Re: [PATCH 00/11] [RFC] Create 'core.size=large' setting to update config defaults

[Junio C Hamano <gitster@pobox.com>]

"Derrick Stolee via GitGitGadget" <gitgitgadget@gmail.com> writes:

> This patch series includes a few new config options we created to speed up
> certain critical commands in VFS for Git. On their own, they would
> contribute little value as it is hard to discover new config variables.
> Instead, I've created this RFC as a goal for probably three sequential patch
> series:
>
>  1. (Patches 1-3) Introduce a new 'core.size' config setting that takes
>     'large' as a value. This enables several config values that are
>     beneficial for large repos. We use a certain set in VFS for Git (see
>     [1]), and most of those are applicable to any repo. This 'core.size'
>     setting is intended for users to automatically receive performance
>     updates as soon as they are stable, but they must opt-in to the setting
>     and can always explicitly set their own config values. The settings to
>     include here are core.commitGraph=true, gc.writeCommitGraph=true,
>     index.version=4, pack.useSparse=true.

... and not the configuration introduced by the other two points in
this list?

"If you set this, these other configuration variables are set to
these default values" is a very valuable usability feature.  It
looks a lot more "meta" or "macro", and certainly is not a good idea
to call it as if it sits next to variables in any existing hierarchy.

I also wonder if this is something we would want to support in
general; random things that come to mind are:

 - should such a "macro" configuration be limited to boolean
   (e.g. the above core.size that takes 'large' is a boolean between
   'large' and 'not large'), or can it be an enum (e.g. choose among
   'large', 'medium' and 'small', and core.bigFileThreshold will be
   set to 1G, 512M and 128M respectively---this silly example is for
   illustration purposes only), and if so, can we express what these
   default values are for each choice without writing a lot of code?

 - if we were to have more than just this 'core.size' macro, can two
   otherwise orthogonal macros both control the same underlying
   variable, and if so, how do we express their interactions?
   "using these two at the same time is forbidden" is a perfectly
   acceptable answer for the first round until we figure out the
   desired semantics, of course.

 - perhaps we may eventually want to allow end users (via their
   ~/.gitconfig) and system administrators (via /etc/gitconfig)
   define such a macro setting (e.g. setting macro.largeRepoSetting
   sets pack.usebitmaps=true, pack.useSpars=true, etc.) *after* we
   figure out what we want to do to the other points in this list.

 - even if we do not allow end users and system administrators futz
   with custom macros, can we specify the macros we ship without
   casting them in code?