Use a simultaneous hashmap to reduce memory usage with the thread pool?

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I'm working with a program that runs lengthy SQL queries and stores the processed results in a HashMap. Currently, to get around the slow execution time of each of the 20-200 queries, I am using a fixed thread pool and a custom callable to do the searching. As a result, each callable is creating a local copy of the data which it then returns to the main program to be included in the report.

I've noticed that 100 query reports, which used to run without issue, now cause me to run out of memory. My speculation is that because these callables are creating their own copy of the data, I'm doubling memory usage when I join them into another large HashMap. I realize I could try to coax the garbage collector to run by attempting to reduce the scope of the callable's table, but that level of restructuring is not really what I want to do if it's possible to avoid.

Could I improve memory usage by replacing the callables with runnables that instead of storing the data, write it to a concurrent HashMap? Or does it sound like I have some other problem here?


Don't create copy of data, just pass references around, ensuring thread safety if needed. If without data copying you still have OOM, consider increasing max available heap for application.

Drawback of above approach not using copy of data is that thread safety is harder to achieve, though.