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Zeroing memory is probably already really fast compared to other things the system is doing. Something that happened to me a couple years ago: I was writing a b

Zeroing memory is probably already really fast compared to other things the system is doing. Something that happened to me a couple years ago: I was writing a benchmark that needed a lot of memory. I just used a vector<char> for that. Of course, the vector zeroes the memory, so I thought hey, I don't actually need it to be zeroed, so why not just use unique_ptr<char[]>? Well it turns out (and most people here probably know what's coming next) that when you reserve memory, you don't actually get any physical pages. They get reserved as you access them. And since apparently the access pattern was worse (or something to that effect), the benchmark ended up running way slower than it did without explicit zeroing, even if I put the zeroing into the benchmark which it hadn't been previously. It turns out compared to reserving memory, zeroing was essentially inconsequential. So I'm not too optimistic that there are a lot of gains to be had here.

nwallin 7y ago
That's not actually what's happening. When you allocate memory that needs to be initialized to zeroes, it doesn't actually allocate memory and zero it. Nor does it allocate memory without mapping the pages, and when a page fault occurs, allocate and zero it. Here's what actually happens. Upon boot up, the OS allocates a page, zeros it, and marks it read only. When pages are requested in the future that need to be zero, it assigns an address and maps the zero page to it. This zero page could be mapped to hundreds, thousands of pages. If you try to read from the page, it happily tells you it contains zeroes. So you still haven't allocated new physical pages to that address, despite the fact that you're actively using it. Only when you try to write to it does a page fault happen. (because you're trying to write to a read only page) Then it allocates a new page of physical RAM, zeroes it, and gives you a real physical page to use. It's clever hacks and abstractions all the down.
- nwmcsween 7y ago
  You're mixing up zeroing here, virtual memory is backed by physical memory and that is what is getting zeroed, in fact Linux recently got zeroing on free or malloc as a security feature.

nwallin 7y ago

That's not actually what's happening. When you allocate memory that needs to be initialized to zeroes, it doesn't actually allocate memory and zero it. Nor does it allocate memory without mapping the pages, and when a page fault occurs, allocate and zero it. Here's what actually happens. Upon boot up, the OS allocates a page, zeros it, and marks it read only. When pages are requested in the future that need to be zero, it assigns an address and maps the zero page to it. This zero page could be mapped to hundreds, thousands of pages. If you try to read from the page, it happily tells you it contains zeroes. So you still haven't allocated new physical pages to that address, despite the fact that you're actively using it. Only when you try to write to it does a page fault happen. (because you're trying to write to a read only page) Then it allocates a new page of physical RAM, zeroes it, and gives you a real physical page to use. It's clever hacks and abstractions all the down.

nwmcsween 7y ago
You're mixing up zeroing here, virtual memory is backed by physical memory and that is what is getting zeroed, in fact Linux recently got zeroing on free or malloc as a security feature.