Re: remap the .text segment into huge pages at run time

On Wed, Jun 21, 2023 at 10:42 AM Andres Freund <andres(at)anarazel(dot)de> wrote:

> So I am wondering if you're encountering a different kind of problem. As I
> mentioned, I have observed that the pages need to be clean for this to
> work. For me adding a "sync path/to/postgres" makes it work on 6.3.8.
Without
> the sync it starts to work a while later (presumably when the kernel got
> around to writing the data back).

Hmm, then after rebooting today, it shouldn't have that problem until a
build links again, but I'll make sure to do that when building. Still same
failure, though. Looking more closely at the manpage for madvise, it has
this under MADV_HUGEPAGE:

"The MADV_HUGEPAGE, MADV_NOHUGEPAGE, and MADV_COLLAPSE operations are
available only if the kernel was configured with
CONFIG_TRANSPARENT_HUGEPAGE and file/shmem memory is only supported if the
kernel was configured with CONFIG_READ_ONLY_THP_FOR_FS."

Earlier, I only checked the first config option but didn't know about the
second...

$ grep CONFIG_READ_ONLY_THP_FOR_FS /boot/config-$(uname -r)
# CONFIG_READ_ONLY_THP_FOR_FS is not set

Apparently, it's experimental. That could be the explanation, but now I'm
wondering why the fallback

madvise(addr, advlen, MADV_HUGEPAGE);

didn't also give an error. I wonder if we could mremap to some anonymous
region and call madvise on that. That would be more similar to the hack I
shared last year, which may be more fragile, but now it wouldn't
need explicit huge pages.

--
John Naylor
EDB: http://www.enterprisedb.com