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Increasing Java Virtual Machine memory for Tomcat

Q. "How do I increase the memory that my Java Virtual Machine in Tomcat can use?" A question asked - not for the first time - by a delegate on todays's Deploying Apache httpd and Tomcat course.

A. Set the JAVA_OPTS environment variable. Longer answer:

If you're running on a Unix or Linux system, edit the setenv.sh file in Tomcat's bin directory to add in a line like:
export JAVA_OPTS="-Xms128m -Xmx512m"
or on a Windows system, add in to setenv.bat something like:
set JAVA_OPTS=-Xms128 -Xmx512
and restart the Tomcat server.

By default (out of the tar), Tomcat starts up with just 64 Mbytes allocated to the JVM, and that's also it's maximum. This is a very low figure, but it's a sensible default for a new install where the person setting the system up isn't into the tuning of the system.

The two extra parameters specified via JAVA_OPTS are as follows:
-Xms - the amount of memory that the JVM starts with.
-Xmx - the maximum memory that the JVM may have.
I have seen advise (but have no proof) that your should keep the -Xms figure rather lower than the -Xmx figure, since Tomcat uses more memory at startup than when it's running. The result (if you have them set to the same value) is a peak and unnecessary memory demand at startup which in an extreme case can cause a failure.

A couple of other tips:

a) You should consider using the -server option too to run your JVM in server rather than (default) client mode (actually, they're two different but related VMs). The server that's selected with -server is slower to load but is said to run faster.

b) We have customers who run multiple instances of Tomcat on the same box (but on different ports) - not a bad idea if you're sharing load between multiple processors. You can front them with httpd using mod-proxy-balance or mod-jk to distribute the requests, but at the same time to provide "sticky sessions" so that a series of requests made by an individual user will all be handled by the same instance.
(written 2008-07-24, updated 2008-07-25)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
A506 - Web Application Deployment - Java - Installing and Deploying optional Technologies
  [3043] Gathering information - logging - with log4j. First steps. - (2010-11-12)
  [2082] Jmeter - a first test case - (2009-03-14)
  [2081] Connecting jconsole remotely - the principles - (2009-03-14)
  [2079] Java - Memory Allocation and garbage collection - (2009-03-14)

A656 - Web Application Deployment - More Tomcat Configuration
  [3018] Tuning Apache httpd and Tomcat to work well together - (2010-10-27)
  [2272] Monitoring and loading tools for testing Apache Tomcat - (2009-07-07)
  [1899] Virtual Hosting under Tomcat - an example - (2008-11-22)
  [1370] Apache Tomcat Performance Tuning - (2007-09-29)
  [466] Separating 'per instance' data from binaries and web sites - (2005-10-16)
  [282] Short weekend - (2005-04-17)

A692 - Web Application Deployment - Monitoring and load testing your server
  [3027] Server logs - drawing a graph of gathered data - (2010-11-03)
  [3019] Apache httpd Server Status - monitoring your server - (2010-10-28)
  [3015] Logging the performance of the Apache httpd web server - (2010-10-25)
  [2080] Using ApacheBench and jconsole to test and monitor Tomcat - (2009-03-14)

A901 - Replaced Page
A912 - Web Application Deployment - Upgrading and tuning Tomcat
  [2060] Database connection Pooling, SSL, and command line deployment - httpd and Tomcat - (2009-03-01)
  [1908] Java CLASSPATH explained - (2008-11-26)
  [1377] Load Balancing with Apache mod_jk (httpd/Tomcat) - (2007-10-02)


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