[PATCH v2 3/5] mm: Default implementation of arch_wants_pte_order()

[Yu Zhao]

On Wed, Jul 5, 2023 at 3:11 AM Ryan Roberts <ryan.roberts at arm.com> wrote:
>
> On 05/07/2023 03:07, Yu Zhao wrote:
> > On Tue, Jul 4, 2023 at 7:20 AM Ryan Roberts <ryan.roberts at arm.com> wrote:
> >>
> >> On 03/07/2023 20:50, Yu Zhao wrote:
> >>> On Mon, Jul 3, 2023 at 7:53 AM Ryan Roberts <ryan.roberts at arm.com> wrote:
> >>>>
> >>>> arch_wants_pte_order() can be overridden by the arch to return the
> >>>> preferred folio order for pte-mapped memory. This is useful as some
> >>>> architectures (e.g. arm64) can coalesce TLB entries when the physical
> >>>> memory is suitably contiguous.
> >>>>
> >>>> The first user for this hint will be FLEXIBLE_THP, which aims to
> >>>> allocate large folios for anonymous memory to reduce page faults and
> >>>> other per-page operation costs.
> >>>>
> >>>> Here we add the default implementation of the function, used when the
> >>>> architecture does not define it, which returns the order corresponding
> >>>> to 64K.
> >>>
> >>> I don't really mind a non-zero default value. But people would ask why
> >>> non-zero and why 64KB. Probably you could argue this is the large size
> >>> all known archs support if they have TLB coalescing. For x86, AMD CPUs
> >>> would want to override this. I'll leave it to Fengwei to decide
> >>> whether Intel wants a different default value.>
> >>> Also I don't like the vma parameter because it makes
> >>> arch_wants_pte_order() a mix of hw preference and vma policy. From my
> >>> POV, the function should be only about the former; the latter should
> >>> be decided by arch-independent MM code. However, I can live with it if
> >>> ARM MM people think this is really what you want. ATM, I'm skeptical
> >>> they do.
> >>
> >> Here's the big picture for what I'm tryng to achieve:
> >>
> >>  - In the common case, I'd like all programs to get a performance bump by
> >> automatically and transparently using large anon folios - so no explicit
> >> requirement on the process to opt-in.
> >
> > We all agree on this :)
> >
> >>  - On arm64, in the above case, I'd like the preferred folio size to be 64K;
> >> from the (admittedly limitted) testing I've done that's about where the
> >> performance knee is and it doesn't appear to increase the memory wastage very
> >> much. It also has the benefits that for 4K base pages this is the contpte size
> >> (order-4) so I can take full benefit of contpte mappings transparently to the
> >> process. And for 16K this is the HPA size (order-2).
> >
> > My highest priority is to get 16KB proven first because it would
> > benefit both client and server devices. So it may be different from
> > yours but I don't see any conflict.
>
> Do you mean 16K folios on a 4K base page system

Yes.

> or large folios on a 16K base
> page system? I thought your focus was on speeding up 4K base page client systems
> but this statement has got me wondering?

Sorry, I should have said 4x4KB.

> >>  - On arm64 when the process has marked the VMA for THP (or when
> >> transparent_hugepage=always) but the VMA does not meet the requirements for a
> >> PMD-sized mapping (or we failed to allocate, ...) then I'd like to map using
> >> contpte. For 4K base pages this is 64K (order-4), for 16K this is 2M (order-7)
> >> and for 64K this is 2M (order-5). The 64K base page case is very important since
> >> the PMD size for that base page is 512MB which is almost impossible to allocate
> >> in practice.
> >
> > Which case (server or client) are you focusing on here? For our client
> > devices, I can confidently say that 64KB has to be after 16KB, if it
> > happens at all. For servers in general, I don't know of any major
> > memory-intensive workloads that are not THP-aware, i.e., I don't think
> > "VMA does not meet the requirements" is a concern.
>
> For the 64K base page case, the focus is server. The problem reported by our
> partner is that the 512M huge page size is too big to reliably allocate and so
> the fauls always fall back to 64K base pages in practice. I would also speculate
> (happy to be proved wrong) that there are many THP-aware workloads that assume
> the THP size is 2M. In this case, their VMAs may well be too small to fit a 512M
> huge page when running on 64K base page system.

Interesting. When you have something ready to share, I might be able
to try it on our ARM servers as well.

> But the TL;DR is that Arm has a partner for which enabling 2M THP on a 64K base
> page system is a very real requirement. Our intent is that this will be the
> mechanism we use to enable it.

Yes, contpte makes more sense for what you described. It'd fit in a
lot better in the hugetlb case, but I guess your partner uses anon.