Re: [PATCH 00/15] [RFC] Maintenance jobs and job runner

From: Johannes Schindelin <johannes.schindelin@gmx.de>
Date: Apr 7, 2020, 22:23 UTC
Message-ID: <nycvar.QRO.7.76.6.2004072355100.46@tvgsbejvaqbjf.bet>
In-Reply-To: <20200407014829.GL6369@camp.crustytoothpaste.net>

Hi brian,

On Tue, 7 Apr 2020, brian m. carlson wrote:

Show 39 quoted lines

> On 2020-04-04 at 00:16:21, Derrick Stolee wrote:
> > On 4/3/2020 5:40 PM, Junio C Hamano wrote:
> > > "Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:
> > >
> > >>  * git run-job <job-name>: This builtin will run a single instance of a
> > >>    maintenance job.
> > >>
> > >>  * git job-runner [--repo=<path>]: This builtin will run an infinite loop
> > >>    that executes git run-job as a subcommand.
> > >
> > > What does this have to do with "git", though?  IOW, why does this
> > > have to be part of Git, so that those who would benefit from having
> > > a mechanism that makes it easy to run regular maintenance tasks but
> > > are not Git users (or those that want to do such maintenance tasks
> > > that are not necessarily tied to "git") must use "git" to do so?
> > >
> > > I'll find out later why it is so after reading thru 15 patches
> > > myself, so no need to give a quick answer to the above; it was just
> > > my knee-jerk reaction.
> >
> > That's a reasonable reaction. The short version of my reasoning is that
> > many many people _use_ Git but are not Git experts. While a Git expert
> > could find the right set of commands to run and at what frequency to
> > keep their repo clean, most users do not want to spend time learning
> > these commands. It's also worth our time as contributors to select what
> > a good set of non-intrusive maintenance tasks could be, and make them
> > easily accessible to users.
> >
> > This series gets us half of the way there: a user interested in doing
> > background maintenance could figure out how to launch "git run-job" on
> > a schedule for their platform, or to launch "git job-runner" at start-
> > up. That's a lot simpler than learning how the commit-graph,
> > multi-pack-index, prune-packed, pack-objects, and fetch builtins work
> > with the complicated sets of arguments.
>
> If there are periodic tasks that should be done, even if only on large
> repos, then let's have a git gc --periodic that does them.  I'm not sure
> that fetch should be in that set, but nothing prevents users from doing
> "git fetch origin && git gc --periodic".

Hmm. Who says that maintenance tasks are essentially only `gc`? With _maaaaaybe_ a `fetch` thrown in?

And about "nothing prevents users from doing ...": while that is true, _even less_ is preventing those same users from _forgetting_ to run them, or _not even knowing_ about the need to run them.

I vividly remember how `git gc --auto` was introduced. A Linux developer too many was complaining about their Git operations becoming slower and slower over the course of some months.

That is a very illustrative example of what we're trying to accomplish here: we are dealing with people whose job it is not to become Git experts, but whose job is to develop software, and it just so happens that they use Git as version control system. For the most part, they are not even all that interested in the internals of Git. Strange, I know.

The solution then, and now, is to teach Git to do these things for the users, without the need to know a specific command and when to run it.

I am not saying that the current state of the design is perfect. What I am saying is that the goal is very clear, and important, and we must try to get there: to teach Git to run maintenance for the user, without much in the way of the user asking for it, but Git knowing when to do what, and trying mostly to stay out of the users' way.

> Let's make it as simple and straightforward as possible.

I fear that we're running the danger of confusing _two_ meanings of "as simple and straightforward as possible" here.

From my perspective, Stolee's patches aim to make using Git as simple and straightforward as possible, and the historical record shows that Scalar (after which they are modeled) does a pretty good job at that, or is at least heading in the right direction.

I get the impression, however, that many reviewers here seem to favor the goal of making the _patches_ as simple and straightforward as possible, however, at the expense of the original goal. Like, totally sacrificing the ease of use in return for "just use a shell script" advice.

> As for handling multiple repositories, the tool to do that could be as
> simple as a shell script which reads from ~/.config/git/repo-maintenance
> (or whatever) and runs the same command on all of the repos it finds
> there, possibly with a subcommand to add and remove repos.

Sure, that is flexible.

And it requires a ton of Git expertise to know what to put into those scripts. And Git updates cannot deliver more value to those scripts.

Which taps right into the above-mentioned focus on _the patches'_ simplicity, as opposed to the original goal of making _using Git_ simpler and less painful with big repositories.

Show 12 quoted lines

> > The second half would be to create a command such as
> >
> > 	git please-run-maintenance-on-this-repo
> >
> > that initializes the background jobs and enables them on the repo they
> > are using. This allows the most casual of Git user to work efficiently
> > on very large repositories.
>
> I'm not opposed to seeing a tool that can schedule periodic maintenance
> jobs, perhaps in contrib, depending on whether other people think it
> should go.  However, I think running periodic jobs is best handled on
> Unix with cron or anacron and not a custom tool or a command in Git.

Okay, here is a challenge for you: design this such that the Windows experience does _not_ feel like a 3rd-class citizen. Go ahead. Yes, there is a scheduler. Yep, it does not do cron-like things. Precisely: you have to feed it an XML to make use of the "advanced" features. Yeah, I also cannot remember what the semantics are regarding missed jobs due to shutdown cycles. Nope, you cannot rely on the XML being an option, that would require Windows 10. The list goes on.

Do you see where I am getting at?

It's really asking the user to take on _all_ the burden of the complexity. The complexity has to live _somewhere_, after all, and if you want to prevent Git from taking care of it, there are only the users left to hold the candle.

I'd rather Git take up that responsibility, and lift that burden from the users' shoulders. It's not like we have a stellar record of usability, so we might just as well welcome Stolee's effort to improve Git in that respect.

> I've dealt with systems that implemented periodic tasks without using
> the existing tools for doing that, and I've found that usually that's a
> mistake.  Despite seeming straightforward, there are a lot of tricky
> edge cases to deal with and it's easy to get wrong.

That might be true for general-purpose periodic taks managers.

But that's not what we need in Git, do we? The scope is a _lot_ more reduced.

But maybe you found one of those issues in Stolee's patches? If so, please do contribute your experience there to point out those issues, so that they can be addressed.

> We also don't have to reimplement all the features in the system
> scheduler and can let expert users use a different tool of their choice
> instead if cron (or the Windows equivalent) is not to their liking.

Do we really want to start relying on `cron`, when the major platform used by the target audience (enterprise software engineers who deal with rather larger repositories than git.git or linux.git) quite obviously _lacks_ support for that?

That would be a decision that would not make sense to me. I mean, why go to all the lengths of making it easier and less involved to manage large repositories, and then just slam the door shut into the faces of all of those developers working on Windows, an operating system with a rather large market share?

Ciao, Dscho