Re: [PATCH 0/2] Support Armv8.9/v9.4 FEAT_HAFT

[Marc Zyngier <maz@kernel.org>]

On Tue, 06 Aug 2024 04:43:52 +0100,
Yicong Yang <yangyicong@huawei.com> wrote:
> 
> On 2024/8/2 18:40, Marc Zyngier wrote:
> > On Fri, 02 Aug 2024 10:34:56 +0100,
> > Yicong Yang <yangyicong@huawei.com> wrote:
> >>
> >> From: Yicong Yang <yangyicong@hisilicon.com>
> >>
> >> This series adds basic support for FEAT_HAFT introduced in Armv8.9/v9.4
> >> and enable ARCH_HAS_NONLEAF_PMD_YOUNG. The latter will be used in
> >> lru-gen aging. Tested with lru-gen in below steps:
> >> 1. Generate a 1GiB workingset by `stress-ng --vm 1`. Then hang the task to
> >>    stop accessing the memory. (AF bit won't be updated)
> >> 2. try to age the memory by /sys/kernel/debug/lru_gen
> >>
> >> Run above steps with LRU_GEN_NONLEAF_YOUNG(0x4) and not respectively
> >> (switching by /sys/kernel/mm/lru_gen/enabled). LRU_GEN_NONLEAF_YOUNG
> >> will clear and test the PMD AF bit on page walking for aging,
> >> otherwise will clear and test the PTE AF bit for aging. In this case
> >> LRU_GEN_NONLEAF_YOUNG will improve the efficiency of page scanning
> >> since pages won't be accessed and we don't need to scan each PTE.
> > 
> > Improve by how much? Can you please publish numbers that demonstrate
> > the effect of this feature?
> > 
> 
> With LRU_GEN_NONLEAF_YOUNG ~40% time saved for 1GiB memory observed on our
> emulated platform.

This certainly looks impressive, but it is a very ad-hoc benchmark,
and emulation numbers don't necessarily result in similar improvement
on actual HW.

How does this translate for a more realistic/useful workload? Even
numbers obtained on another architecture would be useful.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.