Subject: Re: [PATCH v6 00/13] Add support for DAX vmemmap optimization for ppc64

From: "Aneesh Kumar K.V" <[email-masked]>
Date: Thu, 27 Jul 2023 11:07:18 +0530

Andrew Morton <[email-masked]> writes:

> On Wed, 26 Jul 2023 10:59:32 +0530 Aneesh Kumar K V <[email-masked]> wrote:
>
>> On 7/26/23 12:59 AM, Andrew Morton wrote:
>> > On Tue, 25 Jul 2023 00:37:46 +0530 "Aneesh Kumar K.V" <[email-masked]> wrote:
>> > 
>> >> This patch series implements changes required to support DAX vmemmap
>> >> optimization for ppc64.
>> > 
>> > Do we have any measurements to help us understand the magnitude
>> > of this optimization?
>> > 
>> > And any documentation which helps users understand whether and
>> > why they should enable this feature?
>> 
>> That is memory space optimization due to kernel reusing the tail page struct pages. The details
>> of the optimization is documented in patch 11. We document there the impact with both 4k and
>> 64K page size.
>
> I suppose that with sufficient arithmetic one could use
> Documentation/powerpc/vmemmap_dedup.rst to figure out the bottom-line
> savings.
>
> I was more expecting a straightforward statement in the [0/N] overview
> to help people understand why they're reading this patchset at all.
> Like "saves 5% of total memory on my XXX machine".

This is specific to devdax usage and also depends on devdax alignment.
The actual saving details are also documented in mm/vmemmap_dedup.rst.
The saving will be based on the devdax device memory size and aligment.

With 64K page size for 16384 pages added (1G) we save 14 pages
With 4K page size for 262144 pages added (1G) we save 4094 pages
With 4K page size for 512 pages added (2M) we save 6 pages

-aneesh