From: Jeff King <peff@peff.net>
To: Derrick Stolee <dstolee@microsoft.com>
Cc: git@vger.kernel.org, stolee@gmail.com, git@jeffhostetler.com,
sbeller@google.com, gitster@pobox.com
Subject: Re: [PATCH v3 5/5] sha1_name: Minimize OID comparisons during disambiguation
Date: Thu, 5 Oct 2017 05:44:18 -0400 [thread overview]
Message-ID: <20171005094418.irm6omly67bgyvo7@sigill.intra.peff.net> (raw)
In-Reply-To: <20171002145651.204984-6-dstolee@microsoft.com>
On Mon, Oct 02, 2017 at 10:56:51AM -0400, Derrick Stolee wrote:
> +static void find_abbrev_len_for_pack(struct packed_git *p,
> + struct min_abbrev_data *mad)
> +{
> + int match = 0;
> + uint32_t num, last, first = 0;
> + struct object_id oid;
> +
> + open_pack_index(p);
> + num = p->num_objects;
> + last = num;
> + while (first < last) {
> + uint32_t mid = (first + last) / 2;
This can overflow if we are looking past 2^31. Probably not very common
(things seem to get pretty painful at hundreds of millions of objects).
But it can be written as:
uint32_t mid = lo + (hi - lo) / 2;
Sadly, this overflow case seems to be present in many of our binary
searches.
> + const unsigned char *current;
> + int cmp;
> +
> + current = nth_packed_object_sha1(p, mid);
nth_packed_object_sha1() has to do some minor computation to come up
with the correct pointer. The normal packfile lookup precomputes the
initial offset and stride outside the loop. I have no idea if that
micro-optimization is actually noticeable, but I thought I'd mention it
as a possible avenue for investigation since you're obviously interested
in squeezing out performance. ;)
-Peff
next prev parent reply other threads:[~2017-10-05 9:44 UTC|newest]
Thread overview: 47+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-09-25 9:54 [PATCH v2 0/5] Improve abbreviation disambiguation Derrick Stolee
2017-09-25 9:54 ` [PATCH v2 1/5] test-list-objects: List a subset of object ids Derrick Stolee
2017-09-26 9:24 ` Junio C Hamano
2017-10-05 8:42 ` Jeff King
2017-10-05 9:48 ` Junio C Hamano
2017-10-05 10:00 ` Jeff King
2017-10-05 10:16 ` Junio C Hamano
2017-10-05 12:39 ` Derrick Stolee
2017-10-06 14:11 ` Jeff King
2017-10-07 19:12 ` Derrick Stolee
2017-10-07 19:33 ` Jeff King
2017-10-08 1:46 ` Junio C Hamano
2017-09-25 9:54 ` [PATCH v2 2/5] p0008-abbrev.sh: Test find_unique_abbrev() perf Derrick Stolee
2017-09-26 9:27 ` Junio C Hamano
2017-10-05 8:55 ` Jeff King
2017-10-05 8:57 ` Jeff King
2017-09-25 9:54 ` [PATCH v2 3/5] sha1_name: Unroll len loop in find_unique_abbrev_r Derrick Stolee
2017-09-25 9:54 ` [PATCH v2 4/5] sha1_name: Parse less while finding common prefix Derrick Stolee
2017-09-25 23:42 ` Stefan Beller
2017-10-02 14:52 ` Derrick Stolee
2017-09-25 9:54 ` [PATCH v2 5/5] sha1_name: Minimize OID comparisons during disambiguation Derrick Stolee
2017-10-02 14:56 ` [PATCH v3 0/5] Improve abbreviation disambituation Derrick Stolee
2017-10-05 9:49 ` Jeff King
2017-10-02 14:56 ` [PATCH v3 1/5] test-list-objects: List a subset of object ids Derrick Stolee
2017-10-03 4:16 ` Junio C Hamano
2017-10-02 14:56 ` [PATCH v3 2/5] p0008-abbrev.sh: Test find_unique_abbrev() perf Derrick Stolee
2017-10-02 14:56 ` [PATCH v3 3/5] sha1_name: Unroll len loop in find_unique_abbrev_r Derrick Stolee
2017-10-03 10:49 ` Junio C Hamano
2017-10-03 11:26 ` Derrick Stolee
2017-10-04 6:10 ` Junio C Hamano
2017-10-04 13:06 ` Derrick Stolee
2017-10-04 6:07 ` Junio C Hamano
2017-10-04 13:19 ` Derrick Stolee
2017-10-05 1:26 ` Junio C Hamano
2017-10-05 9:13 ` Jeff King
2017-10-05 9:50 ` Junio C Hamano
2017-10-02 14:56 ` [PATCH v3 4/5] sha1_name: Parse less while finding common prefix Derrick Stolee
2017-10-04 6:14 ` Junio C Hamano
2017-10-02 14:56 ` [PATCH v3 5/5] sha1_name: Minimize OID comparisons during disambiguation Derrick Stolee
2017-10-03 15:55 ` Stefan Beller
2017-10-03 17:05 ` Derrick Stolee
2017-10-05 9:44 ` Jeff King [this message]
2017-10-06 13:52 ` [PATCH] cleanup: fix possible overflow errors in binary search Derrick Stolee
2017-10-06 14:18 ` Jeff King
2017-10-06 14:41 ` Derrick Stolee
2017-10-08 18:29 ` [PATCH v2] " Derrick Stolee
2017-10-09 13:33 ` Jeff King
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