On some Linux distributions (Redhat Enterprise Linux Server/CentOS 6.0 or later), the default maximum number of user processes is set to 1024, which is considerably lower than the same parameter on older distributions (e.g., RHEL/CentOS 5.x). The default value of 1024 leads to some JVM memory errors when running multiple servers on a machine due to each Linux thread being counted as a user process.

At startup, the Directory Server and its tools automatically attempt to raise the maximum user processes limit to 16,383 if the value reported by ulimit is less than that. If, for any reason, the server is unable to automatically set the maximum processes limit to 16,383, an error message will be displayed. It is recommended that the limit be set explicitly in /etc/security/limit.conf. For example:
* soft nproc 65535
* hard nproc 65535

The (*) can be replaced with the name of the user under which the software will run. These settings can also be manually configured by setting the NUM_USER_PROCESSES environment variable to 16383 or by setting the same variable in a file named config/num-user-processes.