[[RFC][PATCH] v1 0/2] malloc/realloc with transparent huge page support

[Norbert Manthey]

Dear all,

I have been looking for a way to enable huge pages in a user land process
without privileges, and without modifying the source of the process.

Huge pages had to be used explicitly via linking against special libraries.
An alternative are transparent huge pages, which are easier to obtain, but there
are no guarantees of success. Using transparent huge pages can also result in
slowdowns. Many Linux based distributions enable transparent huge pages via
madvise, so that user land processes. With the madvise system call, a process
can control whether allocated memory should be mapped with huge pages. However,
the process typically only sees the allocations returned via a single malloc or
realloc invocation, most of them being rather small.

For a larger impact, I modified the way allocations are done behind the scenes.
For the allocated memory, I changed the properties to (1) align to 2M instead
of 4K, and (2) to call the madvise system call after the brk system call, to
signal that the used memory area should be mapped with huge pages.

The default behavior is not modified. The modification can currently be enabled,
by specifying the environment variable GLIBC_THP_ALWAYS.

### Testing Done

This change has been tested on top of glibc 2.23 in an Ubuntu 16.04 environment
for programs that benefit from huge pages due to their large memory usage and
pseudo-random memory access patterns (e.g. SAT solvers, model checkers,
optimization tools an others). More details on the performance improvements for
these tools can be found in https://arxiv.org/abs/2004.14378, e.g. page 9.

#### glibc tests

Both "make check" and "make xcheck" have not introduced regressions. I tested
on a Ubuntu 20.04, with gcc-9.3 and an Intel(R) Core(TM) i7-5600U CPU.

### Stable branches

I have a set of these patches to be applied to the 2.23 version of glibc. These
patches had to be slightly adapted to be applicable to the current master
branch. Intermediate versions of glibc might require small adaption. The patches
for glibc-2.23 are available at:
https://github.com/conp-solutions/thp/tree/master/ubuntu16.04-thp-env-2.23

Best,
Norbert

Norbert Manthey (2):
  malloc: support transparent huge pages
  malloc: improve THP effectiveness

 malloc/arena.c  |  14 +++--
 malloc/malloc.c | 144 ++++++++++++++++++++++++++++++++++++++++++++----
 2 files changed, 143 insertions(+), 15 deletions(-)

-- 
2.25.1