bug#44254: Performance of package input rewriting

[Lars-Dominik Braun]

Hi,

this issue is similar to https://issues.guix.gnu.org/41702, but I’m not sure
it’s exactly the same. For guix-science I’m trying to provide some packages
like python-jupyterlab, which depend on a mix of packages from guix proper and
newer versions of packages already included in guix proper. Thus I need to
rewrite inputs of the former to the latter. (Because Python only propagates
dependencies and thus collisions would occur.)

Previously I have been doing this using package-input-rewriting, but starting
an environment containing python-jupyterlab alone took about 20s (warm caches,
all derivations in the store). Manually rewriting inputs by inheriting and
alist-delete’ing brings this down to 3s, which is pretty significant.
--no-grafts has not much of an impact (15s vs 2s) here. See
https://github.com/guix-science/guix-science/commit/972795a23cc9eb5a0bb1a2ffb5681d151fc4d4b0
for the exact changes.

My expectation would be that package-input-rewriting is the preferred, because
easier, solution to this problem and thus should have minimal impact on
performance.

Cheers,
Lars

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