Re: [PATCH v2 7/7] s390/sparsemem: reduce section size to 128 MiB

[David Hildenbrand]

On 14.10.24 19:53, Heiko Carstens wrote:
> On Mon, Oct 14, 2024 at 04:46:19PM +0200, David Hildenbrand wrote:
> > Ever since commit 421c175c4d609 ("[S390] Add support for memory hot-add.")
> > we've been using a section size of 256 MiB on s390 and 32 MiB on s390.
> > Before that, we were using a section size of 32 MiB on both
> > architectures.
> >
> > Likely the reason was that we'd expect a storage increment size of
> > 256 MiB under z/VM back then. As we didn't support memory blocks spanning
> > multiple memory sections, we would have had to handle having multiple
> > memory blocks for a single storage increment, which complicates things.
> > Although that issue reappeared with even bigger storage increment sizes
> > later, nowadays we have memory blocks that can span multiple memory
> > sections and we avoid any such issue completely.
>
> I doubt that z/VM had support for memory hotplug back then already; and the
> sclp memory hotplug code was always written in a way that it could handle
> increment sizes smaller, larger or equal to section sizes.
> > If I remember correctly the section size was also be used to 
represent each
> piece of memory in sysfs (aka memory block). So the different sizes were
> chosen to avoid an excessive number of sysfs entries on 64 bit.
> > This problem went away later with the introduction of 
memory_block_size.
> Even further back in time I think there were static arrays which had
> 2^(MAX_PHYSMEM_BITS - SECTION_SIZE_BITS) elements.

Interesting. I'll drop the "Likely ..." paragraph then!

> I just gave it a try and, as nowadays expected, bloat-o-meter doesn't
> indicate anything like that anymore.
>
> > 128 MiB has been used by x86-64 since the very beginning. arm64 with 4k
> > base pages switched to 128 MiB as well: it's just big enough on these
> > architectures to allows for using a huge page (2 MiB) in the vmemmap in
> > sane setups with sizeof(struct page) == 64 bytes and a huge page mapping
> > in the direct mapping, while still allowing for small hot(un)plug
> > granularity.
> >
> > For s390, we could even switch to a 64 MiB section size, as our huge page
> > size is 1 MiB: but the smaller the section size, the more sections we'll
> > have to manage especially on bigger machines. Making it consistent with
> > x86-64 and arm64 feels like te right thing for now.
>
> That's fine with me.
>
> Acked-by: Heiko Carstens <hca@xxxxxxxxxxxxx>

Thanks!

--
Cheers,

David / dhildenb