Does multi-threading equal less CPU?

Question

I have a small list of rather large files that I want to process, which got me thinking...

In C#, I was thinking of using Parallel.ForEach of TPL to take advantage of modern multi-core CPUs, but my question is more of a hypothetical character;

Does the use of multi-threading in practicality mean that it would take longer time to load the files in parallel (using as many CPU-cores as possible), as opposed to loading each file sequentially (but with probably less CPU-utilization)?

Or to put it in another way (:

What is the point of multi-threading? More tasks in parallel but at a slower rate, as opposed to focusing all computing resources on one task at a time?

Answer 1

In order to not increase latency, parallel computational programs typically only create one thread per core. Applications which aren't purely computational tend to add more threads so that the number of runnable threads is the number of cores (the others are in I/O wait, and not competing for CPU time).

Now, parallelism on disk-I/O bound programs may well cause performance to decrease, if the disk has a non-negligible seek time then much more time will be wasted performing seeks and less time actually reading. This is called "churning" or "thrashing". Elevator sorting helps somewhat, true random access (such as solid state memories) helps more.

Parallelism does almost always increase the total raw work done, but this is only important if battery life is of foremost importance (and by the time you account for power used by other components, such as the screen backlight, completing quicker is often still more efficient overall).