Alex Rivera | Logout

Threads per Processor

Asked 2008-10-18T16:35:18.140
28

In Java, is there a programmatic way to find out how many concurrent threads are supported by a CPU?

Update

To clarify, I'm not trying to hammer the CPU with threads and I am aware of Runtime.getRuntime().availableProcessors() function, which provides me part of the information I'm looking for.

I want to find out if there's a way to automatically tune the size of thread pool so that:

  • if I'm running on a 1-year old server, I get 2 threads (1 thread per CPU x an arbitrary multiplier of 2)
  • if I switch to an Intel i7 quad core two years from now (which supports 2 threads per core), I get 16 threads (2 logical threads per CPU x 4 CPUs x the arbitrary multiplier of 2).
  • if, instead, I use a eight core Ultrasparc T2 server (which supports 8 threads per core), I get 128 threads (8 threads per CPU x 8 CPUs x the arbitrary multiplier of 2)
  • if I deploy the same software on a cluster of 30 different machines, potentially purchased at different years, I don't need to read the CPU specs and set configuration options for every single one of them.
java concurrency
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2 Answers

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Each processor, or processor core, can do exactly 1 thing at a time. With hyperthreading, things get a little different, but for the most part that still remains true, which is why my HT machine at work almost never goes above 50%, and even when it's at 100%, it's not processing twice as much at once.

You'll probably just have to do some testing on common architectures you plan to deploy on to determine how many threads you want to run on each CPU. Just using 1 thread may be too slow if you're waiting for a lot of I/O. Running a lot of threads will slow things down as the processor will have to switch threads more often, which can be quite costly. I'm not sure if there is any hard-coded limit to how many threads you can run, but I gaurantee that your app would probably come to a crawl from too much thread switching before you reached any kind of hard limit. Ultimately, you should just leave it as an option in the configuration file, so that you can easily tune your app to whatever processor you're running it on.

answered 2008-10-18T17:42:54.430
0

This is a function of the VM, not the CPU. It has to do with the amount of heap consumed per thread. When you run out of space on the heap, you're done. As with other posters, I suspect your app becomes unusable before this point if you exceed the heap space because of thread count.

See this discussion.

answered 2008-10-18T16:50:38.500

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