As you keep adding running benches memory consumption will increase.
Check it with 0,1,2,n benches
run command
free -h
Allocate more memory.
Temporary fix, add Swap refer : https://www.digitalocean.com/community/tutorials/how-to-add-swap-space-on-ubuntu-16-04
If you are planning to keep adding more benches, separate file storage, database and compute.
- Get high memory Database (Amazon RDS)
- Mount network filesystem (Amazon EFS) on /home/user/benches/ so that all the benches, site/files will be added on mounted scale-able storage