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QOTD: Java Thread vs. Java Heap Space

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The following question is quite common and is related to OutOfMemoryError: unable to create new native thread problems during the JVM thread creation process and the JVM thread capacity. 

This is also a typical interview question I ask to new technical candidates (senior role). I recommend that you attempt to provide your own response before looking at the answer.


Why can’t you increase the JVM thread capacity (total # of threads) by expanding the Java heap space capacity via -Xmx?


The Java thread creation process requires native memory to be available for the JVM process. Expanding the Java heap space via the –Xmx argument will actually reduce your Java thread capacity since this memory will be “stolen” from the native memory space.

  • For a 32-bit JVM, the Java heap space is in a race with the native heap, including the thread capacity
  • For a 64-bit JVM, the thread capacity will mainly depend of your OS physical & virtual memory availability along with your current OS process related tuning parameters
In order to better understand this limitation, I now propose to you the following video tutorial.

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Published at DZone with permission of Pierre - Hugues Charbonneau, DZone MVB. See the original article here.

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