Find out how many CPUs a system has

[NativeOne]

In my application I need to find out how many processors the host system has, in order to start an appropriate numer of threads (in my case numberOfThreads = numberOfCPUs). Is there a standard means of achieving this in Java? If not, can anyone please give me a hint how to do this platform-independantly with JNI... (I never used native code, so please explain this in a fool-proof manner) Thanks a lot!

[Mark Thornton]

java.lang.Runtime.availableProcessors()

added in 1.4

[NativeOne]

Thanks, that will be optimal. Strange they added this functionality so late...

An other question comes to my mind. What about this new hyperthreading technology of intels P4 (I read of a 25% performance increase): will a multithreaded java application benefit from it? If that's true, I should use the parallel algorithm with two threads even on a single processor system, hmm... how to detect a hyperthreading processor?

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[Mark Thornton]

In my own tests I get a gain from HT nearer 10% --- not enough to be worth worrying about. Possibly because my machine is a 3.06GHz model with only the 533FSB. Allegedly the 800MHz FSB machines show greater benefits from HT. Even so it still looks more like a marketing feature than something of significant value.

I think a HT processor (with it enabled) will show as having 2 processors available. You could run two threads in parallel and see if the elapsed time looks like the result of a genuine dual or a HT processor.

[swpalmer]

FYI: With my company's non-java stuff (media encoding applications and device drivers) we found hyperthreading caused problems.  Some systems only worked with hyperthreading off.  The code works fine on traditional multi-processor systems.

[Jeff]

Our general opinion around Sun, which you can buy or not, is that Intel's so called "HT" is a cheap and cheesy attempt to respond to sun's Sparc CMT  (On-Chip Multi-Tasking), except its not nearly as functional or useful.

[cfmdobbie]

Jeff, I think Intel has proven that the cheap-and-cheerful approach often wins out over precise-but-expensive!

I had a co-worker once who owned a number of very high-quality 2400 baud modems - he complained that today's tacky modems were unreliable.  It seemed rude to point out how many times a 56K can resend any duff data while the 2400 sends the message just once.

But yeah, I personally wouldn't bother optimising Java code to take better advantage of HT.  May as well go back to hand-tuned assembly...

[blahblahblahh]

Our general opinion around Sun, which you can buy or not, is that Intel's so called "HT" is a cheap and cheesy attempt to respond to sun's Sparc CMT  (On-Chip Multi-Tasking), except its not nearly as functional or useful.

Who got DEC's work on this, in the end? I guess Intel (?), but maybe  HPaq managed to hang onto it...? I vaguely recall that the DEC design had good potential, but from what little (nb: very little) I've seen of Intel's current impl, I'd agree it doesn't seem much good.

OTOH, AFAICS Intel still has major problems with the fact that they  don't have a 32-bit multiprocessor architecture (...until they get with the program and start using crossbar switches or some equivalent...), so perhaps HT / MT / HPT (whatever you want to call it ) will prove a way out that doesn't involve 128Mb cache on each CPU just to keep it busy...

[Jeff]

Jeff, I think Intel has proven that the cheap-and-cheerful approach often wins out over precise-but-expensive!..

But not if all youa re "buying' is a name with no real advantage in the hadrware. AIUI this is HT. 99% hype, tiny bit of functionality that a really clever assmebly coder might be able to make some use out of in just the right situations.

Again though I'm NOT a processor expert.  This is just my general imrpession from tlaking to flks who are.

[princec]

...really clever assmebly coder might be able to make some use out of in just the right situations...

That would be, uh, a JVM engineer then?

Cas

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[ChrisRijk]

One thing I have noticed with Hyperthreading is that comparing on a multiprocessor system, the time taken to compile byte-codes is much faster (like 10x faster). Of course, you can just put on -Xbatch and then HotSpot's compiler runs in the foreground, which is more efficient. Basically, if the main thread is on 1 CPU and HotSpot is on the 2nd, then the communication overhead between the threads slows the optimisation down a bit. With Hyperthreading, the optimiser runs on the same CPU as the main thread, so is more efficient.

How much effect Hyperthreading has is highly dependant on your benchmark - with some server benchmarks, a 50% boost is possible, but this is rare.

btw, the latest Pentium 4 (Prescott) has a much longer pipeline making it even less suitable for server work, or more complex code in general. If you want a good development or runtime platform for Java, I'd suggest getting an Opteron or Athlon 64.


If you want more of an introduction to multi-core chip designs (like what Sun is working on), then read my recent article
http://www.aceshardware.com/read.jsp?id=60000312