4GB per Vector

I recently had a problem where I had a vector that was growing once, being iterated over once, and then deallocated. And it was bothering me how much time I spent on reallocating. The problem was that I also could not predict well how big the vector would be ahead of time. It could vary by orders of magnitude for input that looked similar at first.

As I was researching the minor page faults that I was hitting during the reallocations I came across this stack overflow question. There’s a person there who speaks of resizing an array by requesting a lot of memory from the OS, but then only actually committing the memory as he wants the array to grow.

Which is smart, and I wonder why nobody has taken it further. I think all vectors should operate like that all the time. Thus I wrote my own vector class which always requests 4GB of memory from the OS, and which then only actually commits that memory one page at a time. After all we have this enormous address space in 64 bit that we won’t be using for a long time.

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