Recursion calls on different cores?

Question

So I recently was writing program to confirm that Dijkstra mutual exclusion algorithm is working. I decided to use Kotlin coz I didn't want to use C++ and manage memory myself. Today I saw that despite the fact my program is running sequential all cores are in 100% use, is it some JVM optimization? Or maybe Kotlin optimized my recursion calls? I need to mention that my recursion is not tail recursion. Do any of you know why this happens? I did not use threads or coroutines just to be clear.

score 1 · Accepted Answer · answered May 16 '19 at 13:40

Short answer: don't worry!

If you're not using coroutines or explicit threads, then your code should continue to execute in the same thread.

However, there's no guarantee that your thread will always be executed on the same core; the OS is free to schedule it on whichever core it thinks best at each moment. (I don't know what criteria various OSs may use to make those decisions, but it's likely to take account of other threads and processes. And your thread can move between codes quickly enough to confuse any monitoring you may do.)

Also, even if you don't start any other threads, there are likely to be several background threads handling garbage collection, running finalisers, and other housekeeping. If you use a GUI toolkit such as JavaFX or Swing, that will use many threads, as will a framework such as Spring. (These will usually be marked as ‘dæmon’ theads, so they don't prevent the JVM from exiting.)

Finally, the JVM itself is likely to use system threads for background compilation of bytecode, monitoring, and so on. (Different JVM implementations may do this differently, of course.)

(None of this is specific to Kotlin, by the way; it's the same for all Java apps. But it'd almost certainly be different for Kotlin/JS and Kotlin/native.)

So no, activity on multiple cores does not imply that your code has been transformed or rewritten. It just means that you're not running on the bare metal, and should trust the JVM to take care of things!

In this case it was GC that stole my cores, but what u wrote actually clarify a lot of possible questions I could have, thank you — apologiessirnoclue, May 16 '19 at 14:20

score 0 · Answer 2 · answered May 16 '19 at 09:14

0

It was GC, here is Reddit post I created there. https://www.reddit.com/r/Kotlin/comments/bp9y1t/recursion_calls_on_different_cores/

answered May 16 '19 at 09:14

apologiessirnoclue

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Recursion calls on different cores?

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