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Derrick Stolee 091f4cf358 commit: don't use generation numbers if not needed
In 3afc679b "commit: use generations in paint_down_to_common()",
the queue in paint_down_to_common() was changed to use a priority
order based on generation number before commit date. This served
two purposes:

 1. When generation numbers are present, the walk guarantees
    correct topological relationships, regardless of clock skew in
    commit dates.

 2. It enables short-circuiting the walk when the min_generation
    parameter is added in d7c1ec3e "commit: add short-circuit to
    paint_down_to_common()". This short-circuit helps commands
    like 'git branch --contains' from needing to walk to a merge
    base when we know the result is false.

The commit message for 3afc679b includes the following sentence:

    This change does not affect the number of commits that are
    walked during the execution of paint_down_to_common(), only
    the order that those commits are inspected.

This statement is incorrect. Because it changes the order in which
the commits are inspected, it changes the order they are added to
the queue, and hence can change the number of loops before the
queue_has_nonstale() method returns true.

This change makes a concrete difference depending on the topology
of the commit graph. For instance, computing the merge-base between
consecutive versions of the Linux kernel has no effect for versions
after v4.9, but 'git merge-base v4.8 v4.9' presents a performance
regression:

    v2.18.0: 0.122s
v2.19.0-rc1: 0.547s
       HEAD: 0.127s

To determine that this was simply an ordering issue, I inserted
a counter within the while loop of paint_down_to_common() and
found that the loop runs 167,468 times in v2.18.0 and 635,579
times in v2.19.0-rc1.

The topology of this case can be described in a simplified way
here:

  v4.9
   |  \
   |   \
  v4.8  \
   | \   \
   |  \   |
  ...  A  B
   |  /  /
   | /  /
   |/__/
   C

Here, the "..." means "a very long line of commits". By generation
number, A and B have generation one more than C. However, A and B
have commit date higher than most of the commits reachable from
v4.8. When the walk reaches v4.8, we realize that it has PARENT1
and PARENT2 flags, so everything it can reach is marked as STALE,
including A. B has only the PARENT1 flag, so is not STALE.

When paint_down_to_common() is run using
compare_commits_by_commit_date, A and B are removed from the queue
early and C is inserted into the queue. At this point, C and the
rest of the queue entries are marked as STALE. The loop then
terminates.

When paint_down_to_common() is run using
compare_commits_by_gen_then_commit_date, B is removed from the
queue only after the many commits reachable from v4.8 are explored.
This causes the loop to run longer. The reason for this regression
is simple: the queue order is intended to not explore a commit
until everything that _could_ reach that commit is explored. From
the information gathered by the original ordering, we have no
guarantee that there is not a commit D reachable from v4.8 that
can also reach B. We gained absolute correctness in exchange for
a performance regression.

The performance regression is probably the worse option, since
these incorrect results in paint_down_to_common() are rare. The
topology required for the performance regression are less rare,
but still require multiple merge commits where the parents differ
greatly in generation number. In our example above, the commit A
is as important as the commit B to demonstrate the problem, since
otherwise the commit C will sit in the queue as non-stale just as
long in both orders.

The solution provided uses the min_generation parameter to decide
if we should use generation numbers in our ordering. When
min_generation is equal to zero, it means that the caller has no
known cutoff for the walk, so we should rely on our commit-date
heuristic as before; this is the case with merge_bases_many().
When min_generation is non-zero, then the caller knows a valuable
cutoff for the short-circuit mechanism; this is the case with
remove_redundant() and in_merge_bases_many().

