Bad work server. Help Somebody.

I will start on my server settings:
processor phenom x6 1055t . 2.7 Hz
RAM - 16Gb.
OS - Debian 6.0 d64.

I'm from Ukraine,than don't be noise to my English,pls.

My problem with this :
[WARNING] Can't keep up! Did the system time change, or is the server o verloaded?


I know, that This issue is related to server setting and How to start the bukkit server.
I start with *.sh with those properties:
screen -A -m -d -S minecraft java -server -d64 -Xmn4G -Xms8G -Xmx16G -Dfile.encoding=UTF-8 -XX:+UseConcMarkSweepGC -XX:+UseParNewGC -XXarallelGCThreads=5 -XX:+CMSParallelRemarkEnabled -XX:+DisableExplicitGC -XX:MaxGCPauseMillis=500 -XX:SurvivorRatio=16 -XX:TargetSurvivorRatio=90 -jar starter.jar nogui

But, when on my server play on more than 20 players it start to print this massage from hell)
[WARNING] Can't keep up! Did the system time change, or is the server o verloaded?


With command * Top * on debiad i saw, that my bukkit server consumes more than 11Gb Ram *This is when 20+ player's on. * of my hole 16 Gb of Ram. And I was shocked. And my proccecor loaded too.

Have less 30 plugins on bukkit. And that's why it's so weird ....

Pls. Help with this.

up

EDIT by Moderator: merged posts, please use the edit button instead of double posting.