Does multithreading make sense for IO-bound operations?

Most of the answers so far have had to do with the OS scheduler. However, there is a more important factor that I think would lead to your answer. Are you writing to a single physical disk, or multiple physical disks?

Even if you parallelize with multiple threads...IO to a single physical disk is intrinsically a serialized operation. Each thread would have to block, waiting for its chance to get access to the disk. In this case, multiple threads are probably useless...and may even lead to contention problems.

However, if you are writing multiple streams to multiple physical disks, processing them concurrently should give you a boost in performance. This is particularly true with managed disks, like RAID arrays, SAN devices, etc.

I don't think the issue has much to do with the OS scheduler as it has more to do with the physical aspects of the disk(s) your writing to.

That depends on your definition of "I/O bound" but generally multithreading has two effects:

• Use multiple CPUs concurrently (which won't necessarily help if the bottleneck is the disk rather than the CPU[s])

• Use a CPU (with a another thread) even while one thread is blocked (e.g. waiting for I/O completion)

I'm not sure that Konrad's answer is always right, however: as a counter-example, if "I/O bound" just means "one thread spends most of its time waiting for I/O completion instead of using the CPU", but does not mean that "we've hit the system I/O bandwidth limit", then IMO having multiple threads (or asynchronous I/O) might improve performance (by enabling more than one concurrent I/O operation).

I would think it depends on a number of factors, like the kind of application you are running, the number of concurrent users, etc.

I am currently working on a project that has a high degree of linear (reading files from start to finish) operations. We use a NAS for storage, and were concerned about what happens if we run multiple threads. Our initial thought was that it would slow us down because it would increase head seeks. So we ran some tests and found out that the ideal number of threads is the same as the number of cores in the computer.

But your mileage may vary.