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
2018-08-30 11:17:57 -07:00
.github
Documentation commit-graph.txt: update design document 2018-05-22 12:36:34 +09:00
block-sha1
builtin merge: check config before loading commits 2018-05-22 12:36:34 +09:00
ci Merge branch 'sg/travis-linux32-sanity' 2018-02-13 13:39:16 -08:00
compat
contrib coccinelle: avoid wrong transformation suggestions from commit.cocci 2018-05-02 13:38:52 +09:00
ewah
git-gui
gitk-git
gitweb
mergetools
perl
po
ppc
refs Merge branch 'kg/packed-ref-cache-fix' 2018-02-15 14:55:42 -08:00
sha1collisiondetection@19d97bf5af
sha1dc
t merge: check config before loading commits 2018-05-22 12:36:34 +09:00
templates
vcs-svn
xdiff
.clang-format
.gitattributes
.gitignore commit-graph: create git-commit-graph builtin 2018-04-02 14:27:38 -07:00
.gitmodules
.mailmap
.travis.yml
.tsan-suppressions
COPYING
GIT-VERSION-GEN Git 2.16.2 2018-02-15 15:21:23 -08:00
INSTALL
LGPL-2.1
Makefile commit-graph: implement write_commit_graph() 2018-04-02 14:27:38 -07:00
README.md
RelNotes Git 2.16.2 2018-02-15 15:21:23 -08:00
abspath.c
aclocal.m4
advice.c
advice.h
alias.c
alloc.c commit: add generation number to struct commit 2018-05-22 12:36:25 +09:00
apply.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
apply.h
archive-tar.c
archive-zip.c
archive.c
archive.h
argv-array.c
argv-array.h
attr.c
attr.h
base85.c
bisect.c Merge branch 'rs/lose-leak-pending' into maint 2018-02-15 15:18:11 -08:00
bisect.h
blame.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
blame.h
blob.c
blob.h
branch.c
branch.h
builtin.h commit-graph: create git-commit-graph builtin 2018-04-02 14:27:38 -07:00
bulk-checkin.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
bulk-checkin.h
bundle.c Merge branch 'rs/lose-leak-pending' into maint 2018-02-15 15:18:11 -08:00
bundle.h
cache-tree.c Merge branch 'po/object-id' 2018-02-15 14:55:43 -08:00
cache-tree.h
cache.h Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
check-builtins.sh
check-racy.c
check_bindir
checkout.c
checkout.h
color.c
color.h
column.c
column.h
combine-diff.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
command-list.txt commit-graph: create git-commit-graph builtin 2018-04-02 14:27:38 -07:00
commit-graph.c commit-graph: fix UX issue when .lock file exists 2018-05-22 12:38:02 +09:00
commit-graph.h commit-graph: always load commit-graph information 2018-05-22 12:36:34 +09:00
commit-slab.h
commit.c commit: don't use generation numbers if not needed 2018-08-30 11:17:57 -07:00
commit.h commit-graph: always load commit-graph information 2018-05-22 12:36:34 +09:00
common-main.c
config.c commit-graph: add core.commitGraph setting 2018-04-11 10:43:01 +09:00
config.h
config.mak.in
config.mak.uname
configure.ac
connect.c
connect.h
connected.c Merge branch 'jh/partial-clone' 2018-02-13 13:39:04 -08:00
connected.h
convert.c Merge branch 'po/object-id' 2018-02-15 14:55:43 -08:00
convert.h
copy.c
credential-cache--daemon.c
credential-cache.c
credential-store.c
credential.c
credential.h
csum-file.c csum-file: refactor finalize_hashfile() method 2018-04-02 14:27:30 -07:00
csum-file.h csum-file: refactor finalize_hashfile() method 2018-04-02 14:27:30 -07:00
ctype.c
daemon.c
date.c
decorate.c
decorate.h
delta.h
diff-delta.c
diff-lib.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
diff-no-index.c
diff.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
diff.h
diffcore-break.c
diffcore-delta.c diffcore-delta: rename 'new' variables 2018-02-22 10:08:05 -08:00
diffcore-order.c
diffcore-pickaxe.c
diffcore-rename.c Merge branch 'po/object-id' 2018-02-15 14:55:43 -08:00
diffcore.h
dir-iterator.c
dir-iterator.h
dir.c Merge branch 'nd/trace-index-ops' 2018-02-15 14:55:44 -08:00
dir.h
editor.c
entry.c entry: rename 'new' variables 2018-02-22 10:08:05 -08:00
environment.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
exec_cmd.c
exec_cmd.h
fast-import.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
fetch-object.c
fetch-object.h
fetch-pack.c Merge branch 'jh/partial-clone' 2018-02-13 13:39:04 -08:00
fetch-pack.h
fmt-merge-msg.h
fsck.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
fsck.h
fsmonitor.c
fsmonitor.h
generate-cmdlist.sh
gettext.c
gettext.h
git-add--interactive.perl
git-archimport.perl
git-bisect.sh
git-compat-util.h wrapper: rename 'template' variables 2018-02-22 10:08:05 -08:00
git-cvsexportcommit.perl
git-cvsimport.perl
git-cvsserver.perl
git-difftool--helper.sh
git-filter-branch.sh
git-instaweb.sh
git-merge-octopus.sh
git-merge-one-file.sh
git-merge-resolve.sh
git-mergetool--lib.sh
git-mergetool.sh
git-p4.py
git-parse-remote.sh
git-quiltimport.sh
git-rebase--am.sh
git-rebase--interactive.sh
git-rebase--merge.sh
git-rebase.sh
git-remote-testgit.sh
git-request-pull.sh
git-send-email.perl
git-sh-i18n.sh
git-sh-setup.sh
git-stash.sh Merge branch 'tg/stash-with-pathspec-fix' into maint 2018-02-15 15:18:13 -08:00
git-submodule.sh
git-svn.perl Merge branch 'jm/svn-pushmergeinfo-fix' into maint 2018-02-15 15:18:11 -08:00
git-web--browse.sh
git.c commit-graph: create git-commit-graph builtin 2018-04-02 14:27:38 -07:00
git.rc
gpg-interface.c
gpg-interface.h
graph.c
graph.h
grep.c
grep.h
hash.h
hashmap.c
hashmap.h Merge branch 'rb/hashmap-h-compilation-fix' 2018-02-13 13:39:10 -08:00
help.c
help.h
hex.c
http-backend.c
http-fetch.c
http-push.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
http-walker.c
