I suppose it depends on what your concept of threads is. If you're designing a complex system, you're more likely to look at what platforms you must support, a

I suppose it depends on what your concept of threads is. If you're designing a complex system, you're more likely to look at what platforms you must support, and then make up your own (less abstract) notion of threads such that it can be implemented on these systems and that it lets you take more advantage of the capabilities of these systems. I'd be surprised for example if Postgres used pthreads and not its own abstraction, supported by platform-specific implementations.