Re: [PATCH v1 00/10] variable-order, large folios for anonymous memory

[Yu Zhao <yuzhao@google.com>]

On Mon, Jun 26, 2023 at 9:30 PM Yu Zhao <yuzhao@google.com> wrote:
>
> On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
> >
> > Hi All,
> >
> > Following on from the previous RFCv2 [1], this series implements variable order,
> > large folios for anonymous memory. The objective of this is to improve
> > performance by allocating larger chunks of memory during anonymous page faults:
> >
> >  - 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, fewer items on lists, etc. In short, we reduce kernel
> >    overhead. This should benefit all architectures.
> >  - 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 patch set deals with the SW side of things only and based on feedback from
> > the RFC, aims to be the most minimal initial change, upon which future
> > incremental changes can be added. For this reason, the new behaviour is hidden
> > behind a new Kconfig switch, CONFIG_LARGE_ANON_FOLIO, which is disabled by
> > default. Although the code has been refactored to parameterize the desired order
> > of the allocation, when the feature is disabled (by forcing the order to be
> > always 0) my performance tests measure no regression. So I'm hoping this will be
> > a suitable mechanism to allow incremental submissions to the kernel without
> > affecting the rest of the world.
> >
> > The patches are based on top of v6.4 plus Matthew Wilcox's set_ptes() series
> > [2], which is a hard dependency. I'm not sure of Matthew's exact plans for
> > getting that series into the kernel, but I'm hoping we can start the review
> > process on this patch set independently. I have a branch at [3].
> >
> > I've posted a separate series concerning the HW part (contpte mapping) for arm64
> > at [4].
> >
> >
> > Performance
> > -----------
> >
> > Below results show 2 benchmarks; kernel compilation and speedometer 2.0 (a
> > javascript benchmark running in Chromium). Both cases are running on Ampere
> > Altra with 1 NUMA node enabled, Ubuntu 22.04 and XFS filesystem. Each benchmark
> > is repeated 15 times over 5 reboots and averaged.
> >
> > All improvements are relative to baseline-4k. 'anonfolio-basic' is this series.
> > 'anonfolio' is the full patch set similar to the RFC with the additional changes
> > to the extra 3 fault paths. The rest of the configs are described at [4].
> >
> > Kernel Compilation (smaller is better):
> >
> > | kernel          |   real-time |   kern-time |   user-time |
> > |:----------------|------------:|------------:|------------:|
> > | baseline-4k     |        0.0% |        0.0% |        0.0% |
> > | anonfolio-basic |       -5.3% |      -42.9% |       -0.6% |
> > | anonfolio       |       -5.4% |      -46.0% |       -0.3% |
> > | contpte         |       -6.8% |      -45.7% |       -2.1% |
> > | exefolio        |       -8.4% |      -46.4% |       -3.7% |
> > | baseline-16k    |       -8.7% |      -49.2% |       -3.7% |
> > | baseline-64k    |      -10.5% |      -66.0% |       -3.5% |
> >
> > Speedometer 2.0 (bigger is better):
> >
> > | kernel          |   runs_per_min |
> > |:----------------|---------------:|
> > | baseline-4k     |           0.0% |
> > | anonfolio-basic |           0.7% |
> > | anonfolio       |           1.2% |
> > | contpte         |           3.1% |
> > | exefolio        |           4.2% |
> > | baseline-16k    |           5.3% |
>
> Thanks for pushing this forward!
>
> > Changes since RFCv2
> > -------------------
> >
> >   - Simplified series to bare minimum (on David Hildenbrand's advice)
>
> My impression is that this series still includes many pieces that can
> be split out and discussed separately with followup series.
>
> (I skipped 04/10 and will look at it tomorrow.)

I went through the series twice. Here what I think a bare minimum
series (easier to review/debug/land) would look like:
1. a new arch specific function providing a prefered order within (0,
PMD_ORDER).
2. an extended anon folio alloc API taking that order (02/10, partially).
3. an updated folio_add_new_anon_rmap() covering the large() &&
!pmd_mappable() case (similar to 04/10).
4. s/folio_test_pmd_mappable/folio_test_large/ in page_remove_rmap()
(06/10, reviewed-by provided).
5. finally, use the extended anon folio alloc API with the arch
preferred order in do_anonymous_page() (10/10, partially).

The rest can be split out into separate series and move forward in
parallel with probably a long list of things we need/want to do.