From: Duy Nguyen <pclouds@gmail.com>
To: "brian m. carlson" <sandals@crustytoothpaste.net>,
Git Mailing List <git@vger.kernel.org>
Subject: Re: State of NewHash work, future directions, and discussion
Date: Mon, 11 Jun 2018 20:09:47 +0200 [thread overview]
Message-ID: <CACsJy8CJrFCUnVMes=3_gQKNTiyHsKkawWNQ1aB_GCvOh1rKcw@mail.gmail.com> (raw)
In-Reply-To: <20180609205628.GB38834@genre.crustytoothpaste.net>
On Sat, Jun 9, 2018 at 10:57 PM brian m. carlson
<sandals@crustytoothpaste.net> wrote:
>
> Since there's been a lot of questions recently about the state of the
> NewHash work, I thought I'd send out a summary.
>
> == Status
>
> I have patches to make the entire codebase work, including passing all
> tests, when Git is converted to use a 256-bit hash algorithm.
> Obviously, such a Git is incompatible with the current version, but it
> means that we've fixed essentially all of the hard-coded 20 and 40
> constants (and therefore Git doesn't segfault).
This is so cool!
> == Future Design
>
> The work I've done necessarily involves porting everything to use
> the_hash_algo. Essentially, when the piece I'm currently working on is
> complete, we'll have a transition stage 4 implementation (all NewHash).
> Stage 2 and 3 will be implemented next.
>
> My vision of how data is stored is that the .git directory is, except
> for pack indices and the loose object lookup table, entirely in one
> format. It will be all SHA-1 or all NewHash. This algorithm will be
> stored in the_hash_algo.
>
> I plan on introducing an array of hash algorithms into struct repository
> (and wrapper macros) which stores, in order, the output hash, and if
> used, the additional input hash.
I'm actually thinking that putting the_hash_algo inside struct
repository is a mistake. We have code that's supposed to work without
a repo and it shows this does not really make sense to forcefully use
a partially-valid repo. Keeping the_hash_algo a separate variable
sounds more elegant.
> If people are interested, I've done some analysis on availability of
> implementations, performance, and other attributes described in the
> transition plan and can send that to the list.
I quickly skimmed through that document. I have two more concerns that
are less about any specific hash algorithm:
- how does larger hash size affects git (I guess you covered cpu
aspect, but what about cache-friendliness, disk usage, memory
consumption)
- how does all the function redirection (from abstracting away SHA-1)
affects git performance. E.g. hashcmp could be optimized and inlined
by the compiler. Now it still probably can optimize the memcmp(,,20),
but we stack another indirect function call on top. I guess I might be
just paranoid and this is not a big deal after all.
--
Duy
next prev parent reply other threads:[~2018-06-11 18:10 UTC|newest]
Thread overview: 66+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-06-09 20:56 State of NewHash work, future directions, and discussion brian m. carlson
2018-06-09 21:26 ` Ævar Arnfjörð Bjarmason
2018-06-09 22:49 ` Hash algorithm analysis brian m. carlson
2018-06-11 19:29 ` Jonathan Nieder
2018-06-11 20:20 ` Linus Torvalds
2018-06-11 23:27 ` Ævar Arnfjörð Bjarmason
2018-06-12 0:11 ` David Lang
2018-06-12 0:45 ` Linus Torvalds
2018-06-11 22:35 ` brian m. carlson
2018-06-12 16:21 ` Gilles Van Assche
2018-06-13 23:58 ` brian m. carlson
2018-06-15 10:33 ` Gilles Van Assche
2018-07-20 21:52 ` brian m. carlson
2018-07-21 0:31 ` Jonathan Nieder
2018-07-21 19:52 ` Ævar Arnfjörð Bjarmason
2018-07-21 20:25 ` brian m. carlson
2018-07-21 22:38 ` Johannes Schindelin
2018-07-21 23:09 ` Linus Torvalds
2018-07-21 23:59 ` brian m. carlson
2018-07-22 9:34 ` Eric Deplagne
2018-07-22 14:21 ` brian m. carlson
2018-07-22 14:55 ` Eric Deplagne
2018-07-26 10:05 ` Johannes Schindelin
2018-07-22 15:23 ` Joan Daemen
2018-07-22 18:54 ` Adam Langley
2018-07-26 10:31 ` Johannes Schindelin
2018-07-23 12:40 ` demerphq
2018-07-23 12:48 ` Sitaram Chamarty
2018-07-23 12:55 ` demerphq
2018-07-23 18:23 ` Linus Torvalds
2018-07-23 17:57 ` Stefan Beller
2018-07-23 18:35 ` Jonathan Nieder
2018-07-24 19:01 ` Edward Thomson
2018-07-24 20:31 ` Linus Torvalds
2018-07-24 20:49 ` Jonathan Nieder
2018-07-24 21:13 ` Junio C Hamano
2018-07-24 22:10 ` brian m. carlson
2018-07-30 9:06 ` Johannes Schindelin
2018-07-30 20:01 ` Dan Shumow
2018-08-03 2:57 ` Jonathan Nieder
2018-09-18 15:18 ` Joan Daemen
2018-09-18 15:32 ` Jonathan Nieder
2018-09-18 16:50 ` Linus Torvalds
2018-07-25 8:30 ` [PATCH 0/2] document that NewHash is now SHA-256 Ævar Arnfjörð Bjarmason
2018-07-25 8:30 ` [PATCH 1/2] doc hash-function-transition: note the lack of a changelog Ævar Arnfjörð Bjarmason
2018-07-25 8:30 ` [PATCH 2/2] doc hash-function-transition: pick SHA-256 as NewHash Ævar Arnfjörð Bjarmason
2018-07-25 16:45 ` Junio C Hamano
2018-07-25 17:25 ` Jonathan Nieder
2018-07-25 21:32 ` Junio C Hamano
2018-07-26 13:41 ` [PATCH v2 " Ævar Arnfjörð Bjarmason
2018-08-03 7:20 ` Jonathan Nieder
2018-08-03 16:40 ` Junio C Hamano
2018-08-03 17:01 ` Linus Torvalds
2018-08-03 16:42 ` Linus Torvalds
2018-08-03 17:43 ` Ævar Arnfjörð Bjarmason
2018-08-04 8:52 ` Jonathan Nieder
2018-08-03 17:45 ` brian m. carlson
2018-07-25 22:56 ` [PATCH " brian m. carlson
2018-06-11 21:19 ` Hash algorithm analysis Ævar Arnfjörð Bjarmason
2018-06-21 8:20 ` Johannes Schindelin
2018-06-21 22:39 ` brian m. carlson
2018-06-11 18:09 ` Duy Nguyen [this message]
2018-06-12 1:28 ` State of NewHash work, future directions, and discussion brian m. carlson
2018-06-11 19:01 ` Jonathan Nieder
2018-06-12 2:28 ` brian m. carlson
2018-06-12 2:42 ` Jonathan Nieder
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
List information: http://vger.kernel.org/majordomo-info.html
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CACsJy8CJrFCUnVMes=3_gQKNTiyHsKkawWNQ1aB_GCvOh1rKcw@mail.gmail.com' \
--to=pclouds@gmail.com \
--cc=git@vger.kernel.org \
--cc=sandals@crustytoothpaste.net \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this public inbox
https://80x24.org/mirrors/git.git
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).