System

Processes

Overview

PROCESSES specifies the maximum number of operating system user processes that can simultaneously connect to Oracle. Its value should allow for all background processes such as locks, job queue processes, and parallel execution processes.

The default values of the SESSIONS and TRANSACTIONS parameters are derived from this parameter. Therefore, if you change the value of PROCESSES, you should evaluate whether to adjust the values of those derived parameters.

In Linux the nproc parameter in the /etc/security/limits.conf file defines the OS limit for the number of processes per user.

The Oracle docs on this are here.
Redhat discusses this here

Current OS usage: ps h -Led -o user | sort | uniq -c | sort -n

Processes vs. Sessions Fun Facts

The PROCESSES parameter should typically be set lower than the SESSIONS parameter. Each database session may require one or more background or foreground processes to support it, but not every session maps 1:1 to a separate OS-level process.

Oracle uses PROCESSES to define the maximum number of OS processes (like server processes, background workers, etc.), while SESSIONS includes all user and internal sessions.

By default, Oracle sets SESSIONS = (1.1 * PROCESSES) + 5, which accounts for this relationship.

It’s best to keep SESSIONS slightly higher than PROCESSES to avoid unexpected errors or limits being hit.

Display

 SELECT pid, spid, username, terminal, program FROM v$process;

 PID  SPID   USERNAME  TERMINAL        PROGRAM
 ---- ------ --------- --------------- ------------------------------
 116  14486  oracle    UNKNOWN         oracle@lnx01.local (PMON)
 117  14547  oracle    UNKNOWN         oracle@lnx01.local (VKTM)
 118  14549  oracle    UNKNOWN         oracle@lnx01.local (CKPT)
 ...


 SELECT resource_name, current_utilization, max_utilization, limit_value 
 FROM v$resource_limit  WHERE resource_name in ('sessions', 'processes');

 RESOURCE_NAME                  CURRENT_UTILIZATION MAX_UTILIZATION LIMIT_VALU
 ------------------------------ ------------------- --------------- ----------
 processes                                      116             140        300
 sessions                                        76             104        480