Optimizing union.scm

[Mark H Weaver <mhw@netris.org>]

ludo@gnu.org (Ludovic Courtès) writes:

> Mark H Weaver <mhw@netris.org> skribis:
>
>> I want to optimize it anyway, since it takes over 5 minutes to build
>> my profile, which is a bit painful.
>
> Oh, this much?
>
> I have 140 packages in my profile and it takes less than 30s to build
> it; that’s an SSD though, so that probably makes a big difference.

My profile has 155 packages.  I've looked over union.scm and can see
some extreme wastefulness, most notably in the use of 'others-have-it?'.
I guess this typically does N 'lstat' calls for every file or directory
in every package in the resulting profile that cannot be pruned (due to
being in a directory that's only in one package), where N is the number
of packages in the profile.

I'm fairly sure it is possible to replace those N 'lstat' calls with
something that requires 0 system calls and at most O(log N) time.  The
basic idea would be to iterate over all packages in a breadth-first
manner, as follows:

I think we should readdir the top-level directories of every package,
and merge them together into a single map structure (vhash?) that maps
filenames to sets of packages containing that filename.  We don't even
need to 'lstat' them at this point, all we need are the names.  After
we've read the directories of every package, we then iterate over the
map.  For any unique entries, we simply make a symlink in the new
profile.

For duplicates we'd do conflict resolution, which starts with an
'lstat'.  For directories, the default conflict resolution would simply
create a directory in the new profile and then recurse into that
subdirectory, considering only the packages that contained that
subdirectory.

My guess is that this would speed up profile creation by at least an
order of magnitude, maybe more.

What do you think?

      Mark