From: Derrick Stolee <stolee@gmail.com>
To: Jonathan Tan <jonathantanmy@google.com>,
Derrick Stolee <dstolee@microsoft.com>
Cc: git@vger.kernel.org, sandals@crustytoothpaste.net
Subject: Re: [PATCH] sha1_name: use bsearch_hash() for abbreviations
Date: Wed, 21 Mar 2018 09:24:06 -0400 [thread overview]
Message-ID: <3527bd3a-a899-a909-5fda-1d7abeb0e158@gmail.com> (raw)
In-Reply-To: <20180320152505.bd66f0deaecf6d92fa6d62de@google.com>
On 3/20/2018 6:25 PM, Jonathan Tan wrote:
> On Tue, 20 Mar 2018 16:03:25 -0400
> Derrick Stolee <dstolee@microsoft.com> wrote:
>
>> This patch updates the abbreviation code to use bsearch_hash() as defined
>> in [1]. It gets a nice speedup since the old implementation did not use
>> the fanout table at all.
> You can refer to the patch as:
>
> b4e00f7306a1 ("packfile: refactor hash search with fanout table",
> 2018-02-15)
>
> Also, might be worth noting that this patch builds on
> jt/binsearch-with-fanout.
Thanks. That commit is in master and v2.17.0-rc0, so hopefully it isn't
difficult to apply the patch.
>
>> One caveat about the patch: there is a place where I cast a sha1 hash
>> into a struct object_id pointer. This is because the abbreviation code
>> still uses 'const unsigned char *' instead of structs. I wanted to avoid
>> a hashcpy() in these calls, but perhaps that is not too heavy a cost.
> I recall a discussion that there were alignment issues with doing this,
> but I might have be remembering wrongly - in my limited knowledge of C
> alignment, both "unsigned char *" and "struct object_id *" have the same
> constraints, but I'm not sure.
Adding Brian M. Carlson in the CC line for advice on how to do this
translation between old sha1's and new object_ids. If it isn't safe,
then we could do a hashcpy() until the translation makes it unnecessary.
I should have compared the two methods before sending the patch, but
running the "git log --oneline --parents" test with a hashcpy() versus a
cast has no measurable difference in performance for Linux. Probably
best to do the safest thing here if there is no cost to perf.
>
>> + const unsigned char *index_fanout = p->index_data;
> [snip]
>> + return bsearch_hash(oid->hash, (const uint32_t*)index_fanout,
>> + index_lookup, index_lookup_width, result);
> This cast to "const uint32_t *" is safe, because p->index_data points to
> a mmap-ed region (which has very good alignment, as far as I know). I
> wonder if we should document alignment guarantees on p->index_data, and
> if yes, what guarantees to declare.
The existing application in find_pack_entry_one() [1] does similar
pack-index arithmetic before calling. A similar amount of arithmetic and
alignment concerns appear in the commit-graph patch. Where is the right
place to declare these alignment requirements?
In v2, I'll have find_pack_entry_one() call bsearch_pack() so this logic
is not duplicated.
Thanks,
-Stolee
[1]
https://github.com/git/git/blob/c6284da4ff4afbde8211efe5d03f3604b1c6b9d6/packfile.c#L1721-L1749
find_pack_entry_one() in packfile.c
next prev parent reply other threads:[~2018-03-21 13:24 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-20 20:03 [PATCH] sha1_name: use bsearch_hash() for abbreviations Derrick Stolee
2018-03-20 22:25 ` Jonathan Tan
2018-03-21 13:24 ` Derrick Stolee [this message]
2018-03-21 22:42 ` brian m. carlson
2018-03-22 17:40 ` [PATCH v2 0/3] Use " Derrick Stolee
2018-03-22 17:40 ` [PATCH v2 1/3] sha1_name: convert struct min_abbrev_data to object_id Derrick Stolee
2018-03-22 17:40 ` [PATCH v2 2/3] packfile: define and use bsearch_pack() Derrick Stolee
2018-03-22 17:40 ` [PATCH v2 3/3] sha1_name: use bsearch_pack() for abbreviations Derrick Stolee
2018-03-24 16:41 ` [PATCH 4/3] sha1_name: use bsearch_pack() in unique_in_pack() René Scharfe
2018-03-25 16:19 ` Junio C Hamano
2018-03-25 16:32 ` René Scharfe
2018-03-25 18:21 ` Derrick Stolee
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=3527bd3a-a899-a909-5fda-1d7abeb0e158@gmail.com \
--to=stolee@gmail.com \
--cc=dstolee@microsoft.com \
--cc=git@vger.kernel.org \
--cc=jonathantanmy@google.com \
--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).