From: Jonathan Nieder <jrnieder@gmail.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: Brandon Williams <bmwill@google.com>,
git@vger.kernel.org, sbeller@google.com
Subject: Re: [PATCH 2/3] setup: have the_repository use the_index
Date: Wed, 12 Jul 2017 14:33:39 -0700 [thread overview]
Message-ID: <20170712213339.GI93855@aiede.mtv.corp.google.com> (raw)
In-Reply-To: <xmqqmv891jct.fsf@gitster.mtv.corp.google.com>
Hi,
Junio C Hamano wrote:
> Brandon Williams <bmwill@google.com> writes:
>> Since it is a pointer then using a '#define' to replace 'the_index'
>> (which is not a pointer) would be a little more challenging.
>
> The above is merely realizing another downside that stems from the
> earlier design decision that the index field is not a real embedded
> structure, but is a pointer. It does not explain why it is better
> to have a pointer to an allocated structure in the first place.
>
> I am not (yet) telling you to fix the design to have a pointer
> "index" by replacing it with an embedded structure. I may actually
> do so later, but I am first trying to find out if it is a right
> design decision with some advantage.
Consider a command that doesn't need to access the index at all (e.g.,
"git grep --recurse-submodules -e foo HEAD").
In favor of using an embedding instead of a pointer, there is the
advantage that it makes initialization simpler. (It also involves a
tiny speedup by avoiding a pointer indirection on access, but that's
more negligible.) For that reason it was a good choice when there was
only one repository in memory: using such a small bounded portion of
.bss space in exchange for some convenience is a good trade.
When a process has multiple repositories in memory (for example one
per thread), the trade-off becomes different. Instead of .bss, the
unused embedded index is on the stack or heap. Using embedding would
mean that instead of an unused extra word in the per-repository
structure we get an unused ~24 words.
An argument could be made that we wouldn't want to waste either 1 word
or 24 words per in-memory repository object --- we'd want to waste 0
words and separately keep a map from repositories to index_state that
only gets populated when needed. That complicates index access a bit
too much for my taste. 1 word instead of 0 or 24 seems like a
sensible compromise.
All that said, I don't have a strong opinion on this. Both the 1-word
approach (a pointer) and 24-word approach (embedding) are tolerable
and there are reasons to prefer each.
Thanks,
Jonathan
next prev parent reply other threads:[~2017-07-12 21:33 UTC|newest]
Thread overview: 68+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-07-11 22:04 [PATCH 0/3] Convert grep to recurse in-process Brandon Williams
2017-07-11 22:04 ` [PATCH 1/3] repo_read_index: don't discard the index Brandon Williams
2017-07-11 23:51 ` Jonathan Nieder
2017-07-12 17:27 ` Brandon Williams
2017-07-11 23:58 ` Stefan Beller
2017-07-12 17:23 ` Brandon Williams
2017-07-11 22:04 ` [PATCH 2/3] setup: have the_repository use the_index Brandon Williams
2017-07-12 0:00 ` Jonathan Nieder
2017-07-12 0:07 ` Stefan Beller
2017-07-12 17:30 ` Brandon Williams
2017-07-12 0:11 ` Junio C Hamano
2017-07-12 18:01 ` Brandon Williams
2017-07-12 20:38 ` Junio C Hamano
2017-07-12 21:33 ` Jonathan Nieder [this message]
2017-07-12 21:40 ` Junio C Hamano
2017-07-18 21:34 ` Junio C Hamano
2017-07-11 22:04 ` [PATCH 3/3] grep: recurse in-process using 'struct repository' Brandon Williams
2017-07-11 22:44 ` Jacob Keller
