On 25.09.23 10:51, Ryan Roberts wrote:
On 23/09/2023 01:33, John Hubbard wrote:
On 9/22/23 08:48, Ryan Roberts wrote:
...
I never had any feedback on the below; I'm not sure if that means everyone is
happy or that nobody read it??
One can never really know: zero or more people read it, and of those, no
one hated it enough to send out a quick NAK. So that's a *possible*,
lukewarm endorsement of sorts. Success! :)
You really know how to fill a guy with confidence! ;-)
...
BUT I've had yet another idea on the controls front, which would enable exposing
this to user space as an extension to transparent_hugepage, while continuing to
support THP as is and also be able to control THP and ALF (anon large folio)
The new ALF / ANON_LARGE_FOLIO naming looks good to me. The grep aspect
is a nice touch.
Well if we go the route of the newest proposal, then I guess the naming is less
important, because it all attaches to transparent_hugepage.
...
Add 2 controls to sysfs:
/sys/kernel/mm/transparent_hugepage/anon_orders
- bitfield where set bits are orders that will be tried during allocation
- defaults to 1<<PMD_ORDER, which gives current THP behaviour with no ALF
- For now, 1<<PMD_ORDER is highest settable bit, but easy to expand in future
- To enable ALF, set the appropriate lower bits
- To disable THP, clear 1<<PMD_ORDER
- (In future we could add an "auto" option too)
/sys/kernel/mm/transparent_hugepage/anon_always_mask
- orders in (anon_orders & anon_always_mask) are not subject to madvise
- so when enabled=madvise, still try (anon_orders & anon_always_mask) orders
as if enabled=always
- defaults to 0 (all subject to madvise)
I *think* I like this a lot,
On the weight of this lukewarm endorsement, I'm going to code it up and aim to
post something for dicussion end of this week. ;-)
although I have some clarifying question
below. It seems to address the key things that have been complicating
the discussions: the API is now looking more flexible, and yet still
easy to understand and reason about. Nice.
A couple of questions about how this works:
The defaults for those controls give you "legacy THP". But you can modify the
controls to generate policies like this:
For these tables, a small key or legend would help. I've forgotten already
what "S" means, and am also vague about exactly what "THP>ALF>S" behavior
means, too.
THP:
transparent hugepage allocation; specifically PMD sized/aligned/mapped.
ALF:
anonymous large folio allocation; specifically some order between
[PMD_ORDER-1, 1]. Always PTE-mapped.
^ and that's exactly not where we wanted to draw the line to be
future-proof. Ideally we'd create something that is future proof, such
that it could be extended to any folio sizes in the future.
--
Cheers,
David / dhildenb
