afc72b5d3a
The bottom-up mergesort implementation needs to skip through sublists a lot. A recursive version could avoid that, but would require log2(n) stack frames. Explicitly manage a stack of sorted sublists of various lengths instead to avoid fast-forwarding while also keeping a lid on memory usage. While this patch was developed independently, a ranks stack is also used in https://github.com/mono/mono/blob/master/mono/eglib/sort.frag.h by the Mono project. The idea is to keep slots for log2(n_max) sorted sublists, one for each power of 2. Such a construct can accommodate lists of any length up to n_max. Since there is a known maximum number of items (effectively SIZE_MAX), we can preallocate the whole rank stack. We add items one by one, which is akin to incrementing a binary number. Make use of that by keeping track of the number of items and check bits in it instead of checking for NULL in the rank stack when checking if a sublist of a certain rank exists, in order to avoid memory accesses. The first item can go into the empty first slot as a sublist of length 2^0. The second one needs to be merged with the previous sublist and the result goes into the empty second slot as a sublist of length 2^1. The third one goes into vacated first slot and so on. At the end we merge all the sublists to get the result. The new version still performs a stable sort by making sure to put items seen earlier first when the compare function indicates equality. That's done by preferring items from sublists with a higher rank. The new merge function also tries to minimize the number of operations. Like blame.c::blame_merge(), the function doesn't set the next pointer if it already points to the right item, and it exits when it reaches the end of one of the two sublists that it's given. The old code couldn't do the latter because it kept all items in a single list. The number of comparisons stays the same, though. Here's example output of "test-tool mergesort test" for the rand distributions with the most number of comparisons with the ranks stack: $ t/helper/test-tool mergesort test | awk ' NR > 1 && $1 != "rand" {next} $7 > max[$3] {max[$3] = $7; line[$3] = $0} END {for (n in line) print line[n]} ' distribut mode n m get_next set_next compare verdict rand copy 100 32 669 420 569 OK rand dither 1023 64 9997 5396 8974 OK rand dither 1024 512 10007 6159 8983 OK rand dither 1025 256 10993 5988 9968 OK Here are the differences to the results without this patch: distribut mode n m get_next set_next compare rand copy 100 32 -515 -280 0 rand dither 1023 64 -6376 -4834 0 rand dither 1024 512 -6377 -4081 0 rand dither 1025 256 -7461 -5287 0 The numbers of get_next and set_next calls are reduced significantly. NB: These winners are different than the ones shown in the patch that introduced the unriffle mode because the addition of the unriffle_skewed mode in between changed the consumption of rand() values. Here are the distributions with the most comparisons overall with the ranks stack: $ t/helper/test-tool mergesort test | awk ' $7 > max[$3] {max[$3] = $7; line[$3] = $0} END {for (n in line) print line[n]} ' distribut mode n m get_next set_next compare verdict sawtooth unriffle_skewed 100 128 689 632 589 OK sawtooth unriffle_skewed 1023 1024 10230 10220 9207 OK sawtooth unriffle 1024 1024 10241 10240 9217 OK sawtooth unriffle_skewed 1025 2048 11266 10242 10241 OK And here the differences to before: distribut mode n m get_next set_next compare sawtooth unriffle_skewed 100 128 -495 -68 0 sawtooth unriffle_skewed 1023 1024 -6143 -10 0 sawtooth unriffle 1024 1024 -6143 0 0 sawtooth unriffle_skewed 1025 2048 -7188 -1033 0 We get a similar reduction of get_next calls here, but only a slight reduction of set_next calls, if at all. And here are the results of p0071-sort.sh before: 0071.12: llist_mergesort() unsorted 0.36(0.33+0.01) 0071.14: llist_mergesort() sorted 0.15(0.13+0.01) 0071.16: llist_mergesort() reversed 0.16(0.14+0.01) ... and here the ones with this patch: 0071.12: llist_mergesort() unsorted 0.24(0.22+0.01) 0071.14: llist_mergesort() sorted 0.12(0.10+0.01) 0071.16: llist_mergesort() reversed 0.12(0.10+0.01) NB: We can't use t/perf/run to compare revisions in one run because it uses the test-tool from the worktree, not from the revisions being tested. Signed-off-by: René Scharfe <l.s.r@web.de> Signed-off-by: Junio C Hamano <gitster@pobox.com> |
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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-<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). 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://lore.kernel.org/git/, http://marc.info/?l=git and other archival sites.
Issues which are security relevant should be disclosed privately to the Git Security mailing list git-security@googlegroups.com.
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