lhotari opened a new pull request #8944:
URL: https://github.com/apache/pulsar/pull/8944
### Motivation
Log4j2 has a Garbage Free logging mode which reduces GC pressure since the
garbage created by logging is very low ("garbage free"). This mode is the
default of Log4j2 unless there is javax.servlet.Servlet class available at
runtime since the Garbage Free logging mode doesn't support unloading & loading
web applications which would be done in traditional J2EE application servers.
Since Pulsar doesn't use web applications at all, it's the correct thing to
disable Log4j2's web application detection so that the Garbage Free logging
mode can be used.
The performance benefits of the Garbage Free logging mode is described in
the Log4j2 documentation:
https://logging.apache.org/log4j/2.x/manual/garbagefree.html
### Modifications
- Configure log4j2 to disable servlet webapp detection so that Garbage
free logging can be used.
- Log4j2 will disable Garbage free logging if there is
javax.servlet.Servlet class
available at runtime unless "-Dlog4j2.is.webapp=false" is passed
on the command line.
- See source code at
https://github.com/apache/logging-log4j2/blob/master/log4j-api/src/main/java/org/apache/logging/log4j/util/Constants.java#L25-L41
- Documented at
https://logging.apache.org/log4j/2.x/manual/garbagefree.html
"From version 2.6, Log4j runs in "garbage free" mode by default where
objects
and buffers are reused and no temporary objects are allocated as much
as possible.
There is also a "low garbage" mode which is not completely garbage free
but does
not use ThreadLocal fields. This is the default mode when Log4j detects
it is
running in a web application."
...
"ThreadLocal fields holding non-JDK classes can cause memory leaks in
web applications
when the application server's thread pool continues to reference these
fields after
the web application is undeployed. To avoid causing memory leaks, Log4j
will not use
these ThreadLocals when it detects that it is used in a web application
(when the
javax.servlet.Servlet class is in the classpath, or when system
property log4j2.isWebapp
is set to "true")."
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