Why my servers can not support over 1500 concurrent client connections?

Openfire 3.7.1

route.all-resources = true (over 2000 concurrent clients using same account with different resources for receiving special broadcast)

enable LDAP (over 5000 OA users)

httpbind.enabled = false

xmpp.auth.anonymous = false

xmpp.client.idle = 500000

xmpp.client.idle.ping = true

OS1: Windows 2003 R2 sp2, xeon 2.4 GHz processor x2, 3G RAM, JVM 1.6.0_18

OS2: SUSE Linux Enterprise Server 10 (x64), xeon 1.8 GHz processor x4, 4G RAM , JVM 1.6.0_16 64-Bit

Database: Mysql 5.1.52 for win32.

The following JVM parameters are used on OS1 (or OS2),but openfire crashed over and over again when 1500 concurrent same account clients connect to server OS1 (or OS2) in 10 minutes. Could anybody tell me what should I do?

1: -Xms512m -Xmx1024m

2: -Xms1500m -Xmx1500m (OS1) ,-Xms3000m -Xmx3000m (OS2)

3( with 2 ): -Xss128k -Xoss128k -XX:ThreadStackSize=128 -Djava.net.preferIPv4Stack=true -server -XX:+PrintGCDetails -XX:+PrintGCTimeStamps -Xloggc:.\gc.log

4( with 3,2): -XX:+UseAdaptiveSizePolicy-XX:+UseParallelGC -XX:+UseParallelOldGC -XX:ParallelGCThreads=4

Attachment is one of JVM gc view, you can see there is a sudden increase and continuous FULL GC.