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From: Derrick Stolee <derrickstolee@github.com>
To: Elijah Newren <newren@gmail.com>,
	Derrick Stolee via GitGitGadget <gitgitgadget@gmail.com>
Cc: git@vger.kernel.org, jrnieder@gmail.com
Subject: Re: [PATCH 02/30] read-cache: add index.computeHash config option
Date: Mon, 14 Nov 2022 11:30:41 -0500	[thread overview]
Message-ID: <ca39e1d3-b08c-6ed3-8ab5-238efbd1f2dc@github.com> (raw)
In-Reply-To: <CABPp-BEvF3XF+udzTkEtgrtXqYuYEeYi0R65EY5gCespwZgOeg@mail.gmail.com>

On 11/11/2022 6:31 PM, Elijah Newren wrote:
> On Mon, Nov 7, 2022 at 10:48 AM Derrick Stolee via GitGitGadget
> <gitgitgadget@gmail.com> wrote:
>>
>> From: Derrick Stolee <derrickstolee@github.com>
>>
>> The previous change allowed skipping the hashing portion of the
>> hashwrite API, using it instead as a buffered write API. Disabling the
>> hashwrite can be particularly helpful when the write operation is in a
>> critical path.
>>
>> One such critical path is the writing of the index. This operation is so
>> critical that the sparse index was created specifically to reduce the
>> size of the index to make these writes (and reads) faster.
>>
>> Following a similar approach to one used in the microsoft/git fork [1],
>> add a new config option that allows disabling this hashing during the
>> index write. The cost is that we can no longer validate the contents for
>> corruption-at-rest using the trailing hash.
>>
>> [1] https://github.com/microsoft/git/commit/21fed2d91410f45d85279467f21d717a2db45201
>>
>> While older Git versions will not recognize the null hash as a special
>> case, the file format itself is still being met in terms of its
>> structure. Using this null hash will still allow Git operations to
>> function across older versions.
>>
>> The one exception is 'git fsck' which checks the hash of the index file.
>> Here, we disable this check if the trailing hash is all zeroes. We add a
>> warning to the config option that this may cause undesirable behavior
>> with older Git versions.
>>
>> As a quick comparison, I tested 'git update-index --force-write' with
>> and without index.computHash=false on a copy of the Linux kernel
>> repository.
>>
>> Benchmark 1: with hash
>>   Time (mean ± σ):      46.3 ms ±  13.8 ms    [User: 34.3 ms, System: 11.9 ms]
>>   Range (min … max):    34.3 ms …  79.1 ms    82 runs
>>
>> Benchmark 2: without hash
>>   Time (mean ± σ):      26.0 ms ±   7.9 ms    [User: 11.8 ms, System: 14.2 ms]
>>   Range (min … max):    16.3 ms …  42.0 ms    69 runs
>>
>> Summary
>>   'without hash' ran
>>     1.78 ± 0.76 times faster than 'with hash'
>>
>> These performance benefits are substantial enough to allow users the
>> ability to opt-in to this feature, even with the potential confusion
>> with older 'git fsck' versions.
> 
> This is impressive and interesting...but an improvement unrelated to
> this series other than the fact that it builds on some of it.  Perhaps
> pull this patch out?

While patch 1 is required for the packed-refs work, this one is an easy
way to take advantage of it. I'll submit these two patches soon on their
own as the rest of the RFC is discussed.

> Also, would it make sense to integrate index.computeHash with feature.manyFiles?

It would make sense to include in feature.manyFiles and Scalar's recommended
config. I expect that it would be good to have the config available in a Git
release before updating those configs to include it. Perhaps that is too
conservative, though.

Thanks,
-Stolee

  reply	other threads:[~2022-11-14 16:35 UTC|newest]

Thread overview: 56+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-11-07 18:35 [PATCH 00/30] [RFC] extensions.refFormat and packed-refs v2 file format Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 01/30] hashfile: allow skipping the hash function Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 02/30] read-cache: add index.computeHash config option Derrick Stolee via GitGitGadget
2022-11-11 23:31   ` Elijah Newren
2022-11-14 16:30     ` Derrick Stolee [this message]
2022-11-17 16:13   ` Ævar Arnfjörð Bjarmason
2022-11-07 18:35 ` [PATCH 03/30] extensions: add refFormat extension Derrick Stolee via GitGitGadget
2022-11-11 23:39   ` Elijah Newren
2022-11-16 14:37     ` Derrick Stolee
2022-11-07 18:35 ` [PATCH 04/30] config: fix multi-level bulleted list Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 05/30] repository: wire ref extensions to ref backends Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 06/30] refs: allow loose files without packed-refs Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 07/30] chunk-format: number of chunks is optional Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 08/30] chunk-format: document trailing table of contents Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 09/30] chunk-format: store chunk offset during write Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 10/30] chunk-format: allow trailing table of contents Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 11/30] chunk-format: parse " Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 12/30] refs: extract packfile format to new file Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 13/30] packed-backend: extract add_write_error() Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 14/30] packed-backend: extract iterator/updates merge Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 15/30] packed-backend: create abstraction for writing refs Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 16/30] config: add config values for packed-refs v2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 17/30] packed-backend: create shell of v2 writes Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 18/30] packed-refs: write file format version 2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 19/30] packed-refs: read file format v2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 20/30] packed-refs: read optional prefix chunks Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 21/30] packed-refs: write " Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 22/30] packed-backend: create GIT_TEST_PACKED_REFS_VERSION Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 23/30] t1409: test with packed-refs v2 Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 24/30] t5312: allow packed-refs v2 format Derrick Stolee via GitGitGadget
2022-11-07 18:35 ` [PATCH 25/30] t5502: add PACKED_REFS_V1 prerequisite Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 26/30] t3210: require packed-refs v1 for some tests Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 27/30] t*: skip packed-refs v2 over http tests Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 28/30] ci: run GIT_TEST_PACKED_REFS_VERSION=2 in some builds Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 29/30] p1401: create performance test for ref operations Derrick Stolee via GitGitGadget
2022-11-07 18:36 ` [PATCH 30/30] refs: skip hashing when writing packed-refs v2 Derrick Stolee via GitGitGadget
2022-11-09 15:15 ` [PATCH 00/30] [RFC] extensions.refFormat and packed-refs v2 file format Derrick Stolee
2022-11-11 23:28 ` Elijah Newren
2022-11-14  0:07   ` Derrick Stolee
2022-11-15  2:47     ` Elijah Newren
2022-11-16 14:45       ` Derrick Stolee
2022-11-17  4:28         ` Elijah Newren
2022-11-18 23:31     ` Junio C Hamano
2022-11-19  0:41       ` Elijah Newren
2022-11-19  3:00         ` Taylor Blau
2022-11-30 15:31       ` Derrick Stolee
2022-11-28 18:56 ` Han-Wen Nienhuys
2022-11-30 15:16   ` Derrick Stolee
2022-11-30 15:38     ` Phillip Wood
2022-11-30 16:37     ` Taylor Blau
2022-11-30 18:30     ` Han-Wen Nienhuys
2022-11-30 18:37       ` Sean Allred
2022-12-01 20:18       ` Derrick Stolee
2022-12-02 16:46         ` Han-Wen Nienhuys
2022-12-02 18:24           ` Ævar Arnfjörð Bjarmason
2022-11-30 22:55     ` Junio C Hamano

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