Re: transparent huge pages

On Thu, 5 Sep 2024 13:45:13 +0000, Anton Ertl wrote:

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.

My 66000 supports large pages with one advancement--the bits not used
to specify a page address are a count of the number of pages allocated
to this large page-1. Thus one could start out with a heap with 1 page
on a large boundary, and then incrementally add pages to the single
PTE by incrementing the pages-count portion in the PTE.

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