X minor pagefault time turned out to be huge

t_sysmmap_c.c

+/* This program benchmarks pagefault time.
+ *
+ * Unfortunately as of 2017-Mar-20 for data in pagecache the situation is as
+ * follows (i7-6600U, Linux 4.9.13):
+ *
+ * 1. minor pagefault:			~ 1200ns
+ *    (this program)
+ *
+ * 2. read syscall + whole page copy:	~  215ns
+ *    (https://github.com/golang/go/issues/19563#issuecomment-287423654)
+ *
+ * 3. it is not possible to mmap(MAP_POPULATE | MAP_NONBLOCK) (i.e. prefault
+ *    those PTE that are already in pagecache).
+ *    ( http://www.spinics.net/lists/linux-man/msg11420.html,
+ *      https://git.kernel.org/linus/54cb8821de07f2ffcd28c380ce9b93d5784b40d7 )
+ *
+ * 4. (Q) I'm not sure a mechanism exists in the kernel to automatically
+ *    subscribe a VMA so that when a page becomes pagecached, associated PTE is
+ *    adjusted so that programs won't need to pay minor pagefault time on
+ *    access.
+ *
+ * unless 3 and 4 are solved mmap unfortunately seems to be slower choice
+ * compared to just pread.
+ */
 #define _GNU_SOURCE
 #include <sys/types.h>
 #include <sys/stat.h>
@@ -47,6 +71,7 @@ int main() {
 		abort();
 	}
 
+#if 1
 	// make sure RAM is actually allocated
 	Tstart = microtime();
 	err = fallocate(fd, /*mode*/0, 0, size);
@@ -56,9 +81,12 @@ int main() {
 		abort();
 	}
 	printf("T(fallocate):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
+#endif
 
 	Tstart = microtime();
-	addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE, fd, 0);
+	addr = mmap(NULL, size, PROT_READ, MAP_SHARED, fd, 0);
+	//addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE, fd, 0);
+	//addr = mmap(NULL, size, PROT_READ, MAP_SHARED | MAP_POPULATE | MAP_NONBLOCK, fd, 0);
 	if (addr == MAP_FAILED) {
 		perror("mmap");
 		abort();
@@ -67,6 +95,7 @@ int main() {
 	printf("T(mmap):\t%.1f\t%6.1f ns / page\n", Tend - Tstart, (Tend - Tstart) * 1E9 / NITER);
 
 	Tstart = microtime();
+	//for (int j=0; j < 100; j++)
 	for (i=0; i<NITER; i++) {
 		sum += addr[i*PAGE_SIZE];
 	}