MEM_RESERVE gobs of memory on Windows happens instantly.

test.c

/* See https://ghc.haskell.org/trac/ghc/ticket/9706 */

#include <windows.h>
#include <stdio.h>

int main() {
    printf("How many GB to reserve:");
    SIZE_T numGB = 1;
    scanf("%Iu", &numGB);
    SIZE_T toReserve = numGB*1024*1024*1024;
    printf("Trying to reserve %Iu bytes...\n", toReserve);
    LPVOID addr = VirtualAlloc(NULL, toReserve, MEM_RESERVE, PAGE_NOACCESS);
    printf("Got addr: %p\nPress any key...\n", addr);
    getc(stdin);

    return 0;
}