Ordinarily all struct pages are initialised during early boot in a
single thread. On very large machines this can take a considerable
amount of time. If this option is set, large machines will bring up
a subset of memmap at boot and then initialise the rest in parallel
by starting one-off "pgdatinitX" kernel thread for each node X. This
has a potential performance impact on processes running early in the
lifetime of the system until these kthreads finish the
initialisation.