Re: [PATCH 0/3] x86: Make 5-level paging support unconditional for x86-64

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jul 31, 2024 at 11:15:05AM +0200, Thomas Gleixner wrote:
> On Wed, Jul 31 2024 at 14:27, Shivank Garg wrote:
> > lmbench:lat_pagefault: Metric- page-fault time (us) - Lower is better
> >                 4-Level PT              5-Level PT		% Change
> > THP-never       Mean:0.4068             Mean:0.4294		5.56
> >                 95% CI:0.4057-0.4078    95% CI:0.4287-0.4302
> >
> > THP-Always      Mean: 0.4061            Mean: 0.4288		% Change
> >                 95% CI: 0.4051-0.4071   95% CI: 0.4281-0.4295	5.59
> >
> > Inference:
> > 5-level page table shows increase in page-fault latency but it does
> > not significantly impact other benchmarks.
> 
> 5% regression on lmbench is a NONO.
> 
> 5-level page tables add a cost in every hardware page table walk. That's
> a matter of fact and there is absolutely no reason to inflict this cost
> on everyone.
> 
> The solution to this to make the 5-level mechanics smarter by evaluating
> whether the machine has enough memory to require 5-level tables and
> select the depth at boot time.

I gotta mention (again) that its a pain we can't mix and match like
s390. They default run their userspace on 4 level, even if the kernel
runs 5. Only silly daft userspace that needs more than insane amounts of
memory get 5 level.