Scrubber processes getting killed by OOM killer

On the production server, some scrubber processes are killed by the OOM killer

Jul 10 18:37:59 scrubber1 kernel: Out of memory: Kill process 3192 (swh) score 116 or sacrifice child
Jul 10 18:37:59 scrubber1 kernel: Killed process 3192 (swh) total-vm:297888kB, anon-rss:229668kB, file-rss:24kB, shmem-rss:0kB
Jul 10 18:37:59 scrubber1 kernel: oom_reaper: reaped process 3192 (swh), now anon-rss:0kB, file-rss:0kB, shmem-rss:0kB
Jul 10 18:37:59 scrubber1 swh[28310]: INFO:swh.scrubber.storage_checker:Processing revision range bd30e8 to bd30e9
Jul 10 18:37:59 scrubber1 swh[10665]: INFO:swh.scrubber.storage_checker:Processing revision range 48b4e6 to 48b4e7
Jul 10 18:37:59 scrubber1 systemd[1]: swh-scrubber-checker-postgres@directory-3.service: Main process exited, code=killed, status=9/KILL

 free -h
              total        used        free      shared  buff/cache   available
Mem:          978Mi       756Mi       109Mi       0.0Ki       112Mi        47Mi
Swap:         975Mi       962Mi        13Mi

Despite the ballooning allowed to 4g, it seems the server remains to 1Go.

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Migrated from T4387 (view on Phabricator)