madv-dontneet.c

#include <sys/mman.h>
#include <sys/types.h>
#include <time.h>
#include <stdio.h>
#include <stdint.h>
#include <stdlib.h>

int main(void)
{
	void *addr = mmap(0, (4 << 20) - 4096, PROT_NONE,
			  MAP_ANONYMOUS|MAP_PRIVATE, 0, 0);
	void *rva;
	int rv;
	if (addr == MAP_FAILED) {
		perror("mmap");
		abort();
	}
	addr = (void *)(((uintptr_t)addr + (2 << 20) - 1) & ~((2 << 20) - 1));
	printf("addr = %p\n", addr);

	rva = mmap(addr, 2 << 20, PROT_READ|PROT_WRITE,
		   MAP_ANONYMOUS|MAP_PRIVATE|MAP_FIXED, 0, 0);
	if (rva == MAP_FAILED) {
		perror("mmap");
		abort();
	}

	rv = madvise(addr, 2 << 20, MADV_HUGEPAGE);
	if (rv) {
		perror("madvise");
		abort();
	}
	*(volatile char *)(addr) = 0;

	/*
         * printf("getchar1\n");
	 * char ch = getchar();
	 *
	 * {
	 * 	rv = madvise(addr, 2 << 20, MADV_DONTNEED);
	 * 	if (rv) {
	 * 		perror("madvise");
	 * 		abort();
	 * 	}
	 * 	printf("getchar2\n");
	 * 	ch = getchar();
	 * 	*(volatile char *)(addr) = 0;
	 * }
         */

	clock_t before, after;
	uintptr_t iters = (1ULL << 18);
	before = clock();
	for (uintptr_t i = 0; i < iters; i++) {
		rv = madvise(addr, 2 << 20, MADV_DONTNEED);
		if (rv) {
			perror("madvise");
			abort();
		}
		if (*(volatile char *)(addr)) {
			abort();
		}
		*(volatile char *)(addr) = 1;
	}
	after = clock();
	printf("iteration took %f us\n",
	       ((double)after - before)/CLOCKS_PER_SEC*1E6/iters);
	return 0;
}