Re: Getting rid of the mandb profile hook?

[Maxim Cournoyer <maxim.cournoyer@gmail.com>]

Hi Ludovic,

Ludovic Courtès <ludo@gnu.org> writes:

> Hi Guix!
>
> I was inspired by Michael Stapelberg’s talk recently shared on IRC¹
> (well worth watching!).  One of the takeaways for me is that many
> actions should be done lazily, in particular populating caches.
>
> ‘guix install’ & co. spend a significant time populating such caches, in
> particular the XDG caches² and the manual page database (mandb).
>
> I’m thinking we could get rid of the mandb hook.  However, the
> functionality matters IMO (we need good tools so users can browse local
> documentation; mandb is not that good but better than no search
> mechanism.)  Here are several options that come to mind:
>
>   1. Provide a ‘man’ wrapper or modify the ‘man-db’ package such that
>      the database gets built on the first use of ‘man -k’, unless it’s
>      already up-to-date.

That would mean the database would live in some user-specific writable
area of the file system correct (where?), right?  And could use the
common 'update' mechanism of man-db to make it as fast as possible.

This sounds good from a performance perpective, but could introduce
cache issues every now and then (if man-db changes a lot).  I wouldn't
expect much problem given how mature man-db is, but that's one thing to
consider.

>   2. Add a phase in gnu-build-system.scm that creates a per-package
>      database.  Change the mandb profile hook such that all it needs to
>      do is “concatenate” all these GDBM databases (which should be much
>      faster than browsing all the man pages as it currently does).

I like that idea better, but I don't know how feasible it would be.

> There are crazier option that came to mind but let’s ignore them for
> now.

What is taking so much time anyway?  Why is generating this database so
compute intensive?  I don't grok why it should be so inefficient to scan
a union'd tree for expected prefixes and append a bunch of file names
together.

> Thoughts?  :-)

Lazily doing things seems a good idea in general to make the experience
more snappy.  Thanks for looking into it!

Maxim