Re: [PATCH v8 0/3] memory,x86,acpi: hotplug memory alignment advisement

[David Hildenbrand]

On 02.04.25 11:39, Mike Rapoport wrote:
> On Tue, Apr 01, 2025 at 09:08:31PM +0200, David Hildenbrand wrote:
> > On 01.04.25 20:53, Oscar Salvador wrote:
> > > On Mon, Jan 27, 2025 at 10:34:02AM -0500, Gregory Price wrote:
> > > > v8: nits and tag pickups
> > > >
> > > > When physical address regions are not aligned to memory block size,
> > > > the misaligned portion is lost (stranded capacity).
> > > >
> > > > Block size (min/max/selected) is architecture defined. Most architectures
> > > > tend to use the minimum block size or some simplistic heurist. On x86,
> > > > memory block size increases up to 2GB, and is otherwise fitted to the
> > > > alignment of non-hotplug (i.e. not special purpose memory).
> > >
> > > I wonder if something like this could help us in improving the
> > > ridiculous situation of having 16MB memory-block size on powerpc.
> >
> > They have this granularity because ... they want to add/remove memory in
> > 16MiB on some powerpc dlpar machines :(
>
> I'm not sure they do it today, there's a comment in near define of that 16M
> in arch/powerpc/mm/init_64.c:
>
> /*
>   * Outside hotplug the kernel uses this value to map the kernel direct map
>   * with radix. To be compatible with older kernels, let's keep this value
>   * as 16M which is also SECTION_SIZE with SPARSEMEM. We can ideally map
>   * things with 1GB size in the case where we don't support hotplug.
>   */
>   > and their SECTION_SIZE didn't change since 2005.
> Quite possible that they'll be fine with increasing their
> DEFAULT_MEMORY_BLOCK_SIZE.

At least modern PowerVM on Power10 seems to support LMBs of 128 MiB. 
"Based on this data, Power10 initially only supported LMB sizes of 128MB 
and 256MB." The default usually seems to be 256 MiB.

In reality, the expectation is that the hypervisor will always 
communicate the LMB such that the memory block size will be set to that.

Assuming we'd increase DEFAULT_MEMORY_BLOCK_SIZE to 128MiB, we might 
also be able to increase SECTION_SIZE to 128 MiB I assume.

Not sure about older PowerVM / systems.

[1] 
https://community.ibm.com/community/user/power/blogs/pete-heyrman1/2024/03/06/power10-lmb-sizes

--
Cheers,

David / dhildenb