Can static memory be lazily allocated?

Question

Having a static array in a C program:

#define MAXN (1<<13)
void f() {
    static int X[MAXN];
    //...
}

Can the Linux kernel choose to not map the addresses to physical memory until the each page is actually used? How can X be full of 0s then, is the memory zeroed when each page is accessed? How does that not impact the performance of the program?

Answer 1

Can the Linux kernel choose to not map the addresses to physical memory until the each page is actually used?

Yes, it does this for all memory (except special memory used by drivers and the kernel itself).

How can X be full of 0s then, is the memory zeroed when each page is accessed?

You're supposed to ignore this detail. As long as the memory is full of zeroes when you access it, we say it's full of zeroes.

How does that not impact the performance of the program?

It does.

Answer 2

Can the Linux kernel choose to not map the addresses to physical memory until the each page is actually used?

Yes, with userspace memory it is always done.

How can X be full of 0s then, is the memory zeroed when each page is accessed?

The kernel maintains a page full of 0s, when the user asks for a new page of the static array (static thus full of 0s before first use), the kernel provides the zeroed page, without permissions for the program to write. Writing to the array causes the copy-on-write mechanism to trigger: a page fault occurs, the kernel then allocates a writable page, maps it and resumes the program from the last instruction (the one that couldn't complete because of the page fault). Note that prezeroing optimizations change the implementation details here, but the theory's the same.

How does that not impact the performance of the program?

The program doesn't have to zero a (potentially) lot of pages on start, and the kernel doesn't actually have to have the memory (one can ask for more memory than the system's got, as long as you don't use it). Page faults will be generated during the execution of the program, but they can be minimized, see mmap() and madvise() with MADV_SEQUENTIAL. Remember that the Translation Lookaside Buffer is not infinite, there are so many entries it can maintain.

Sources: A linux memory FAQ, Introduction to Memory Management in Linux by Alan Ott