What is the optimal number of threads for performing IO operations in java?

Question

In Goetz\'s \"Java Concurrency in Practice\", in a footnote on page 101, he writes \"For computational problems like this that do not I/O and access no shared data, Ncpu or

Answer 1

Yes, 20 threads can definitely write to disk faster than 4 threads on a 4 CPU machine. Many real programs are I/O bound more than CPU bound. However, it depends in great detail on your disks and how much CPU work your other threads are doing before they, too, end up waiting on those disks.

If all of your threads are solely writing to disk and doing nothing else, then it may well be that 1 thread on a 4 CPU machine is actually the fastest way to write to disk. It depends entirely on how many disks you have, how much data you're writing, and how good your OS is at I/O scheduling. Your specific question suggests you want 4 threads all writing to the same file. That doesn't make much sense, and in any practical scenario I can't think how that'd be faster. (You'd have to allocate the file ahead of time, then each thread would seek() to a different position, and you'd end up just thrashing the write head as each thread tried to write some blocks.)

The advantage of multithreading is much simpler when you're network bound. Ie: waiting on a database server, or a web browser, or the like. There you're waiting on multiple external resources.