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From: Derrick Stolee <stolee@gmail.com>
To: Jakub Narebski <jnareb@gmail.com>
Cc: "Mike Hommey" <mh@glandium.org>,
	"SZEDER Gábor" <szeder.dev@gmail.com>,
	git@vger.kernel.org
Subject: Re: Revision walking, commit dates, slop
Date: Mon, 20 May 2019 07:20:01 -0400	[thread overview]
Message-ID: <cfa2c367-5cd7-add5-0293-caa75b103f34@gmail.com> (raw)
In-Reply-To: <86mujhpewj.fsf@gmail.com>

On 5/20/2019 7:02 AM, Jakub Narebski wrote:
>
> Are there any blockers that prevent the switch to this
> "generation number v2"?
> 
> - Is it a problem with insufficient data to choose the correct numbering
>   as "generation number v2' (there can be only one)?
> - Is it a problem with selected "generation number v2" being
>   incompatibile with gen v2, and Git failing when new version of
>   commit-graph is used instead of softly just not using commit-graph?
> - Or is it something else?

The switch becomes a bit complicated.

First, the plan was to version the commit-graph file to v2, and that would
include a byte in the header for the "reachability index version" [1]. Since
older clients fail hard on a newer file version, we are switching to instead
including the reachability index version as a value in a more flexible 
"metadata chunk" [2]. Using the generation number column for the corrected
commit-date offsets (assuming we also guarantee the offset is strictly
increasing from parent to child), these new values will be backwards-
compatible _except_ for 'git commit-graph verify'.

Second, we need to pull the reachability index value into a commit slab.
The generation value is currently 32 bits, but we will expand that to
64 as it stores a timestamp. The commit struct is a bit bloated already,
so this will reduce the required memory space even when not using the
commit-graph. But, it requires some refactoring, including every place
where we pass a "min_generation" needs to change type and name.

Third and finally, we need to calculate the new values and change the
read logic to sum the offset and commit-date (when the metadata chunk
says we are using corrected commit date).

While none of this is _incredibly_ hard to do, it does require a bit
of care. It's on my list to get to at some point, but making the file
incremental is higher priority to me.

Thanks,
-Stolee

[1] https://public-inbox.org/git/pull.112.git.gitgitgadget@gmail.com/
[2] https://public-inbox.org/git/87h8acivkh.fsf@evledraar.gmail.com/

  reply	other threads:[~2019-05-20 11:20 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-18  0:54 Mike Hommey
2019-05-18  1:50 ` SZEDER Gábor
2019-05-18  3:58   ` Mike Hommey
2019-05-18  4:17     ` Mike Hommey
2019-05-18 12:01       ` SZEDER Gábor
2019-05-19 22:28         ` Jakub Narebski
2019-05-20  1:33       ` Derrick Stolee
2019-05-20 11:02         ` Jakub Narebski
2019-05-20 11:20           ` Derrick Stolee [this message]
2019-05-20 13:42             ` Jakub Narebski
2019-05-20 23:27               ` Jakub Narebski
2019-05-21  1:20                 ` Derrick Stolee
2019-05-22 18:29                   ` Jakub Narebski
2019-05-22 19:06                     ` Derrick Stolee
2019-05-23 21:04                       ` Jakub Narebski
2019-06-25  7:51               ` Jakub Narebski
2019-06-25 10:54                 ` Derrick Stolee
2019-09-18  8:43                   ` [RFC/PATCH] commit-graph: generation v5 (backward compatible date ceiling) Jakub Narebski
2019-09-18 12:26                     ` Derrick Stolee
2019-05-21 13:14         ` [PATCH] revision: use generation for A..B --topo-order queries Derrick Stolee
2019-05-21 13:59           ` [PATCH 2/2] revision: keep topo-walk free of unintersting commits Derrick Stolee
2019-05-22  2:19             ` Mike Hommey
2019-05-21  2:00   ` Revision walking, commit dates, slop Jonathan Nieder

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