http.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
http.h
ident.c
imap-send.c imap-send: rename 'new' variables 2018-02-22 10:08:05 -08:00
iterator.h
khash.h
kwset.c
kwset.h
levenshtein.c
levenshtein.h
line-log.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
line-log.h
line-range.c
line-range.h
list-objects-filter-options.c
list-objects-filter-options.h
list-objects-filter.c
list-objects-filter.h
list-objects.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
list-objects.h
list.h
ll-merge.c
ll-merge.h
lockfile.c
lockfile.h
log-tree.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
log-tree.h
mailinfo.c Merge branch 'jc/mailinfo-cleanup-fix' 2018-02-13 13:39:14 -08:00
mailinfo.h
mailmap.c
mailmap.h
match-trees.c
merge-blobs.c
merge-blobs.h
merge-recursive.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
merge-recursive.h
merge.c
mergesort.c
mergesort.h
name-hash.c
notes-cache.c
notes-cache.h
notes-merge.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
notes-merge.h
notes-utils.c
notes-utils.h
notes.c
notes.h
object.c commit-graph: always load commit-graph information 2018-05-22 12:36:34 +09:00
object.h Merge branch 'rs/lose-leak-pending' into maint 2018-02-15 15:18:11 -08:00
oidmap.c
oidmap.h
oidset.c
oidset.h
pack-bitmap-write.c csum-file: refactor finalize_hashfile() method 2018-04-02 14:27:30 -07:00
pack-bitmap.c
pack-bitmap.h
pack-check.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
pack-objects.c
pack-objects.h
pack-revindex.c
pack-revindex.h
pack-write.c csum-file: refactor finalize_hashfile() method 2018-04-02 14:27:30 -07:00
pack.h
packfile.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
packfile.h commit-graph: read only from specific pack-indexes 2018-04-11 10:43:02 +09:00
pager.c
parse-options-cb.c
parse-options.c
parse-options.h
patch-delta.c
patch-ids.c
patch-ids.h
path.c
path.h
pathspec.c
pathspec.h
pkt-line.c
pkt-line.h
preload-index.c
pretty.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
pretty.h
prio-queue.c
prio-queue.h
progress.c
progress.h
prompt.c
prompt.h
protocol.c
protocol.h
quote.c
quote.h
reachable.c object: rename function 'typename' to 'type_name' 2018-02-14 13:10:05 -08:00
reachable.h
read-cache.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
ref-filter.c ref-filter: use generation number for --contains 2018-05-22 12:36:34 +09:00
ref-filter.h
reflog-walk.c
reflog-walk.h
refs.c
refs.h
remote-curl.c
remote-testsvn.c
remote.c remote: rename 'new' variables 2018-02-22 10:08:05 -08:00
remote.h
replace_object.c
repository.c Merge branch 'tg/split-index-fixes' 2018-02-13 13:39:13 -08:00
repository.h
rerere.c
rerere.h
resolve-undo.c
resolve-undo.h
revision.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
revision.h Merge branch 'jk/cached-commit-buffer' into HEAD 2018-03-13 13:35:25 -07:00
run-command.c
run-command.h
send-pack.c
send-pack.h
sequencer.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
sequencer.h Merge branch 'pw/sequencer-in-process-commit' 2018-02-13 13:39:15 -08:00
server-info.c
setup.c
sh-i18n--envsubst.c
sha1-array.c
sha1-array.h
sha1-lookup.c packfile: refactor hash search with fanout table 2018-02-15 13:08:55 -08:00
sha1-lookup.h packfile: refactor hash search with fanout table 2018-02-15 13:08:55 -08:00
sha1_file.c commit-graph: always load commit-graph information 2018-05-22 12:36:34 +09:00
sha1_name.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
sha1dc_git.c
sha1dc_git.h
shallow.c
shell.c
shortlog.h
show-index.c
sideband.c
sideband.h
sigchain.c
sigchain.h
split-index.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
split-index.h split-index: rename 'new' variables 2018-02-22 10:08:05 -08:00
strbuf.c
strbuf.h
streaming.c
streaming.h
string-list.c
string-list.h
sub-process.c
sub-process.h
submodule-config.c
submodule-config.h
submodule.c submodule: rename 'new' variables 2018-02-22 10:08:05 -08:00
submodule.h submodule: rename 'new' variables 2018-02-22 10:08:05 -08:00
symlinks.c
tag.c object: rename function 'typename' to 'type_name' 2018-02-14 13:10:05 -08:00
tag.h
tar.h
tempfile.c tempfile: rename 'template' variables 2018-02-22 10:08:05 -08:00
tempfile.h tempfile: rename 'template' variables 2018-02-22 10:08:05 -08:00
thread-utils.c
thread-utils.h
tmp-objdir.c
tmp-objdir.h
trace.c
trace.h
trailer.c trailer: rename 'template' variables 2018-02-22 10:08:05 -08:00
trailer.h
transport-helper.c Merge branch 'jh/partial-clone' 2018-02-13 13:39:04 -08:00
transport-internal.h
transport.c Merge branch 'jh/partial-clone' 2018-02-13 13:39:04 -08:00
transport.h Merge branch 'jh/partial-clone' 2018-02-13 13:39:04 -08:00
tree-diff.c
tree-walk.c
tree-walk.h
tree.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
tree.h
unicode_width.h
unimplemented.sh
unix-socket.c
unix-socket.h
unpack-trees.c Merge branch 'bw/c-plus-plus' into ds/lazy-load-trees 2018-04-11 10:46:32 +09:00
unpack-trees.h
upload-pack.c Merge branch 'jh/partial-clone' 2018-02-13 13:39:04 -08:00
url.c
url.h
urlmatch.c
urlmatch.h
usage.c
userdiff.c
userdiff.h
utf8.c
utf8.h
varint.c
varint.h
version.c
version.h
versioncmp.c
walker.c treewide: replace maybe_tree with accessor methods 2018-04-11 10:47:16 +09:00
walker.h
wildmatch.c
wildmatch.h
worktree.c
worktree.h
wrap-for-bin.sh
wrapper.c wrapper: rename 'template' variables 2018-02-22 10:08:05 -08:00
write_or_die.c
ws.c
wt-status.c
wt-status.h
xdiff-interface.c
xdiff-interface.h
zlib.c

