> On Mar 1, 2024, at 12:29, Matthew Wilcox <willy@xxxxxxxxxxxxx> 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.
