原因是参考ruby实现在Linux上关闭了透明巨大页面（THP）。此设置在execve()中保留，如man page中所述：

PR_SET_THP_DISABLE (since Linux 3.15)
              Set  the  state  of  the  "THP  disable"  flag for the calling
              thread.  If arg2 has a nonzero value, the flag is set,  other‐
              wise  it  is cleared.  Setting this flag provides a method for
              disabling transparent huge pages for jobs where the code  can‐
              not  be  modified,  and using a malloc hook with madvise(2) is
              not an option (i.e., statically allocated data).  The  setting
              of  the "THP disable" flag is inherited by a child created via
              fork(2) and is preserved across execve(2).

关闭透明的大页面：

$ echo never | sudo tee /sys/kernel/mm/transparent_hugepage/enabled

将使运行时一致。
添加

prctl(PR_SET_THP_DISABLE, 0 /* or 1 */, 0, 0, 0);

测试程序也将使运行时一致。
The change that introduced this in ruby可能没有考虑到这个设置是跨execve()保留的，因此它会对子进程中的大分配重工作负载产生负面影响。