bug#44175: [optimization] Grafting is too slow

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

Hello Lars,

Lars-Dominik Braun <ldb@leibniz-psychology.org> writes:

> Hi Maxim,
>
>> Judging from the above, it seems this issue has been resolved.
> grafting is still a performance issue imo. Compare for example:
>
> $ time guix environment --ad-hoc  --search-paths r-learnr
> guix environment --ad-hoc --search-paths r-learnr  5,90s user 0,09s system 210% cpu 2,844 total
> $ time guix environment --ad-hoc  --search-paths r-learnr --no-grafts
> guix environment --ad-hoc --search-paths r-learnr --no-grafts  2,03s user 0,08s system 164% cpu 1,277 total

I'm opening a new issue to track optimizing the grafting code, since
it's independent of environments (grafts are applied anytime a
derivation is built, AFAICT).  Grafting is inherently IO-bound, since it
must scan all the built artifacts of a grafted package (from its
dependency that was 'replaced' all the way up), if I understood
correctly.  "info (guix) Security Updates" gives some information.

Perhaps we could benchmark how fast our code currently is for grafting,
and compare it with the fastest sed-like utility out there, as a
starting point, to have an idea of how much there is to gain from
optimization.

Maxim