Real differences between “java -server” and “java -client”?

Question

Is there any real practical difference between \"java -server\" and \"java -client\"?

All I can find on Sun\'s site is a vague

\"-server st

Answer 1

I've not noticed any difference in startup time between the 2, but clocked a very minimal improvement in application performance with "-server" (Solaris server, everyone using SunRays to run the app). That was under 1.5.

Answer 2

Oracle’s online documentation provides some information for Java SE 7.

On the java – the Java application launcher page for Windows, the -client option is ignored in a 64-bit JDK:

Select the Java HotSpot Client VM. A 64-bit capable jdk currently ignores this option and instead uses the Java HotSpot Server VM.

However (to make things interesting), under -server it states:

Select the Java HotSpot Server VM. On a 64-bit capable jdk only the Java HotSpot Server VM is supported so the -server option is implicit. This is subject to change in a future release.

The Server-Class Machine Detection page gives information on which VM is selected by OS and architecture.

I don’t know how much of this applies to JDK 6.

Answer 3

One difference I've just noticed is that in "client" mode, it seems the JVM actually gives some unused memory back to the operating system, whereas with "server" mode, once the JVM grabs the memory, it won't give it back. Thats how it appears on Solaris with Java6 anyway (using prstat -Z to see the amount of memory allocated to a process).

Answer 4

This is really linked to HotSpot and the default option values (Java HotSpot VM Options) which differ between client and server configuration.

From Chapter 2 of the whitepaper (The Java HotSpot Performance Engine Architecture):

The JDK includes two flavors of the VM -- a client-side offering, and a VM tuned for server applications. These two solutions share the Java HotSpot runtime environment code base, but use different compilers that are suited to the distinctly unique performance characteristics of clients and servers. These differences include the compilation inlining policy and heap defaults.

Although the Server and the Client VMs are similar, the Server VM has been specially tuned to maximize peak operating speed. It is intended for executing long-running server applications, which need the fastest possible operating speed more than a fast start-up time or smaller runtime memory footprint.

The Client VM compiler serves as an upgrade for both the Classic VM and the just-in-time (JIT) compilers used by previous versions of the JDK. The Client VM offers improved run time performance for applications and applets. The Java HotSpot Client VM has been specially tuned to reduce application start-up time and memory footprint, making it particularly well suited for client environments. In general, the client system is better for GUIs.

So the real difference is also on the compiler level:

The Client VM compiler does not try to execute many of the more complex optimizations performed by the compiler in the Server VM, but in exchange, it requires less time to analyze and compile a piece of code. This means the Client VM can start up faster and requires a smaller memory footprint.

The Server VM contains an advanced adaptive compiler that supports many of the same types of optimizations performed by optimizing C++ compilers, as well as some optimizations that cannot be done by traditional compilers, such as aggressive inlining across virtual method invocations. This is a competitive and performance advantage over static compilers. Adaptive optimization technology is very flexible in its approach, and typically outperforms even advanced static analysis and compilation techniques.

Note: The release of jdk6 update 10 (see Update Release Notes:Changes in 1.6.0_10) tried to improve startup time, but for a different reason than the hotspot options, being packaged differently with a much smaller kernel.

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G. Demecki points out in the comments that in 64-bit versions of JDK, the -client option is ignored for many years.
See Windows java command:

-client

Selects the Java HotSpot Client VM.
A 64-bit capable JDK currently ignores this option and instead uses the Java Hotspot Server VM.

Answer 5

IIRC the server VM does more hotspot optimizations at startup so it runs faster but takes a little longer to start and uses more memory. The client VM defers most of the optimization to allow faster startup.

Edit to add: Here's some info from Sun, it's not very specific but will give you some ideas.

Answer 6

When doing a migration from 1.4 to 1.7("1.7.0_55") version.The thing that we observed here is, there is no such differences in default values assigned to heapsize|permsize|ThreadStackSize parameters in client & server mode.

By the way, (http://www.oracle.com/technetwork/java/ergo5-140223.html). This is the snippet taken from above link.

initial heap size of 1/64 of physical memory up to 1Gbyte
maximum heap size of ¼ of physical memory up to 1Gbyte

ThreadStackSize is higher in 1.7, while going through Open JDK forum,there are discussions which stated frame size is somewhat higher in 1.7 version. It is believed real difference could be possible to measure at run time based on your behavior of your application