On 2024/5/8 22:28, Daniel Gomez wrote:
On Wed, May 08, 2024 at 01:58:19PM +0200, David Hildenbrand wrote:
On 08.05.24 13:39, Daniel Gomez wrote:
On Mon, May 06, 2024 at 04:46:24PM +0800, Baolin Wang wrote:
Anonymous pages have already been supported for multi-size (mTHP) allocation
through commit 19eaf44954df, that can allow THP to be configured through the
sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
However, the anonymous shared pages will ignore the anonymous mTHP rule
configured through the sysfs interface, and can only use the PMD-mapped
THP, that is not reasonable. Many implement anonymous page sharing through
mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios,
therefore, users expect to apply an unified mTHP strategy for anonymous pages,
also including the anonymous shared pages, in order to enjoy the benefits of
mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat
than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc.
The primary strategy is similar to supporting anonymous mTHP. Introduce
a new interface '/mm/transparent_hugepage/hugepage-XXkb/shmem_enabled',
which can have all the same values as the top-level
'/sys/kernel/mm/transparent_hugepage/shmem_enabled', with adding a new
additional "inherit" option. By default all sizes will be set to "never"
except PMD size, which is set to "inherit". This ensures backward compatibility
with the shmem enabled of the top level, meanwhile also allows independent
control of shmem enabled for each mTHP.
I'm trying to understand the adoption of mTHP and how it fits into the adoption
of (large) folios that the kernel is moving towards. Can you, or anyone involved
here, explain this? How much do they overlap, and can we benefit from having
both? Is there any argument against the adoption of large folios here that I
might have missed?
mTHP are implemented using large folios, just like traditional PMD-sized THP
are. (you really should explore the history of mTHP and how it all works
internally)
I'll check more in deep the code. By any chance are any of you going to be at
LSFMM this year? I have this session [1] scheduled for Wednesday and it would
be nice to get your feedback on it and if you see this working together with
mTHP/THP.
[1] https://lore.kernel.org/all/4ktpayu66noklllpdpspa3vm5gbmb5boxskcj2q6qn7md3pwwt@kvlu64pqwjzl/
Great. I'm also interested in tmpfs support for large folios (or mTHP),
so please CC me if you plan to send a new version.
As David mentioned, this patchset is mainly about adding mTHP support
for anonymous shmem, and I think that some of the swap support for large
folios could work together.
