~/src/git/git --no-pager log \ -L:build_restore_work_registers:arch/mips/mm/tlbex.c v5.2 Before: real 3m50.033s user 3m48.041s sys 0m0.300s After: real 0m2.599s user 0m2.466s sys 0m0.157s That's just over 88x speedup. [1] Line-level log's rename following is quite similar to 'git log --follow path', with the notable differences that it does handle multiple paths at once as well, and that it doesn't show the commit performing the rename if it's an exact rename. [2] This slowness might not have been apparent initially, because back when the line-level log feature was introduced rename detection was not yet enabled by default; 12da1d1f6f (Implement line-history search (git log -L), 2013-03-28) and 5404c116aa (diff: activate diff.renames by default, 2016-02-25). Signed-off-by: SZEDER Gábor Signed-off-by: Junio C Hamano --- t/t4211-line-log.sh | 82 +++++++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 82 insertions(+) (limited to 't') diff --git a/t/t4211-line-log.sh b/t/t4211-line-log.sh index 1db7bd0f59..8319163744 100755 --- a/t/t4211-line-log.sh +++ b/t/t4211-line-log.sh @@ -132,4 +132,86 @@ test_expect_success '--raw is forbidden' ' test_must_fail git log -L1,24:b.c --raw ' +test_expect_success 'setup for checking fancy rename following' ' + git checkout --orphan moves-start && + git reset --hard && + + printf "%s\n" 12 13 14 15 b c d e >file-1 && + printf "%s\n" 22 23 24 25 B C D E >file-2 && + git add file-1 file-2 && + test_tick && + git commit -m "Add file-1 and file-2" && + oid_add_f1_f2=$(git rev-parse --short HEAD) && + + git checkout -b moves-main && + printf "%s\n" 11 12 13 14 15 b c d e >file-1 && + git commit -a -m "Modify file-1 on main" && + oid_mod_f1_main=$(git rev-parse --short HEAD) && + + printf "%s\n" 21 22 23 24 25 B C D E >file-2 && + git commit -a -m "Modify file-2 on main #1" && + oid_mod_f2_main_1=$(git rev-parse --short HEAD) && + + git mv file-1 renamed-1 && + git commit -m "Rename file-1 to renamed-1 on main" && + + printf "%s\n" 11 12 13 14 15 b c d e f >renamed-1 && + git commit -a -m "Modify renamed-1 on main" && + oid_mod_r1_main=$(git rev-parse --short HEAD) && + + printf "%s\n" 21 22 23 24 25 B C D E F >file-2 && + git commit -a -m "Modify file-2 on main #2" && + oid_mod_f2_main_2=$(git rev-parse --short HEAD) && + + git checkout -b moves-side moves-start && + printf "%s\n" 12 13 14 15 16 b c d e >file-1 && + git commit -a -m "Modify file-1 on side #1" && + oid_mod_f1_side_1=$(git rev-parse --short HEAD) && + + printf "%s\n" 22 23 24 25 26 B C D E >file-2 && + git commit -a -m "Modify file-2 on side" && + oid_mod_f2_side=$(git rev-parse --short HEAD) && + + git mv file-2 renamed-2 && + git commit -m "Rename file-2 to renamed-2 on side" && + + printf "%s\n" 12 13 14 15 16 a b c d e >file-1 && + git commit -a -m "Modify file-1 on side #2" && + oid_mod_f1_side_2=$(git rev-parse --short HEAD) && + + printf "%s\n" 22 23 24 25 26 A B C D E >renamed-2 && + git commit -a -m "Modify renamed-2 on side" && + oid_mod_r2_side=$(git rev-parse --short HEAD) && + + git checkout moves-main && + git merge moves-side && + oid_merge=$(git rev-parse --short HEAD) +' + +test_expect_success 'fancy rename following #1' ' + cat >expect <<-EOF && + $oid_merge Merge branch '\''moves-side'\'' into moves-main + $oid_mod_f1_side_2 Modify file-1 on side #2 + $oid_mod_f1_side_1 Modify file-1 on side #1 + $oid_mod_r1_main Modify renamed-1 on main + $oid_mod_f1_main Modify file-1 on main + $oid_add_f1_f2 Add file-1 and file-2 + EOF + git log -L1:renamed-1 --oneline --no-patch >actual && + test_cmp expect actual +' + +test_expect_success 'fancy rename following #2' ' + cat >expect <<-EOF && + $oid_merge Merge branch '\''moves-side'\'' into moves-main + $oid_mod_r2_side Modify renamed-2 on side + $oid_mod_f2_side Modify file-2 on side + $oid_mod_f2_main_2 Modify file-2 on main #2 + $oid_mod_f2_main_1 Modify file-2 on main #1 + $oid_add_f1_f2 Add file-1 and file-2 + EOF + git log -L1:renamed-2 --oneline --no-patch >actual && + test_cmp expect actual +' + test_done -- cgit v1.2.3