Re: [LSF/MM/BPF TOPIC] Multi-sized THP performance benchmarks and analysis on ARM64

[Matthew Wilcox <willy@infradead.org>]

On Thu, Apr 04, 2024 at 11:57:03AM -0700, Christoph Lameter (Ampere) wrote:
> On Mon, 1 Apr 2024, Jonathan Cameron wrote:
> 
> > Sounds like useful data, but is it a suitable topic for LSF-MM?
> > What open questions etc is it raising?
> 
> 
> mTHP is new functionality that will require additional work to support more
> use cases. It is also unclear at this point in what usecases mTHP is useful
> and where no benefit can so far be seen. Also the effect of coalescing
> multiple PTE entries into one TLB entry is new to MM (CONT_PTE).
> 
> Ultimately it would be useful to have mTHP support also provide larger
> blocksize capabilities for filesystem etc etc. mTHP needs to mature and an
> analysis of the arguable a bit experimental state of affairs can help a lot
> in getting there.

Have you been paying attention to anything that's been happening in Linux
development in the last three years?  7b230db3b8d3 introduced folios
in December 2020 (was merged in November 2021 for v5.16).  v5.17 (March
2022) did everything short of enabling large folios for the page cache,
which landed in v5.18 (May 2022).  We started using cont-PTEs for large
folios in August 2023.  Again, the page cache led the way here and we're
just adding support for anonymous large folios (called mTHP) now.

There's still a ton of work to do, but we've been busy doing it since
LSFMM in Puerto Rico (2019) with READ_ONLY_THP_FOR_FS being the very
first result from the group of interested developers.

And if you haven't seen the results that Ryan Roberts has posted for
the tests he's run, I suggest you look them up.  He does a great job
of breaking down how much benefit he sees from the hardware side (use of
contPTE) vs the software side (shorter LRU lists, fewer atomic ops).