Re: [LSF/MM/BPF TOPIC] Hugetlb Unifications

[Muchun Song <muchun.song@linux.dev>]

> On Mar 1, 2024, at 12:29, Matthew Wilcox <willy@infradead.org> wrote:
> 
> On Thu, Feb 29, 2024 at 05:37:23PM -0800, James Houghton wrote:
>> - It has HVO (which can hopefully be dropped in a memdesc world)
> 
> I've spent a bit of time thinking about this.  I'll keep this x86-64
> specific just to have concrete numbers.
> 
> Currently a 2MB htlb page without HVO occupies 64 * 512 = 32kB.  With HVO,
> it's reduced to 8kB.  A 1GB htlb page occupies 64 * 256k = 8MB, with HVO,
> it's still 8kB (right?)

Correct in the past. In the first version, HVO needs 2 pages (8k) for
vmemmap, however, it only needs only one page (4k) for it whatever the
huge page sizes (2MB or 1GB) now.

> 
> In a memdesc world, a 2MB page without HVO consumes 8 * 512 = 4kB.
> There's no room for savings here.  But a 1GB page takes 8 * 256k = 2MB.
> There's still almost 2MB of savings to be had here, so I suspect some
> people will still want it.

Agree. With 2MB pages, there is no savings with HVO, but it saves a lot
for 1GB huge pages.

> 
> Hopefully Yu Zhao's zone proposal lets us enable HVO for THP.  At least
> 1GB ones.

Hopefully see it.

Thanks.

> 
> I do have a proposal to turn mmap into a much more dynamic data structure
> where we'd go from a fixed 8 bytes per page to around 16 bytes per
> allocation.  But it depends on memdescs working first, and we haven't
> demonstrated that yet, so it's not worth talking about.  It's much more
> complicated than 8 bytes per page, so it may not be worth doing.