Document performance implications of high volume System.out/System.err usage

[kgibm]

In a recent performance competition with another application server, an application with incredibly heavy System.out/System.err usage highlighted the fact that Liberty defaults to sending System.out/System.err messages to both messages.log and process stdout/stderr (e.g. cosnole.log):

copySystemStreams
com.ibm.ws.logging.copy.system.streams
If this setting is set to true, messages that are written to the System.out and System.err streams are copied to process stdout and stderr streams and so appear in the console.log file. If this setting is set to false, those messages are written to configured logs such as the messages.log file or trace.log file, but they are not copied to stdout and stderr and do not appear in console.log. The default value is true.

This will be worse performance compared to just sending to messages.log and the performance overhead will be proportional to the volume of System.out/System.err usage.

It would be useful to highlight this in the Liberty Performance tuning page.

[kgibm]

Suggested documentation (CC @jhanders34 @gjdeval for thoughts/review):

Tune Logging

By default, messages sent to System.out and System.err are sent to both messages.log and process stdout/stderr (e.g. console.log) because copySystemStreams / com.ibm.ws.logging.copy.system.streams default to true. With high volume System.out/System.err usage, this duplicated output may have a significant performance impact. You may disable sending System.out/System.err to process stdout/stderr by setting com.ibm.ws.logging.copy.system.streams=false in bootstrap.properties.

If System.out/System.err logging is excessive and causing a performance problem even sending just to messages.log, then they may be disabled temporarily with <logging traceSpecification="*=info:SystemOut=off:SystemErr=off"/>; however, this generally isn't recommended long-term because some components in the Java Class Library (and others) may have critical diagnostics that write to System.out and/or System.err. Instead, it's best to fix such excessive System.out/System.err usage.