alt Hacker NewsEarly BSD VM pre-allocated swap backing for every anonymous page — you couldn't allocate virtual memory without a swap slot reserved for it, even if the page was never paged out.
When a process forks, the child needed swap reservations for the parent's entire address space (before exec replaces it). A large process forking temporarily needs double its swap allocation. If your working set is roughly equal to physical RAM, fork alone gets you to 2x.
This was the practical bottleneck people actually hit. Your system had enough RAM, swap wasn't full, but fork() failed because there wasn't enough contiguous swap to reserve. 2x was the number that made fork() stop failing on a reasonably loaded system.
The later overcommit/copy-on-write changes made this less relevant, but the rule of thumb outlived the technical reason. Most people repeating "2x RAM" today are running systems where anonymous pages aren't swap-backed until actually paged out.
Today swap is no longer about extending your address space, it's about giving the kernel room to page out cold anonymous pages so that RAM can be used for disk cache.
A little swap makes the system faster even when you're nowhere near running out of memory, because the kernel can evict pages it hasn't touched in hours and use that RAM for hot file data instead.
The exception is hibernation — you need swap >= RAM for that, which is why Ubuntu's recommendations are higher than RedHat's 20% of RAM.
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Today's swap also is not preallocated by the user. It is entirely handled by the OS itself. If it needs swap space to hibernate it will go ahead and allocate it itself.
TBF, I think overcommit was and remains an ugliness in how we manage memory. I wish we'd solved the fork commit-charge-spike issue by encouraging vfork (and later, posix_spawn) more heavily, not by making the OS lie about the availability of memory.
The ship's long sailed though, so even I run with overcommit enabled and only grumble about what might have been.