Re: transparent huge pages

anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:

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.

 Yes, to use fixed huge pages, you need to allocate sufficient RAM at
boot time to support the alignment requirements.