Modern CPUs have many features that increase multi-threaded performance
(eg: hyperthreading on Pentium 4s and similar features on recent PowerPC chips).
Over the next year the trend towards multi-cored CPUs from AMD, Intel and Sun will accelerate multithreaded performance while single threaded performance will begin to level off.

Java has always had excellent threading support, but JDK 1.5 introduces a whole new set of concurrency libraries which make multi-threaded programming easier. In theory these libraries should mesh well with modern CPUs, since (on a hyperthreading CPU) each thread appears as an extra CPU.

I've written a program to investigate multithreaded performance under Java. Does hyperthreading help multithreaded Java programs, or is the VM unable to use it properly?

My program is pretty simple – it generates random numbers and converts them to a string inside a loop a set amount of time. This loop is executed four time – twice by a single thread, and then twice by two threads simultanously. We would hope to see the multithreaded version run quicker on a hyperthreaded CPU.

Results:

2984.2 MHz Intel Pentium 4 3 GHz (Hyperthreading On):

Java Environment
    Sun Microsystems Inc. Java HotSpot(TM) Client VM 1.5.0_01-b08
Native Environment
    Windows XP 5.1 on x86
    2 CPU(s) detected
Please wait. Running Tests..
Single Threaded Test completed.
Dual Threaded Test completed.

Results
----------------------------------
Single Thread Time = 49241 ms.
Dual Thread Time = 35776 ms.

2984.2 MHz Intel Pentium 4 3 GHz (Hyperthreading Off):

Results
----------------------------------
Single Thread Time = 46101 ms.
Dual Thread Time = 50646 ms.

1668.8 MHz AMD Athlon(tm) XP 2000+:

Java Environment
Sun Microsystems Inc. Java HotSpot(TM) Client VM 1.5.0_01-b08
Native Environment
Windows 2000 5.0 on x86
1 CPU(s) detected
Please wait. Running Tests..
Single Threaded Test completed.
Dual Threaded Test completed.

Results
----------------------------------
Single Thread Time = 54844 ms.
Dual Thread Time = 66937 ms. 

As you can see, hyperthreading really does work (27% quicker). However, we shouldn't just fire off threads everywhere possible, because multithreaded code runs will run significantly slower (9.8% on the P4 and 22% on the Athlon) than single threaded code on conventional CPUs.

Code similar to the following may be a suitable strategy:

int numThreads = osMBean.getAvailableProcessors();
ExecutorService executor = Executors.newFixedThreadPool(numThreads);
while (tasksNotExecuting()) {
   executor.submit(someLongRunningTask());
}

I'd be interested in other people results with the same tests. The more the better, but I'm especially interested in some more exotic environments – PowerPC5 based server, multi CPU machines and G5 Macs.

My program is available as a jar (run using java -jar threadtest.jar under Java 5). I use CPUChk to get the CPU id. Please record your results in the comments, with the machine CPU used to generate them and (if possible) the hyperthreading status (it can usually be enabled & disabled in the bios).