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I'm reading about fork and exec for an exam, and my book says that whenever it is needed to run a new (different) process in unix systems, you would fork the current process followed by an execve.

However, it also says that whenever fork is called, the whole memory image of the parent is copied to the new process.

Then my question is: What if you have a process with a really big memory image, and you just want to run a new process? Isn't it a waste of resources to copy all the data from the parent process if you are just going to replace it immediately?

bobbaluba
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    +1 good question, thank you. Often wonder how the answers get rewarded so much more than the question. Without the question there was no answer. And the question was in my head as well :) – humanityANDpeace Aug 29 '13 at 08:01

3 Answers3

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Usually the fork does not actually copy all the memory, but uses a "copy on write" which means that as long as the memory is not modified the same pages are used. However, to avoid not having enough memory later on (should the process write to the memory) enough memory must be allocated.

This means that forking from large process on systems that do not allow overcommitting memory the memory must be available. So, if you have a 8 GB process forking, then for at least a short period of time 16 GB must be available.

See also vfork and posix_spawn for other solutions.

Roger Lindsjö
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    This answer is unfortunately wrong - overcommitting is not part of unix, but an extension by some systems. The most common ones (Linux for example) do not by default require the full 16GB to be available on fork. – Remember Monica Jan 20 '15 at 08:13
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    Even if an OS kernel is wanting to check that there is enough memory available that doesn't mean that it would need the full 8 GB available since many of those pages can be shared by virtue of read only and read/execute only pages. – nategoose Jun 24 '15 at 21:13
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Some systems that are either very old (early unix), or very special (mmu-less linux) or very crappy (windows via cygwin) do need to make a full copy of all pages ("every byte") on fork, so the potential is there.

Modern unix kernels do not copy all the process memory, instead chosing to make a virtual copy. While this involves only a fraction of the copying (page tables need to be copied), this can still be many megabytes and take substantial time.

So the answer is, yes in general, but most modern implementations use hardware to make a fast virtual copy, but even that virtual copy isn't free.

Both old and some modern systems implement a special vfork() call which has somewhat strict limitations (although less strict than the POSIX requireemnts for vfork) but avoid this copy, for performance reasons.

To give some actual numbers, on my GNU/Linux system, I can fork+exit 1340 times per second from a 20MB process, but only 235 times/s on a 2000MB process. In both cases it is faster to vfork+execve, which is somewhat unintuitive, because many people think "fork is fast" and "execve must be slow".

Remember Monica
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No memory copy is made unless one of the process modified the memory, in which case the page will be copied, and if you are calling exec() in the child process right after fork() is called, no copy is made.

Actually I think to make sure exec() is always called before the father process ever write to memory, the child process always run first.

I think you can find this in Advanced Programming in the UNIX

yuyichao
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  • You think wrong, the parent may continue before the child, both in theory and practise. Also, more importantly, a memory copy is still made. – Remember Monica Jan 20 '15 at 08:23
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    This answer most closely describes vfork, rather than fork in some aspects. In vfork the memory is not copied at all, so the host process *is* halted until the child exist or execs. In fork the memory is copied on older systems or marked for copy on write in newer systems, and the host process is able to resume quickly in a multiprocessor system. – Gem Taylor Apr 11 '19 at 16:56