On 18.10.24 21:44, Yang Shi wrote:
On Fri, Oct 18, 2024 at 12:11 PM Zi Yan <ziy@xxxxxxxxxx> wrote:
On 18 Oct 2024, at 14:42, David Hildenbrand wrote:
On 09.10.24 00:37, Zi Yan wrote:
Hi all,
Hi!
Matthew and I have discussed about a different way of splitting large
folios. Instead of split one folio uniformly into the same order smaller
ones, doing buddy allocator like split can reduce the total number of
resulting folios, the amount of memory needed for multi-index xarray
split, and keep more large folios after a split. In addition, both
Hugh[1] and Ryan[2] had similar suggestions before.
The patch is an initial implementation. It passes simple order-9 to
lower order split tests for anonymous folios and pagecache folios.
There are still a lot of TODOs to make it upstream. But I would like to gather
feedbacks before that.
Interesting, but I don't see any actual users besides the debug/test interface wired up.
Right. I am working on it now, since two potential users, anon large folios
and truncate, might need more sophisticated implementation to fully take
advantage of this new split.
For anon large folios, this might be open to debate, if only a subset of
orders are enabled, I assume folio_split() can only split to smaller
folios with the enabled orders. For example, to get one order-0 from
an order-9, and only order-4 (64KB on x86) is enabled, folio_split()
can only split the order-9 to 16 order-0s, 31 order-4s, unless we are
OK with anon large folios with not enabled orders appear in the system.
For anon large folios, deferred split may be a problem too. The
deferred split is typically used to free the unmapped subpages by, for
example, MADV_DONTNEED. But we don't know which subpages are unmapped
without reading their _mapcount by iterating every subpages.
Yeah, and I am still hoping we can get rid of the _mapcounts.
If you know a folio is exclusive, at least synchronously during
MADV_DONTNEED you know what you can try split. Deferred would require an
rmap walk -- if it's really worth it.
--
Cheers,
David / dhildenb
