add /etc/default/jenkins with modified JAVA_ARGS

etc/default/jenkins

+# defaults for jenkins continuous integration server
+
+# pulled in from the init script; makes things easier.
+NAME=jenkins
+
+# location of java
+JAVA=/usr/bin/java
+
+# arguments to pass to java
+#JAVA_ARGS="-Xmx512m"  # + MaxPermSize is 128m normally
+JAVA_ARGS="-Xmx1024m -XX:MaxPermSize=256m"
+#JAVA_ARGS="-Djava.net.preferIPv4Stack=true" # make jenkins listen on IPv4 address
+
+PIDFILE=/var/run/jenkins/jenkins.pid
+
+# user id to be invoked as (otherwise will run as root; not wise!)
+JENKINS_USER=jenkins
+
+# location of the jenkins war file
+JENKINS_WAR=/usr/share/jenkins/jenkins.war
+
+# jenkins home location
+JENKINS_HOME=/var/lib/jenkins
+
+# set this to false if you don't want Hudson to run by itself
+# in this set up, you are expected to provide a servlet container
+# to host jenkins.
+RUN_STANDALONE=true
+
+# log location.  this may be a syslog facility.priority
+JENKINS_LOG=/var/log/jenkins/$NAME.log
+#HUDSON_LOG=daemon.info
+
+# OS LIMITS SETUP
+#   comment this out to observe /etc/security/limits.conf
+#   this is on by default because http://github.com/jenkinsci/jenkins/commit/2fb288474e980d0e7ff9c4a3b768874835a3e92e
+#   reported that Ubuntu's PAM configuration doesn't include pam_limits.so, and as a result the # of file
+#   descriptors are forced to 1024 regardless of /etc/security/limits.conf
+MAXOPENFILES=8192
+
+# port for HTTP connector (default 8080; disable with -1)
+HTTP_PORT=8080
+
+# port for AJP connector (disabled by default)
+AJP_PORT=-1
+
+# servlet context, important if you want to use apache proxying  
+PREFIX=/jenkins
+
+# arguments to pass to jenkins.
+# --javahome=$JAVA_HOME
+# --httpPort=$HTTP_PORT (default 8080; disable with -1)
+# --httpsPort=$HTTP_PORT
+# --ajp13Port=$AJP_PORT
+# --argumentsRealm.passwd.$ADMIN_USER=[password]
+# --argumentsRealm.roles.$ADMIN_USER=admin
+# --webroot=~/.jenkins/war
+# --prefix=$PREFIX
+
+JENKINS_ARGS="--webroot=/var/cache/jenkins/war --httpPort=$HTTP_PORT --ajp13Port=$AJP_PORT"