Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

David Hildenbrand
2023-12-05T17:22:41Z

On 04.12.23 11:20, Ryan Roberts wrote:
> Hi All,
>
> A new week, a new version, a new name... This is v8 of a series to implement
> multi-size THP (mTHP) for anonymous memory (previously called "small-sized THP"
> and "large anonymous folios"). Matthew objected to "small huge" so hopefully
> this fares better.
>
> The objective of this is to improve performance by allocating larger chunks of
> memory during anonymous page faults:
>
> 1) Since SW (the kernel) is dealing with larger chunks of memory than base
> pages, there are efficiency savings to be had; fewer page faults, batched PTE
> and RMAP manipulation, reduced lru list, etc. In short, we reduce kernel
> overhead. This should benefit all architectures.
> 2) Since we are now mapping physically contiguous chunks of memory, we can take
> advantage of HW TLB compression techniques. A reduction in TLB pressure
> speeds up kernel and user space. arm64 systems have 2 mechanisms to coalesce
> TLB entries; "the contiguous bit" (architectural) and HPA (uarch).
>
> This version changes the name and tidies up some of the kernel code and test
> code, based on feedback against v7 (see change log for details).
>
> By default, the existing behaviour (and performance) is maintained. The user
> must explicitly enable multi-size THP to see the performance benefit. This is
> done via a new sysfs interface (as recommended by David Hildenbrand - thanks to
> David for the suggestion)! This interface is inspired by the existing
> per-hugepage-size sysfs interface used by hugetlb, provides full backwards
> compatibility with the existing PMD-size THP interface, and provides a base for
> future extensibility. See [8] for detailed discussion of the interface.
>
> This series is based on mm-unstable (715b67adf4c8).

I took a look at the core pieces. Some things might want some smaller
tweaks, but nothing that should stop this from having fun in
mm-unstable, and replacing the smaller things as we move forward.

--
Cheers,

David / dhildenb