Re: [PATCH 00/21] Maintenance builtin, allowing 'gc --auto' customization

From: Son Luong Ngoc <sluongng@gmail.com>
Date: Jul 13, 2020, 06:18 UTC
Message-ID: <CAL3xRKfTTr=Tkq2NY7zUwCSWGHXO2FAwEHKvV2-4AavX9Cs7ow@mail.gmail.com>

Hi Derrick,

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> This is a second attempt at redesigning Git's repository maintenance
> patterns. The first attempt [1] included a way to run jobs in the background
> using a long-lived process; that idea was rejected and is not included in
> this series. A future series will use the OS to handle scheduling tasks.
>
> [1]
> https://lore.kernel.org/git/pull.597.git.1585946894.gitgitgadget@gmail.com/
>
> As mentioned before, git gc already plays the role of maintaining Git
> repositories. It has accumulated several smaller pieces in its long history,
> including:
>
>  1. Repacking all reachable objects into one pack-file (and deleting
>     unreachable objects).
>  2. Packing refs.
>  3. Expiring reflogs.
>  4. Clearing rerere logs.
>  5. Updating the commit-graph file.

It's worth mentioning 'git worktree prune' as well.

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>
> While expiring reflogs, clearing rererelogs, and deleting unreachable
> objects are suitable under the guise of "garbage collection", packing refs
> and updating the commit-graph file are not as obviously fitting. Further,
> these operations are "all or nothing" in that they rewrite almost all
> repository data, which does not perform well at extremely large scales.
> These operations can also be disruptive to foreground Git commands when git
> gc --auto triggers during routine use.
>
> This series does not intend to change what git gc does, but instead create
> new choices for automatic maintenance activities, of which git gc remains
> the only one enabled by default.
>
> The new maintenance tasks are:
>
>  * 'commit-graph' : write and verify a single layer of an incremental
>    commit-graph.
>  * 'loose-objects' : prune packed loose objects, then create a new pack from
>    a batch of loose objects.
>  * 'pack-files' : expire redundant packs from the multi-pack-index, then
>    repack using the multi-pack-index's incremental repack strategy.
>  * 'fetch' : fetch from each remote, storing the refs in 'refs/hidden//'.

As some of the previous discussions [1] have raised, I think 'prefetch' would communicate the refs' purpose better than just 'hidden'. In-fact, I would suggest naming the task 'prefetch' instead, just to avoid potential communication issue between 'git fetch' and 'git maintenance fetch'.

[1]: https://lore.kernel.org/git/xmqqeet1y8wy.fsf@gitster.c.googlers.com/

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>
> These tasks are all disabled by default, but can be enabled with config
> options or run explicitly using "git maintenance run --task=". There are
> additional config options to allow customizing the conditions for which the
> tasks run during the '--auto' option. ('fetch' will never run with the
> '--auto' option.)
>
>  Because 'gc' is implemented as a maintenance task, the most dramatic change
> of this series is to convert the 'git gc --auto' calls into 'git maintenance
> run --auto' calls at the end of some Git commands. By default, the only
> change is that 'git gc --auto' will be run below an additional 'git
> maintenance' process.
>
> The 'git maintenance' builtin has a 'run' subcommand so it can be extended
> later with subcommands that manage background maintenance, such as 'start',
> 'stop', 'pause', or 'schedule'. These are not the subject of this series, as
> it is important to focus on the maintenance activities themselves.
>
> An expert user could set up scheduled background maintenance themselves with
> the current series. I have the following crontab data set up to run
> maintenance on an hourly basis:
>
> 0 * * * * git -C /<path-to-repo> maintenance run --no-quiet >>/<path-to-repo>/.git/maintenance.log

Perhaps the logging should be included inside the maintenance command instead of relying on the append here? Given that we have 'gc.log', I would imagine 'maintenance.log' is not too far-fetched?

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>
> My config includes all tasks except the 'gc' task. The hourly run is
> over-aggressive, but is sufficient for testing. I'll replace it with daily
> when I feel satisfied.
>
> Hopefully this direction is seen as a positive one. My goal was to add more
> options for expert users, along with the flexibility to create background
> maintenance via the OS in a later series.
>
> OUTLINE
> =======
>
> Patches 1-4 remove some references to the_repository in builtin/gc.c before
> we start depending on code in that builtin.
>
> Patches 5-7 create the 'git maintenance run' builtin and subcommand as a
> simple shim over 'git gc' and replaces calls to 'git gc --auto' from other
> commands.
>
> Patches 8-15 create new maintenance tasks. These are the same tasks sent in
> the previous RFC.
>
> Patches 16-21 create more customization through config and perform other
> polish items.
>
> FUTURE WORK
> ===========
>
>  * Add 'start', 'stop', and 'schedule' subcommands to initialize the
>    commands run in the background.
>
>
>  * Split the 'gc' builtin into smaller maintenance tasks that are enabled by
>    default, but might have different '--auto' conditions and more config
>    options.
>
>
>  * Replace config like 'gc.writeCommitGraph' and 'fetch.writeCommitGraph'
>    with use of the 'commit-graph' task.
>
>
>
> Thanks, -Stolee

Thanks, Son Luong.