Threads seem to be blocking on class org.apache.loggin.log4j.core.pattern.DatePatternConverter. It's short lived but is visible in profiler. It also is adding on to CPU. Here is the mail conversation on the mailing list:
Ralph Goers firstname.lastname@example.org via logging.apache.org
The converter uses a SimpleDateFormat which is not thread safe and so is synchronized. I am sure there might be minor optimizations that could be done to this
What I would do is modify DatePatternConverter to
a) use Java 8’s java.time.format.DateTimeFormatter if running on Java 8
b) use Joda Time’s DateTimeFormat if it is present.
c) create a pool of SimpleDateFormat objects and use those.
Please create a Jira issue for this.
One alternative that was suggested on the mailing list is to use commons lang FastDateFormat to format log timestamps.