Does multithreading make sense for IO-bound operations?

Question

When performing many disk operations, does multithreading help, hinder, or make no difference?

For example, when copying many files from one folder to another.

Clarification: I understand that when other operations are performed, concurrency will obviously make a difference. If the task was to open an image file, convert to another format, and then save, disk operations can be performed concurrently with the image manipulation. My question is when the only operations performed are disk operations, whether concurrently queuing and responding to disk operations is better.

Answer 1

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.

Answer 2

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).