Take this:

public class Hello {
public static void main(String... args) throws Throwable {
System.out.println("Hello World");
System.in.read();
}
}

Config system to accept THP:

$ cat /sys/kernel/mm/transparent_hugepage/{enabled,defrag}
always [madvise] never
always defer defer+madvise [madvise] never

Run with:

$ java -XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC -Xmx100g -Xms100g -XX:+AlwaysPreTouch -XX:+UseTransparentHugePages Hello

Observe heap mapping in /proc/$pid/smaps:

7fd958000000-7ff2bc000000 rw-p 00000000 00:00 0
Size: 106496000 kB
KernelPageSize: 4 kB
MMUPageSize: 4 kB
Rss: 315712 kB
Pss: 315712 kB
Shared_Clean: 0 kB
Shared_Dirty: 0 kB
Private_Clean: 0 kB
Private_Dirty: 315712 kB
Referenced: 315712 kB
Anonymous: 315712 kB
LazyFree: 0 kB
AnonHugePages: 0 kB
ShmemPmdMapped: 0 kB
Shared_Hugetlb: 0 kB
Private_Hugetlb: 0 kB
Swap: 0 kB
SwapPss: 0 kB
Locked: 0 kB
VmFlags: rd wr mr mw me ac sd

It is not really committed (RSS/PSS is low), and no THP is used (AnonHugePages is zero).

JDK-8213927 fixed the similar issue in G1, we need to fix it in Shenandoah as well.

On one hand, this is AlwaysPreTouch problem, but we also have to communicate heap alignment to heap reservation code to use any flavor of large pages.