How to increase Openfire capacity?

I saw a couple of posts saying one Openfire node is able to handle up to 200000 concurrent users or at least 50000. However, no one really explained how to achieve such a performance.

Our Openfire installation had 4000 real concurrent users and Java Memory was always 95-99%.

We switched to 64-bit JVM to let it have more memory but it looks like once we have 5000-6000 users we’ll have problems again. Tried to play with cache settings a little bit – no real improvement, maybe it wasn’t the right direction.

I have 2 questions:

1. What should I do to make it work with 50000 users?
2. How many concurrent users can this installation handle?

Our current configuration:

Windows Server 2008 R2

CPU: Quad-Core AMD 2.20 GHz (2 processors)

4 Gb RAM

Openfire 3.8.1

Java: 1.6.0_33 (64-bit) (-Xms1024m; -Xmx3200m; -Xss128k; -Xoss128k)

Database: MySQL 5.5.28 (max 45 connections), running on the same machine

Average roster is 1 to 3 users. No groups. Not more than 1 entry in Vcard.

PEP disabled. Proxy disabled.

Cache settings:

cache.group.size                          200000

cache.lastActivity.size                    1500000

cache.offlinePresence.size            100000

cache.routeUserSessions.size       400000

cache.userCache.size                        1000000

cache.username2roster.size          1000000

cache.vcardCache.size                  100000