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copied!<p>Processes are usually created with <code>fork</code>, threads (lightweight processes) are usually created with <code>clone</code> nowadays. However, anecdotically, there exist 1:N thread models, too, which don't do either.</p> <p>Both <code>fork</code> and <code>clone</code> map to the same kernel function <code>do_fork</code> internally. This function can create a lightweight process that shares the address space with the old one, or a separate process (and many other options), depending on what flags you feed to it. The <code>clone</code> syscall is more or less a direct forwarding of that kernel function (and used by the higher level threading libraries) whereas <code>fork</code> wraps <code>do_fork</code> into the functionality of the 50 year old traditional Unix function.</p> <p>The important difference is that <code>fork</code> guarantees that a complete, separate copy of the address space is made. This, as Basil points out correctly, is done with copy-on-write nowadays and therefore is not nearly as expensive as one would think.<br> When you create a thread, it just reuses the original address space and the same memory.</p> <p>However, one should not assume that creating processes is generally "lightweight" on unix-like systems because of copy-on-write. It is somewhat less heavy than for example under Windows, but it's nowhere near free.<br> One reason is that although the actual pages are not copied, the new process still needs a copy of the page table. This can be several kilobytes to megabytes of memory for processes that use larger amounts of memory. Another reason is that although copy-on-write is invisible and a clever optimization, it is not free, and it cannot do magic. When data is modified by either process, which inevitably happens, the affected pages fault.</p> <p>Redis is a good example where you can see that <code>fork</code> is everything but lightweight (it uses <code>fork</code> to do background saves).</p>

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