performance - Threads vs Processes in Linux
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Linux uses a 1-1 threading model, with (to the kernel) no distinction between processes and threads -- everything is simply a runnable task. *
On Linux, the system call clone clones a task, with a configurable level of sharing, among which are:

CLONE_FILES: share the same file descriptor table (instead of creating a copy)
CLONE_PARENT: don't set up a parent-child relationship between the new task and the old (otherwise, child's getppid() = parent's getpid())
CLONE_VM: share the same memory space (instead of creating a COW copy)

fork() calls clone(least sharing) and pthread_create() calls clone(most sharing). **
forking costs a tiny bit more than pthread_createing because of copying tables and creating COW mappings for memory, but the Linux kernel developers have tried (and succeeded) at minimizing those costs.
Switching between tasks, if they share the same memory space and various tables, will be a tiny bit cheaper than if they aren't shared, because the data may already be loaded in cache.  However, switching tasks is still very fast even if nothing is shared -- this is something else that Linux kernel developers try to ensure (and succeed at ensuring).
In fact, if you are on a multi-processor system, not sharing may actually be beneficial to performance: if each task is running on a different processor, synchronizing shared memory is expensive.

* Simplified.  CLONE_THREAD causes signals delivery to be shared (which needs CLONE_SIGHAND, which shares the signal handler table).
** Simplified.  There exist both SYS_fork and SYS_clone syscalls, but in the kernel, the sys_fork and sys_clone are both very thin wrappers around the same do_fork function, which itself is a thin wrapper around copy_process.  Yes, the terms process, thread, and task are used rather interchangeably in the Linux kernel...
