transparent huge pages (was: Address bits again)

scott@slp53.sl.home (Scott Lurndal) writes:

Linux supports THP (Transparent Huge Pages) were the OS automatically
updates the translation table to coalsce smaller pages into larger blocks
on intel/amd/arm64 processors.

I have seen good effects from THP a number of years ago: I has seen a
matrix multiply program miss the TLB every time on one of its input
matrices.  And then I ran it again, and the effect was gone.
Eventually I found out that this is due to THP, and I have to disable
THP if I want to demonstrate that effect to my students.

Recently, I thought that by using a 2MB size for an mmap()-allocated
memory block I would get THP on Linux.  Unfortunately, mmap() does not
aligng 2MB blocks to 2MB boundaries, and if it does not, the block is
not eligible for THP.  This was disappointing.

- anton
--
'Anyone trying for "industrial quality" ISA should avoid undefined behavior.'
  Mitch Alsup, <c17fcd89-f024-40e7-a594-88a85ac10d20o@googlegroups.com>