README.md

Git - fast, scalable, distributed revision control system

Git is a fast, scalable, distributed revision control system with an unusually rich command set that provides both high-level operations and full access to internals.

Git is an Open Source project covered by the GNU General Public License version 2 (some parts of it are under different licenses, compatible with the GPLv2). It was originally written by Linus Torvalds with help of a group of hackers around the net.

Please read the file INSTALL for installation instructions.

Many Git online resources are accessible from https://git-scm.com/ including full documentation and Git related tools.

See Documentation/gittutorial.txt to get started, then see Documentation/giteveryday.txt for a useful minimum set of commands, and Documentation/git-.txt for documentation of each command. If git has been correctly installed, then the tutorial can also be read with man gittutorial or git help tutorial, and the documentation of each command with man git-<commandname> or git help <commandname>.

CVS users may also want to read Documentation/gitcvs-migration.txt (man gitcvs-migration or git help cvs-migration if git is installed).

The user discussion and development of Git take place on the Git mailing list -- everyone is welcome to post bug reports, feature requests, comments and patches to git@vger.kernel.org (read Documentation/SubmittingPatches for instructions on patch submission). To subscribe to the list, send an email with just "subscribe git" in the body to majordomo@vger.kernel.org. The mailing list archives are available at https://public-inbox.org/git/, http://marc.info/?l=git and other archival sites.

The maintainer frequently sends the "What's cooking" reports that list the current status of various development topics to the mailing list. The discussion following them give a good reference for project status, development direction and remaining tasks.

The name "git" was given by Linus Torvalds when he wrote the very first version. He described the tool as "the stupid content tracker" and the name as (depending on your mood):

  • random three-letter combination that is pronounceable, and not actually used by any common UNIX command. The fact that it is a mispronunciation of "get" may or may not be relevant.
  • stupid. contemptible and despicable. simple. Take your pick from the dictionary of slang.
  • "global information tracker": you're in a good mood, and it actually works for you. Angels sing, and a light suddenly fills the room.
  • "goddamn idiotic truckload of sh*t": when it breaks