One fine day you you as a BO Admin will find the Tomcat is going down frequently ot couple to few times a week. And you may not have any clue what's going.
In our case we had this issue and hence i thought to share it with you all.
This is likely due to the compression bug in tomcat .
Or check the logs and see if there is any indication of a problem with max permgen being reached (OOM - out of memory).
As a resolution First thing is to disable compression in server.xml (/path/to/tomcat/server.xml ) - Look for the connectors that have
compression=on and change it to compression=off .
Second thing to do will be to up the max permgen space for Tomcat. This can typically be done in the environment shell script (env.sh/setenv.sh) .
Knowing all these, we modified the server.xml from Tomact/conf/server.xml to off the compression. From compression=on to compression=off.
We cleared out the cache of tomcat Tomcat/work/catalina/localhost.
Then, we were able to login to CMC. 
Then we changed the value for -Xmx to 4096 and added one parameter
-Xms to 256 in following variable-
JAVA_OPTS in tomcat/bin/setenv.sh script.
For more , we can refer SAP note-1750952 - BI4 Setting JAVA_OPTS for tomcat