We upgraded from v4.2.1 to v6.0.5 to increase performance..it did help but I think we can fine tune further..currently at high loads, number of connections on the JVM goes high and JIRA goes slow.
We have about 800,000 issues, 10,000+ users, ~300 projects.
We have 12 cores, 20 gigs of heap space. Here are are the starup parameters --
-Djava.util.logging.config.file=/usr/local/atlassian-jira-6.0.5-standalone/conf/logging.properties
-XX:MaxPermSize=384m
-Xms20480m
-Xmx20480m
-Dcom.sun.management.jmxremote
-XX:HeapDumpPath=/appl/heapdumps
-XX:+HeapDumpOnOutOfMemoryError
-XX:+PrintGCDateStamps
-XX:+PrintGCTimeStamps
-verbose:gc
-Xloggc:/appl/jira/data/log/atlassian-jira-gc.20130908-233033.log
-Dorg.apache.jasper.runtime.BodyContentImpl.LIMIT_BUFFER=true
-Dmail.mime.decodeparameters=true
-Dsvnkit.http.methods=Basic,Digest,Negotiate,NTLM
-XX:NewSize=8192m
-XX:+UseParallelOldGC
-XX:+UseCompressedOops
-Dcom.sun.management.jmxremote.port=8066
-Dcom.sun.management.jmxremote.authenticate=true
-Dcom.sun.management.jmxremote.password.file=/usr/local/tomcat/conf/jmx-password.txt
-Dcom.sun.management.jmxremote.access.file=/usr/local/tomcat/conf/jmx-access.txt
-Dcom.sun.management.jmxremote.ssl=false
-Djava.awt.headless=true
-Datlassian.standalone=JIRA
-Dorg.apache.jasper.runtime.BodyContentImpl.LIMIT_BUFFER=true
-Dmail.mime.decodeparameters=true
-Datlassian.plugins.enable.wait=300
-XX:+PrintGCDateStamps
-Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager
-Djava.endorsed.dirs=/usr/local/tomcat/endorsed
-classpath
/usr/local/tomcat/bin/bootstrap.jar:/usr/local/atlassian-jira-6.0.5-standalone/bin/tomcat-juli.jar
-Dcatalina.base=/usr/local/atlassian-jira-6.0.5-standalone
-Dcatalina.home=/usr/local/tomcat
-Djava.io.tmpdir=/usr/local/atlassian-jira-6.0.5-standalone/temp
org.apache.catalina.startup.Bootstrap
start