How to speed up external merge sort in Java?

You might wish to merge k>2 segments at a time. This reduces the amount of I/O from n log k / log 2 to n log n / log k.

I would use memory mapped files. It can be as much as 10x faster than using this type of IO. I suspect it will be much faster in this case as well.

The mapped buffers use virtual memory rather heap space to store data and can be larger than your available physical memory.

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