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

[David Hildenbrand]

On 06.12.23 11:13, Ryan Roberts wrote:
> On 05/12/2023 17:21, David Hildenbrand wrote:
> > 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.
>
> Thanks! I'll address your comments and see if I can post another (final??)
> version next week.

It's always possible to do incremental changes on top that Andrew will 
squash in the end. I even recall that he prefers that way once a series 
has been in mm-unstable for a bit, so one can better observe the diff 
and which effects they have.

--
Cheers,

David / dhildenb