2017-07-12 18:54 ` Brandon Williams
2017-07-12 0:04 ` Stefan Beller
2017-07-12 18:56 ` Brandon Williams
2017-07-12 0:25 ` Jonathan Nieder
2017-07-12 18:49 ` Brandon Williams
2017-07-12 7:42 ` [PATCH 0/3] Convert grep to recurse in-process Jeff King
2017-07-12 18:06 ` Brandon Williams
2017-07-12 18:17 ` Jeff King
2017-07-12 18:24 ` Jonathan Nieder
2017-07-12 18:33 ` Jeff King
2017-07-12 18:09 ` Jonathan Nieder
2017-07-12 18:17 ` Stefan Beller
2017-07-12 18:27 ` Jeff King
2017-07-14 22:28 ` [PATCH v2 " Brandon Williams
2017-07-14 22:28 ` [PATCH v2 1/3] repo_read_index: don't discard the index Brandon Williams
2017-07-14 22:28 ` [PATCH v2 2/3] repository: have the_repository use the_index Brandon Williams
2017-07-14 22:28 ` [PATCH v2 3/3] grep: recurse in-process using 'struct repository' Brandon Williams
2017-07-18 19:05 ` [PATCH v3 00/10] Convert grep to recurse in-process Brandon Williams
2017-07-18 19:05 ` [PATCH v3 01/10] repo_read_index: don't discard the index Brandon Williams
2017-07-18 19:05 ` [PATCH v3 02/10] repository: have the_repository use the_index Brandon Williams
2017-07-18 19:05 ` [PATCH v3 03/10] cache.h: add GITMODULES_FILE macro Brandon Williams
2017-07-31 23:11 ` [PATCH] convert any hard coded .gitmodules file string to the MACRO Stefan Beller
2017-08-01 13:14 ` Jeff Hostetler
2017-08-01 17:35 ` Stefan Beller
2017-08-01 20:26 ` Junio C Hamano
2017-08-02 17:26 ` Brandon Williams
2017-08-02 17:46 ` Brandon Williams
2017-07-18 19:05 ` [PATCH v3 04/10] config: add config_from_gitmodules Brandon Williams
2017-07-18 19:05 ` [PATCH v3 05/10] submodule: remove submodule.fetchjobs from submodule-config parsing Brandon Williams
2017-07-18 19:05 ` [PATCH v3 06/10] submodule: remove fetch.recursesubmodules " Brandon Williams
2017-07-18 19:05 ` [PATCH v3 07/10] submodule: check for unstaged .gitmodules outside of config parsing Brandon Williams
2017-07-31 23:41 ` Stefan Beller
2017-08-02 17:41 ` Brandon Williams
2017-08-02 18:00 ` Brandon Williams
2017-07-18 19:05 ` [PATCH v3 08/10] submodule: check for unmerged " Brandon Williams
2017-07-18 19:05 ` [PATCH v3 09/10] submodule: merge repo_read_gitmodules and gitmodules_config Brandon Williams
2017-07-18 19:05 ` [PATCH v3 10/10] grep: recurse in-process using 'struct repository' Brandon Williams
2017-07-18 19:36 ` [PATCH v3 00/10] Convert grep to recurse in-process Junio C Hamano
2017-07-18 20:06 ` Brandon Williams
2017-08-02 19:49 ` [PATCH v4 " Brandon Williams
2017-08-02 19:49 ` [PATCH v4 01/10] repo_read_index: don't discard the index Brandon Williams
2017-08-02 19:49 ` [PATCH v4 02/10] repository: have the_repository use the_index Brandon Williams
2017-08-02 19:49 ` [PATCH v4 03/10] cache.h: add GITMODULES_FILE macro Brandon Williams
2017-08-02 19:49 ` [PATCH v4 04/10] config: add config_from_gitmodules Brandon Williams
2017-08-02 19:49 ` [PATCH v4 05/10] submodule: remove submodule.fetchjobs from submodule-config parsing Brandon Williams
2017-08-02 19:49 ` [PATCH v4 06/10] submodule: remove fetch.recursesubmodules " Brandon Williams
2017-08-02 19:49 ` [PATCH v4 07/10] submodule: check for unstaged .gitmodules outside of config parsing Brandon Williams
2017-08-02 19:49 ` [PATCH v4 08/10] submodule: check for unmerged " Brandon Williams
2017-08-02 19:49 ` [PATCH v4 09/10] submodule: merge repo_read_gitmodules and gitmodules_config Brandon Williams
2017-08-02 19:49 ` [PATCH v4 10/10] grep: recurse in-process using 'struct repository' Brandon Williams
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