They are not the same thing. Threads share address space and processes don't and that's a crucial difference.
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I said same thing with different flavors. Literally, they are both represented by the same data structure in the kernel. The salient point of the discussion - cost of forking a process versus cost of spawning a thread - is the same. They require different styles of programming, but I was responding to your point on cost.
If you ever need to reason about scheduling in the Linux kernel, then you need to understand this concept. In the eyes of the scheduler, they're all the same.
I understand I was less than clear on my concern about the cost of starting a process versus starting a thread, but that was not all of my point. Besides the convenient shared memory space, threads also should take a little less to start because they don't need a new memory context. In long running threads, this is not much of a problem but if you decide to start a lot of threads, that could make a difference. I don't know if the Linux kernel makes a big deal out of this but I am sure that, as programs get more threaded, the cost of starting a thread will approach the theoretical minimum thanks to our skillful kernel developer friends.
I am not familiar with current (less than 10 year-old) processor architectures, but, back when I was familiar with that stuff, switching context between threads of the same process was a lot less costly than switching between different processes because the memory context is the same and set-up and tear-down was a somewhat costly operation. As processors start getting more cores, it also allows schedulers to keep threads of the same process to the same cores, reducing context-switch overhead (or to power-down less used cores to conserve energy).
If I had a very good threading/multi-core support, I would probably go with Solaris on a 64-thread T2 SPARC thingie, not Linux on 6-thread x86. And that's a good reason for Sun to invest some money on helping convince Guido the GIL is bad and will become worse with time. Because Intel will probably catch up.