Run garbage collection once when the main process is waiting for

[kkeundotnet]

In #5376, @dra27 suggested running Gc.compact when the main process is waiting for the children processes for the first time.

what I was thinking here was that possibly at the point where opamProcess first is going to wait
for the completion of running jobs, we could add a Gc.compact?

In my local running on opam install ppxlib with this PR, "GC compact" ran in the middle of parallel processing of actions.

The following actions will be performed:
=== install 1 package
  ∗ ppxlib 0.28.0
00:07.216  XSYS                            Adding to env { LC_ALL=C }
00:09.099  STATE                           depexts loaded in 1.883s
00:09.100  SOLUTION                        parallel_apply
00:09.100  SOLUTION                        Regroup shared source packages: {}

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><>  🐫
00:09.106  PARALLEL                        Iterate over 3 task(s) with 11 process(es)
00:09.106  PARALLEL                        Starting job 444950918 (worker -/11 -/1 1/3): ⬇ ppxlib.0.28.0
00:09.106  SOLUTION                        Fetching sources for ppxlib.0.28.0
00:09.106  ACTION                          download_package: ppxlib.0.28.0
00:09.106  SYSTEM                          rmdir /Users/kkeundotnet/.opam/4.14.0/.opam-switch/sources/ppxlib.0.28.0
00:09.109  SYSTEM                          mkdir /var/folders/gx/6809fgsd3nndyyf69y3h12_h0000gn/T/opam-14220-67d907
00:09.110  PARALLEL                        Next task in job 444950918: /usr/bin/tar xfj /Users/kkeundotnet/.opam/download-cache/sha256/d8/d87ae5f9a081206308ca964809b50a66aeb8e83d254801e8b9675448b60cf377 -C /var/folders/gx/6809fgsd3nndyyf69y3h12_h0000gn/T/opam-14220-67d907
00:09.619  PARALLEL                        GC compact (heap 490 MB -> 328 MB)
00:09.619  PARALLEL                        Collected task for job 444950918 (ret:0)
00:10.158  SYSTEM                          rmdir /var/folders/gx/6809fgsd3nndyyf69y3h12_h0000gn/T/opam-14220-67d907
⬇ retrieved ppxlib.0.28.0  (cached)
00:10.321  PARALLEL                        Job 444950918 finished

Similar to #5376, this PR enabled my 1GB-RAM machine to install ppxlib or js_of_ocaml without OOM.

[rjbou]

Thanks for the PR! we'll look at it

[kit-ty-kate]

@kkeundotnet was there any specific reason why you closed this PR? Sorry we haven’t had time to review this properly but there is only 1 person working on opam full-time and we’ve been extremely busy working on the release of 2.2.0 which includes Windows support (which is not an easy task)

[kkeundotnet]

No, just because it has been stcuk long time and the maintainers look busy to address this minor issue, supporting 1GB-ram machine by running GC once at starting. Also, I found 1GB is small enough to OOM on building js_of_ocaml even with this PR these days. So, I need to find another way anyhow.

I reopen the PR, but feel free to close it if you have not much of time. Actually, "Windows support" sounds much much cooler, more exciting, and important!!

[rjbou]

Thanks for the PR!

[kit-ty-kate]

Thanks!