WIP: Use MAP_POPULATE

For benchmarking it's quite annoying that the first time a memory is touched
has completely different perf characteristics than subsequent accesses. Using
MAP_POPULATE reduces that substantially.

Author: anarazel

src/backend/port/sysv_shmem.c

@@ -620,7 +620,7 @@ CreateAnonymousSegment(Size *size)
 			allocsize += hugepagesize - (allocsize % hugepagesize);
 
 		ptr = mmap(NULL, allocsize, PROT_READ | PROT_WRITE,
-				   PG_MMAP_FLAGS | mmap_flags, -1, 0);
+				   PG_MMAP_FLAGS | MAP_POPULATE | mmap_flags, -1, 0);
 		mmap_errno = errno;
 		if (huge_pages == HUGE_PAGES_TRY && ptr == MAP_FAILED)
 			elog(DEBUG1, "mmap(%zu) with MAP_HUGETLB failed, huge pages disabled: %m",