If I have a program with multithreading and 1 dual-core cpu on working machine, how much threads can I create parallel for these 2 cores? In some articles I saw that cpu cor can handle only on thread. Does it mean that I can create only 2 threads? Or I can create multiple threads, but only 2 of them will be executed by core in a tact? I know, that this question is simple, but, I'm confused a little bit
Modern hardware and OS combinations can easily handle thousands of them. And they can all be in the 'running state'.
However, let's say you have 2000 threads, 2 CPUs in the box, and each CPU has 16 cores, then certainly at most 32 of your running threads are truly 'executing stuff' - the other ones are ready to go if any of he 32 currently actually executing threads do something that isn't CPU-dependent (for example, waiting for some bytes to flow in from the network, or bytes to be read from a disk), or just time passes (eventually the CPU+OS+JVM work together to pre-empt an executing thread - to pause it so that it doesn't hog the resources).
The bottleneck you're most likely to run into first is memory: Each thread needs a stack, and stacks take memory. If for example you are working with 1MB stacks, creating 2000 threads means you now have 2GB worth of stackspace alone, and the java heap (Where all objects live) probably also takes a GB or 2, that's 4. On your average provisioned IAAS box you don't have that much.
A simple solution to that problem is that you can actually control the stack size when you create a new thread object. You can use this to make much smaller stacks, for example, 64k stacks - now 2000 threads only take an eighth of a GB. Of course, a 64k stack is not capable of running particularly deeply nested methods - usually if you're creating thousands of threads, the code they actually end up running should be relatively simple and not create large stacks or do recursive calling.
If you mess this up, you'll get StackOverflowError. Then you know to either adjust the code or increase the stack sizes.
If you're not making thousands of threads (merely, say, hundreds), don't worry about it. Just.. make em and run em, trust the OS+CPU+JVM to sort it out for you.