Server capping at one core?

Hi. I run a minecraft server and when it gets to around 50-60 players it begins to lag. Going into the linux box itself I see that it seems to be capped with 1 CPU core, but I have 2 cores available. The server does not seem to be making use of the second core.

I want to try and find out what is using up the whole one core and not threading at all when we get to half our capacity for the linux box. Is it a plugin? Is something not configured properly to use multiple cores?

I am using Centos 6.4 x64

Code:

java version "1.7.0_17"
Java(TM) SE Runtime Environment (build 1.7.0_17-b02)
Java HotSpot(TM) 64-Bit Server VM (build 23.7-b01, mixed mode)

No wrappers

Code:

Manifest-Version: 1.0
Archiver-Version: Plexus Archiver
Created-By: Apache Maven
Built-By: jenkins
Build-Jdk: 1.7.0_03
Specification-Title: Bukkit
Main-Class: org.bukkit.craftbukkit.Main
Specification-Version: unknown
Specification-Vendor: Bukkit Team
Implementation-Version: git-Spigot-903
Implementation-Vendor: Bukkit Team
Sealed: true
Implementation-Title: CraftBukkit
 
Name: net/bukkit/
Sealed: true
 
Name: com/bukkit/
Sealed: true
 
Name: org/bukkit/
Sealed: true

Let me know if more information is needed. I would like to discuss and see how I can debug this.

EDIT: Oh yeah, the command I use to run the server is:

Code:

java -Xincgc -Xmx3G -jar craftbukkit.jar nogui

Bump

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

 

That is because the server doesn't use the other core for the heavy lifting (ie the core of the server), this is normal expected behavior out of the server