[dataflow]

> Solaris and Windows both do strict accounting by default, and overcommit is opt-in at a fine-grain API level.

Not sure what you mean by this - I don't think Windows has overcommit in any form, whether opt-in or opt-out. What it does have is virtual address space reservation, but that's separate from commitment; reserved virtual memory is not backed by any page, no matter how much free RAM you have, until you explicitly tell the system to commit physical memory to it.

In fact I'm not even sure 'opt-in' to overcommit is possible in principle. Because if you opt-in to overcommit, you jeopardize other applications' integrity—who likely did not opt-in.

[wahern]

I thought there was a flag or commonly used library function that would do VirtualAlloc(MEM_RESERVER) and then from an in-process page fault handler attempt VirtualAlloc(MEM_COMMIT). But I guess I was wrong? I assume it's possible, just not as common as I thought.

[dataflow]

I don't know of a common (or uncommon) function like this, though I think you could indeed implement it if you really want to (likely via AddVectoredExceptionHandler). It still requires explicitly telling the OS to commit just-in-time, so it's not "overcommitting". The closest built-in thing to this that I know of is PAGE_GUARD, which is internally used for stack extension, but that's all I can think of. The use cases for such a thing would be incredibly niche though—like kind of high-performance sparse page management where every single memory access instruction counts. Like maybe if you're writing a VM or emulator or something. Something that's only appropriate for << 1% of programs.