Allow other jobs to run in unused cores

The current select mechanism was using the memory too as a consumable
resource, which by default only sets 1 MiB per node. As each job already
requests 1 MiB, it prevents other jobs from running.

As we are not really concerned with memory usage, we only use the unused
cores in the select criteria.

Reviewed-by: Aleix Boné <abonerib@bsc.es>

m/module/slurm-client.nix

@@ -83,6 +83,14 @@ in {
 
       # Reduce port range so we can allow only this range in the firewall
       SrunPortRange=60000-61000
+
+      # Use cores as consumable resources. In SLURM terms, a core may have
+      # multiple hardware threads (or CPUs).
+      SelectType=select/cons_tres
+
+      # Ignore memory constraints and only use unused cores to share a node with
+      # other jobs.
+      SelectTypeParameters=CR_Core
     '';
   };