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

Andrew Morton [off-list ref] writes:

On Wed, 26 Jul 2023 10:59:32 +0530 Aneesh Kumar K V [off-list ref] wrote:

quoted

On 7/26/23 12:59 AM, Andrew Morton wrote:

quoted

On Tue, 25 Jul 2023 00:37:46 +0530 "Aneesh Kumar K.V" [off-list ref] wrote:

quoted

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