Hi,

I’ve noticed that `guix environment` can be very very slow for some packages.
Whereas usually a call like

	time guix environment --ad-hoc <package> -- true

takes 600ms to 1.5s, it takes 8.4s for the package r-learnr on my Ryzen 5 3600
with NVMe SSD, 32G of RAM and a warm cache. This seems to accumulate when
trying to install multiple unrelated “slow” packages into the same environment,
i.e. jupyter (2.9s) + r-rlearnr (8.4s) + r-stm (2.9s) = 12.1s. What these
packages seem to have in common is a deep chain of propagated inputs.

And in fact, if I create artificial test packages where test-999 propagates
test-998, test-998→test-997 and so on the command above takes 1m 9s. A single
package propagating 999 packages takes 4.5s.

Instrumenting guix-environment’s with-store call with statprof I see a lot of
time is spent in the garbage collector:

Total time: 24.672604202 seconds (19.431122691 seconds in GC)
./pre-inst-env guix environment --ad-hoc r-learnr -- true  25,18s user 0,24s system 308% cpu 8,248 total

More specifically in an anonymous function and reap-pipes, which is a gc hook,
I believe:

%     cumulative   self
time   seconds    seconds   calls   procedure
 33.41     14.49      8.24                            anon #xbb8480
 27.95      6.90      6.90                            ice-9/popen.scm:145:0:reap-pipes
  4.37      1.08      1.08                            anon #xbbdcd8
  3.28      0.86      0.81                            ice-9/vlist.scm:539:0:vhash-assq
  2.40      2.37      0.59                            guix/grafts.scm:202:22

Not sure if that helps tracking the issue down though.

Cheers,
Lars

PS: Here’s `guix describe`:
Generation 1	Jun 03 2020 10:38:17	(current)
  guix 9ab2790
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: 9ab2790e2acf872c0f2cf41ec180de80d1eee3f3
  zpid bab33c1
    repository URL: https://github.com/leibniz-psychology/guix-zpid.git
    branch: master
    commit: bab33c19e0f0304d7aa82751653eb85d8cad8c0a