When running multiple Vivado sessions on a Red Hat 6 (RHEL6) machine, the following error messages can be observed:
There is a known issue with RHEL6 (https://bugzilla.redhat.com/show_bug.cgi?id=919793) where it limits the threads/processes per user to 1024.
Vivado takes up to 50 threads, so it is not possible to run more than 20 Vivado sessions at any given time.
You can resolve this issue by increasing the number in /etc/security/limits.d/90-nproc.conf.
This file and limit are not present in RHEL5.