Linux threads reported as processes

I've noticed in some Linuces, threads are reported as separate processes with separate PIDs. I have a few questions for Anonymous:

1. Why?
2. How can I tell what's the main one?
3. I suppose global memory shared by all threads is now system shared memory, so it won't be reported with ps -el, right? Any way to tell how much memory are these threads/processes sharing?
4. PROTIPs

Thanks, Anonymous.

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