Re: [PATCH 2/4] ARM: Disable HIGHPTE on PREEMPT_RT kernels

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Dec 11, 2024, at 15:04, Sebastian Andrzej Siewior wrote:
> On 2024-12-11 14:48:11 [+0100], To Arnd Bergmann wrote:
>> I guess if you have boxes with 4GiB+ and can proof that the performance
>> improves without HIGHPTE (since you don't have to map the page table).
>> The question is then how much of low mem has to be used instead and when
>> does it start to hurt.
>
> Some numbers have been been documented in commit
>    14315592009c1 ("x86, mm: Allow highmem user page tables to be 
> disabled at boot time")
>
> and I would like cite:
> | We could probably handwave up an argument for a threshold at 16G of total
> | RAM.
>
> which means HIGHPTE would make sense with >= 16GiB of memory.

Very useful, thanks!

On x86, that means we can definitely remove HIGHPTE along with
CONFIG_HIGHMEM64G on x86.

On 32-bit ARM, we still need to support LPAE for systems that
require 64-bit addressing. LPAE supports 36 bits of addressing
(up to 64GB), but the largest actual size I've seen mentioned
is 16GB (Hisilicon HiP04, Calxeda Midway servers) and I'm
certain nobody actually requires these to perform well
given that they are no longer useful for the workloads they
were designed for.

There are also a small number of embedded systems with 8GB
(Ti Keystone2, NVidia Tegra3, Marvell Armada XP), but they
are rare enough that turning off HIGHPTE is completely safe.

      Arnd