parallel processing with infinite stream in Java

Question

Why the below code doesn\'t print any output whereas if we remove parallel, it prints 0, 1?

IntStream.iterate(0, i -> ( i + 1 ) % 2)
         .parallel()
         


        

Answer 1

This code has a major problem, even without the parallel: After .distinct(), the stream will only have 2 elements- so the limit never kicks in- it will print the two elements and then continue wasting your CPU time indefinitely. This might have been what you intended though.

With parallel and limit, I believe the problem is exacerbated due to the way the work is divided. I haven't traced all the way through the parallel stream code, but here is my guess:

The parallel code divides the work between multiple threads, which all chug along indefinitely as they never fill their quota. The system probably waits for each thread to finish so it can combine their results to ensure distinctness in order- but this will never happen in the case you provide.

Without the order requirement, results from each worker thread can be immediately uses after being checked against a global distinctness set.

Without limit, I suspect that different code is used to handle infinite streams: instead of waiting for the required 10 to fill up, results are reported as discovered. Its a little like making an iterator that reports hasNext() = true, first produces 0, then 1, then the next() call hangs forever without producing a result- in the parallel case something is waiting for several reports so it can properly combine/order them before outputting, while in the serial case it does what it can then hangs.

Ill try to find the exact difference in call stack with and without distinct() or limit(), but so far it seems very difficult to navigate the rather complex stream library calling sequence.