1b9e9be8b4
Update hashwrite() and friends to use the unsafe_-variants of hashing
functions, calling for e.g., "the_hash_algo->unsafe_update_fn()" instead
of "the_hash_algo->update_fn()".
These callers only use the_hash_algo to produce a checksum, which we
depend on for data integrity, but not for cryptographic purposes, so
these callers are safe to use the unsafe (non-collision detecting) SHA-1
implementation.
To time this, I took a freshly packed copy of linux.git, and ran the
following with and without the OPENSSL_SHA1_UNSAFE=1 build-knob. Both
versions were compiled with -O3:
$ git for-each-ref --format='%(objectname)' refs/heads refs/tags >in
$ valgrind --tool=callgrind ~/src/git/git-pack-objects \
--revs --stdout --all-progress --use-bitmap-index <in >/dev/null
Without OPENSSL_SHA1_UNSAFE=1 (that is, using the collision-detecting
SHA-1 implementation for both cryptographic and non-cryptographic
purposes), we spend a significant amount of our instruction count in
hashwrite():
$ callgrind_annotate --inclusive=yes | grep hashwrite | head -n1
159,998,868,413 (79.42%) /home/ttaylorr/src/git/csum-file.c:hashwrite [/home/ttaylorr/src/git/git-pack-objects]
, and the resulting "clone" takes 19.219 seconds of wall clock time,
18.94 seconds of user time and 0.28 seconds of system time.
Compiling with OPENSSL_SHA1_UNSAFE=1, we spend ~60% fewer instructions
in hashwrite():
$ callgrind_annotate --inclusive=yes | grep hashwrite | head -n1
59,164,001,176 (58.79%) /home/ttaylorr/src/git/csum-file.c:hashwrite [/home/ttaylorr/src/git/git-pack-objects]
, and generate the resulting "clone" much faster, in only 11.597 seconds
of wall time, 11.37 seconds of user time, and 0.23 seconds of system
time, for a ~40% speed-up.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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.
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See Documentation/gittutorial.txt to get started, then see
Documentation/giteveryday.txt for a useful minimum set of commands, and
Documentation/git-<commandname>.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 and Documentation/CodingGuidelines).
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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