Real differences between "java -server" and "java -client"?

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.

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.

2022: Holger references in the comments the JavaSE6 / Server-Class Machine Detection, adding:

Only on 32 bit Windows systems, -client was ever chosen unconditionally. Other systems checked whether the machine was “server class” which was fulfilled when having at least 2 cores and at least 2GiB of memory. Which explains why almost everything uses -server for quite some time now. Even the cheapest computers you can find, are “server class” machines. The Sun/Oracle 64 builds did not even ship with a client JVM.

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The most visible immediate difference in older versions of Java would be the memory allocated to a -client as opposed to a -server application. For instance, on my Linux system, I get:

$ java -XX:+PrintFlagsFinal -version 2>&1 | grep -i -E 'heapsize|permsize|version'
uintx AdaptivePermSizeWeight               = 20               {product}
uintx ErgoHeapSizeLimit                    = 0                {product}
uintx InitialHeapSize                     := 66328448         {product}
uintx LargePageHeapSizeThreshold           = 134217728        {product}
uintx MaxHeapSize                         := 1063256064       {product}
uintx MaxPermSize                          = 67108864         {pd product}
uintx PermSize                             = 16777216         {pd product}
java version "1.6.0_24"

as it defaults to -server, but with the -client option I get:

$ java -client -XX:+PrintFlagsFinal -version 2>&1 | grep -i -E 'heapsize|permsize|version'
uintx AdaptivePermSizeWeight               = 20               {product}
uintx ErgoHeapSizeLimit                    = 0                {product}
uintx InitialHeapSize                     := 16777216         {product}
uintx LargePageHeapSizeThreshold           = 134217728        {product}
uintx MaxHeapSize                         := 268435456        {product}
uintx MaxPermSize                          = 67108864         {pd product}
uintx PermSize                             = 12582912         {pd product}
java version "1.6.0_24"

so with -server most of the memory limits and initial allocations are much higher for this java version.

These values can change for different combinations of architecture, operating system and jvm version however. Recent versions of the jvm have removed flags and re-moved many of the distinctions between server and client.

Remember too that you can see all the details of a running jvm using jvisualvm. This is useful if you have users who or modules which set JAVA_OPTS or use scripts which change command line options. This will also let you monitor, in real time, heap and permgen space usage along with lots of other stats.

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the -client and -server systems are different binaries. They are essentially two different compilers (JITs) interfacing to the same runtime system. The client system is optimal for applications which need fast startup times or small footprints, the server system is optimal for applications where the overall performance is most important. In general the client system is better suited for interactive applications such as GUIs

https://i.stack.imgur.com/zkWjn.gif

We run the following code with both switches:

package com.blogspot.sdoulger;

public class LoopTest {
    public LoopTest() {
        super();
    }

    public static void main(String[] args) {
        long start = System.currentTimeMillis();
        spendTime();
        long end = System.currentTimeMillis();
        System.out.println("Time spent: "+ (end-start));

        LoopTest loopTest = new LoopTest();
    }

    private static void spendTime() {
        for (int i =500000000;i>0;i--) {
        }
    }
}

Note: The code is been compiled only once! The classes are the same in both runs!

With -client: java.exe -client -classpath C:\mywork\classes com.blogspot.sdoulger.LoopTest Time spent: 766

With -server: java.exe -server -classpath C:\mywork\classes com.blogspot.sdoulger.LoopTest Time spent: 0

It seems that the more aggressive optimazation of the server system, remove the loop as it understands that it does not perform any action!

Reference

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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).

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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.

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From Goetz - Java Concurrency in Practice:

Debugging tip: For server applications, be sure to always specify the -server JVM command line switch when invoking the JVM, even for development and testing. The server JVM performs more optimization than the client JVM, such as hoisting variables out of a loop that are not modified in the loop; code that might appear to work in the development environment (client JVM) can break in the deployment environment (server JVM). For example, had we “forgotten” to declare the variable asleep as volatile in Listing 3.4, the server JVM could hoist the test out of the loop (turning it into an infinite loop), but the client JVM would not. An infinite loop that shows up in development is far less costly than one that only shows up in production.

Listing 3.4. Counting sheep. volatile boolean asleep; ... while (!asleep) countSomeSheep();

My emphasis. YMMV

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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.

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IIRC, it involves garbage collection strategies. The theory is that a client and server will be different in terms of short-lived objects, which is important for modern GC algorithms.

Here is a link on server mode. Alas, they don't mention client mode.

Here is a very thorough link on GC in general; this is a more basic article. Not sure if either address -server vs -client but this is relevant material.

At No Fluff Just Stuff, both Ken Sipe and Glenn Vandenburg do great talks on this kind of thing.

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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.

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Last time I had a look at this, (and admittedly it was a while back) the biggest difference I noticed was in the garbage collection.

IIRC:

The server heap VM has a differnt number of generations than the Client VM, and a different garbage collection algorithm. This may not be true anymore

The server VM will allocate memory and not release it to the OS

The server VM will use more sophisticated optimisation algorithms, and hence have bigger time and memory requirements for optimisation

If you can compare two java VMs, one client, one server using the jvisualvm tool, you should see a difference in the frequency and effect of the garbage collection, as well as in the number of generations.

I had a pair of screenshots that showed the difference really well, but I can't reproduce as I have a 64 bit JVM which only implements the server VM. (And I can't be bothered to download and wrangle the 32 bit version on my system as well.)

This doesn't seem to be the case anymore, having tried running some code on windows with both server and client VMs, I seem to get the same generation model for both...

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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

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