Tomcat memory usage (PermGen)

Tomcat memory usage (PermGen)

I am using Tomcat for deploying a web application withe following PermGen
configurations:
JAVA_OPTS="-Djava.awt.headless=true -Dfile.encoding=UTF-8
-server -Xms1536m -Xmx1536m
-XX:NewSize=256m -XX:MaxNewSize=256m -XX:PermSize=256m
-XX:MaxPermSize=256m -XX:+DisableExplicitGC"
Please I want to know the following:
Is the whole memory used by Tomcat should be 256MB or Tomcat consumes
other memory for other reasons ?
How can I monitor the memory used by Tomcat to avoid reaching PemGen max
size ?
Also please be noticed that I have 3 Tomcats with the same previous
PermGen configurations, running on the same physical server with 8 GB Ram
which make me afraid of increasing PermGen max size to 512MB not to affect
the physical server memory.