Question

For example if I use dual core processor and write a java program without using threads. Does it mean the program execution is sequential and it will only use single core among the dual?

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For example if I use dual core processor and write a java program using threads and synchronisation. Does it mean the program execution is parallel and it will use all available cores(in this case two cores)?

If my reasoning is totally wrong then , what is the relation between threading,cores and parallelism?

Answer 1

First, the JVM has a number of background threads that will use multiple CPUs and cores even if the user code never forks another thread. The garbage collector for example will run concurrently in another CPU if possible regardless of the user code.

If your user code never forks another thread, the JVM will never run your code concurrently in multiple CPUs. If you do write your program with multiple threads there is no guarantee that it will be run in multiple CPUs but it is certainly more likely. It depends a lot on what else is running on on the OS and how blocked your threads are. If you threads are consuming a lot of CPU cycles and run for any length of time on a modern OS then yes, your program will use both CPUs.

You can verify this on a Linux OS (and other Unixen) by watching to see if your process consumes more than 100% of CPU at any one time. You can also use ps options to show the underlying threads and their CPU usage. See my answer here: Concurrency of posix threads in multiprocessor machine