Sorry yep, didn't see your reply earlier.

That's not to say threads aren't expensive, they still require several heavyweight struct allocations on the kernel side, e.g. struct task_struct, which I counted to 1k before getting bored (and wasn't even quarter way through the fields)

Edit: Linux git HEAD with Debian unstable .config:

(gdb) print sizeof(struct task_struct)

$1 = 1904