Virtual memory clarification - allocation of large contiguous memory

Question

I have an application where I have to allocate on Windows (using operator new) quite a large memory space (hundreds of MB). The application is 32 bit (we don't use 64 bit for now, even on 64 bit systems) and I enabled /LARGEADDRESSAWARE linker option to be able to use 4 GB of user space memory.

Question If I need to allocate, say 450 MB of contiguous memory does the virtual address space of the process need to have a contiguous large enough space and additionally the physical memory does not have to be fragmented on the system ? I ask this because I can make it so that my application reserves a large enough contiguous space but don't know if other applications on the system can affect me in this way. Does OS page tables need to translate contiguous virtual addresses seen by the application into contiguous physical addresses ?

Answer 1

If the memory is simply used in your software, then your 450MB allocation will only need a hole of 450MB in the virtual space. It can be satisfied with pages from every corner of the memory system [as long as there is at least 450MB available somewhere in the system - including swapspace].

Your system will get a little bit better performance if the OS is able to allocate the pages in contiguous blocks of 2MB a piece [using "large pages" of 2MB at a time]. But the system will fall back to individual 4KB pages if it needs to.

One of several benefits with a paged memory architecture is that any physical page can be placed at any virtual address. In some systems, for example Xen virutalization manager in Debug mode, pages are INTENTIONALLY allocated out of sequence, to make it easier to detect when the system makes assumptions about memory pages being contiguous.

Answer 2

You don't need to be concerned about contiguity of the physical memory. That's one thing that virtual to physical address translation helps you with. As long as you can reserve a chunk of the address space and back it with physical memory, wherever it happens to be, things are going to work.