The Application Server produces writes log messages and exception stack trace output to the log file in the logs directory of the instance, appserver-root/domains/domain-name/logs. Naturally, the volume of log activity can impact server performance; particularly in benchmarking situations.
In general, writing to the system log slows down performance slightly; and increased disk access (increasing the log level, decreasing the file rotation limit or time limit) also slows down the application.
Also, make sure that any custom log handler doesn’t log to a slow device like a network file system since this can adversely affect performance.
Set the log level for the server and its subsystems in the Admin Console Logger Settings page, Log Levels tab. The page enables you to specify the default log level for the server (labeled Root), the default log level for javax.enterprise.system subsystems (labeled Server) such as the EJB Container, MDB Container, Web Container, Classloader, JNDI naming system, and Security, and for each individual subsystem.
Log levels vary from FINEST, which provides maximum log information, through SEVERE, which logs only events that interfere with normal program execution. The default log level is INFO. The individual subsystem log level overrides the Server setting, which in turn overrides the Root setting.
For example, the MDB container can produce log messages at a different level than server default. To get more debug messages, set the log level to FINE, FINER, or FINEST. For best performance under normal conditions, set the log level to WARNING. Under benchmarking conditions, it is often appropriate to set the log level to SEVERE.