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Jeff King 5827a03545 fetch: use "quick" has_sha1_file for tag following When we auto-follow tags in a fetch, we look at all of the tags advertised by the remote and fetch ones where we don't already have the tag, but we do have the object it peels to. This involves a lot of calls to has_sha1_file(), some of which we can reasonably expect to fail. Since 45e8a74 (has_sha1_file: re-check pack directory before giving up, 2013-08-30), this may cause many calls to reprepare_packed_git(), which is potentially expensive. This has gone unnoticed for several years because it requires a fairly unique setup to matter: 1. You need to have a lot of packs on the client side to make reprepare_packed_git() expensive (the most expensive part is finding duplicates in an unsorted list, which is currently quadratic). 2. You need a large number of tag refs on the server side that are candidates for auto-following (i.e., that the client doesn't have). Each one triggers a re-read of the pack directory. 3. Under normal circumstances, the client would auto-follow those tags and after one large fetch, (2) would no longer be true. But if those tags point to history which is disconnected from what the client otherwise fetches, then it will never auto-follow, and those candidates will impact it on every fetch. So when all three are true, each fetch pays an extra O(nr_tags * nr_packs^2) cost, mostly in string comparisons on the pack names. This was exacerbated by 47bf4b0 (prepare_packed_git_one: refactor duplicate-pack check, 2014-06-30) which uses a slightly more expensive string check, under the assumption that the duplicate check doesn't happen very often (and it shouldn't; the real problem here is how often we are calling reprepare_packed_git()). This patch teaches fetch to use HAS_SHA1_QUICK to sacrifice accuracy for speed, in cases where we might be racy with a simultaneous repack. This is similar to the fix in 0eeb077 (index-pack: avoid excessive re-reading of pack directory, 2015-06-09). As with that case, it's OK for has_sha1_file() occasionally say "no I don't have it" when we do, because the worst case is not a corruption, but simply that we may fail to auto-follow a tag that points to it. Here are results from the included perf script, which sets up a situation similar to the one described above: Test HEAD^ HEAD ---------------------------------------------------------- 5550.4: fetch 11.21(10.42+0.78) 0.08(0.04+0.02) -99.3% Reported-by: Vegard Nossum <vegard.nossum@oracle.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>		2016-10-14 11:31:32 -07:00
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argv-array.c
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base85.c
bisect.c
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blob.c
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branch.c
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date.c
decorate.c
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diff-no-index.c
diff.c
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diffcore-break.c
diffcore-delta.c
diffcore-order.c
diffcore-pickaxe.c
diffcore-rename.c
diffcore.h
dir.c
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editor.c
entry.c
environment.c
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name-hash.c
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object.c
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pack-bitmap-write.c
pack-bitmap.c
pack-bitmap.h
pack-check.c
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pack-revindex.c
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patch-ids.h
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quote.h
reachable.c
reachable.h
read-cache.c
README.md
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ref-filter.h
reflog-walk.c
reflog-walk.h
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RelNotes
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remote-testsvn.c
remote.c
remote.h
replace_object.c
rerere.c
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resolve-undo.c
resolve-undo.h
revision.c
revision.h
run-command.c
run-command.h
send-pack.c
send-pack.h
sequencer.c
sequencer.h
server-info.c
setup.c
sh-i18n--envsubst.c
sha1_file.c
sha1_name.c
sha1-array.c
sha1-array.h
sha1-lookup.c
sha1-lookup.h
shallow.c
shell.c
shortlog.h
show-index.c
sideband.c
sideband.h
sigchain.c
sigchain.h
split-index.c
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strbuf.c
strbuf.h
streaming.c
streaming.h
string-list.c
string-list.h
submodule-config.c
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submodule.c
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symlinks.c
tag.c
tag.h
tar.h
tempfile.c
tempfile.h
thread-utils.c
thread-utils.h
trace.c
trace.h
trailer.c
trailer.h
transport-helper.c
transport.c
transport.h
tree-diff.c
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tree.c
tree.h
unicode_width.h
unimplemented.sh
unix-socket.c
unix-socket.h
unpack-trees.c
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update_unicode.sh
upload-pack.c
url.c
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urlmatch.c
urlmatch.h
usage.c
userdiff.c
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utf8.c
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varint.c
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worktree.c
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wrapper.c
write_or_die.c
ws.c
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wt-status.h
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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 http://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 http://news.gmane.org/gmane.comp.version-control.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