Running tomcat with -server JVM option

Question

I am testing my tomcat server 4, with -server jvm option. My JDK is 1.5 on FreeBSD.

I don't see any noticable difference or any issues. If I am to turn this option on on my prod systems, what kind of improvement can I expect and what kind of issues should I lookout for?

I have read What's the effect of -server option for the HotSpot JVM?, but it does not discuss this in detail.

Answer 1

It is quite possible that the JVM was already running in server mode. For JDK 5/6 on Linux, the JVM will default to server-mode on a server-class machine:

"[...] the definition of a server-class machine is one with at least 2 CPUs and at least 2GB of physical memory. "

This is documented here (and here for Java 6).

It is not spelled out what happens on FreeBsd, but I expect that it is the FreeBsd JVMs are server-mode-only or that they defaults to server-mode on a server-class machine as with Linux.

Answer 2

The trick is that modern JVM can "autodetect" a server-like machine. So if you do not specify -server or -client, a JVM1.5+ will choose the best method - and it simply may be that it will run in server mode even if you do not use the attribute.

In that case, you may notice a difference if you start the JVM with the -client option.

Reference

• Server-Class Machine Detection