Does it make sense to spawn more than one thread per processor?

Question

From a logical point of view an application may need dozens or hundreds of threads, some of which will we sleeping most of the time, but a very few will be always running co

Answer 1

Sure. If your software makes frequent use of disk or network IO, you can often improve throughput by adding a few more threads. Those extra threads will be awake and doing stuff while the other threads are blocking on IO.

Answer 2

One of the benefits is when you upgrade your hardware, which will likely get more processors/cores.

Answer 3

Everytime you have a task waiting for an I/O operation it does make a sense to enclose it in a thread and fire it up. There's a great probability that your thread will be suspended while waiting for the I/O operation to finish. When it gets waken up the result will be waiting for it.

Answer 4

I found that when writing data parsers which handle larger sets of data over a network it is best to create a thread for each letter of the alphabet (pertaining to the data) and get the program to be more CPU and memory bound. The I/O boundedness inherit with network and disk operations is a major bottleneck so you may as well "get started" on the other data files instead of doing the work sequentially.

On a quad core, it would certainly make sense to start more than four threads. It is unlikely that those 4 threads would be spread across more than one of the cores, especially with todays processor speeds.

Answer 5

Other have talked about situations in which it almost certainly does make sense (when you are doing any kind of slow IO).

It might not be a good idea if:

• your threads are doing CPU bound work

and

• the threads each want to use a lot (i.e. significant compared to the cache size) of memory that does not overlap

In this case there is the possibility of causing unnecessary cache misses.

Answer 6

Short answer is "yes".

Even thought you could gain more from multithreading on a multiprocessor environement, it's still a useful technology on a single processor machine, mainly because it means you'll delegate some work to the process scheduler, who should have much better information than you have.

If you don't multithread, you'll end up doing the scheduling job yourself, which could be a good thing if that's what you need, but will most probably be both tedious and inefficient