CONTROL: WHERE THIS FILE'S PAGE CACHE ACTUALLY LIVES - Board A

The clustering result is worthless on its own. If the kernel happened to place
this file's page cache inside a 42 MiB window, then finding all the faults there
says nothing about DRAM. So we measured the real distribution: map 1 GiB of the
same file, fault every page in, and read the frames out of /proc/self/pagemap.
Source is attached as tools/pfnspread.py. Read-only; the mapping is dropped.

file window mapped   : 1024 MiB (262144 pages)
pages present        : 262144 (100.0% of the window)
PFN range            : 0x106620000 .. 0x2fee73000
PHYSICAL SPAN        : 8072.3 MiB

fault window         : 0x1c40a7000 .. 0x1c6a99000 (41.9 MiB)
cache pages INSIDE it: 423 of 262144 (0.161%)

distribution across 256 MiB bins of physical address space:
   0x100000000     2506 pages     9.8 MiB
   0x110000000     9435 pages    36.9 MiB
   0x120000000    26366 pages   103.0 MiB
   0x130000000    26349 pages   102.9 MiB
   0x140000000    20799 pages    81.2 MiB
   0x150000000    13436 pages    52.5 MiB
   0x160000000    13998 pages    54.7 MiB
   0x170000000     6544 pages    25.6 MiB
   0x180000000    15286 pages    59.7 MiB
   0x190000000    22321 pages    87.2 MiB
   0x1a0000000    27643 pages   108.0 MiB
   0x1b0000000    40712 pages   159.0 MiB
   0x1c0000000    13091 pages    51.1 MiB
   0x1d0000000     1685 pages     6.6 MiB
   0x1e0000000      635 pages     2.5 MiB
   0x1f0000000    17486 pages    68.3 MiB
   0x2f0000000     3852 pages    15.0 MiB

=== conclusion ===
The cache is spread over an 8,072 MiB span covering both high System RAM banks,
in every 256 MiB bin across it. It is emphatically not confined to the fault
window: only 0.161% of the cache falls inside. That 0.161% is the correct null
hypothesis here - not a uniform-over-RAM assumption - because it is measured from
the same allocator, the same file and the same machine as the faults.

So if faults were placed the way the page cache is placed, we would expect 0.242
of the 150 frames to land in that window. We observed 148.
