Re: [PATCH RFC 0/9] mm, sparse-vmemmap: Introduce compound pagemaps

[David Hildenbrand <david@redhat.com>]

On 08.12.20 18:28, Joao Martins wrote:
> Hey,
> 
> This small series, attempts at minimizing 'struct page' overhead by
> pursuing a similar approach as Muchun Song series "Free some vmemmap
> pages of hugetlb page"[0] but applied to devmap/ZONE_DEVICE. 
> 
> [0] https://lore.kernel.org/linux-mm/20201130151838.11208-1-songmuchun@bytedance.com/
> 
> The link above describes it quite nicely, but the idea is to reuse tail
> page vmemmap areas, particular the area which only describes tail pages.
> So a vmemmap page describes 64 struct pages, and the first page for a given
> ZONE_DEVICE vmemmap would contain the head page and 63 tail pages. The second
> vmemmap page would contain only tail pages, and that's what gets reused across
> the rest of the subsection/section. The bigger the page size, the bigger the
> savings (2M hpage -> save 6 vmemmap pages; 1G hpage -> save 4094 vmemmap pages).
> 
> In terms of savings, per 1Tb of memory, the struct page cost would go down
> with compound pagemap:
> 
> * with 2M pages we lose 4G instead of 16G (0.39% instead of 1.5% of total memory)
> * with 1G pages we lose 8MB instead of 16G (0.0007% instead of 1.5% of total memory)
> 

That's the dream :)

> Along the way I've extended it past 'struct page' overhead *trying* to address a
> few performance issues we knew about for pmem, specifically on the
> {pin,get}_user_pages* function family with device-dax vmas which are really
> slow even of the fast variants. THP is great on -fast variants but all except
> hugetlbfs perform rather poorly on non-fast gup.
> 
> So to summarize what the series does:
> 
> Patches 1-5: Much like Muchun series, we reuse tail page areas across a given
> page size (namely @align was referred by remaining memremap/dax code) and
> enabling of memremap to initialize the ZONE_DEVICE pages as compound pages or a
> given @align order. The main difference though, is that contrary to the hugetlbfs
> series, there's no vmemmap for the area, because we are onlining it.

Yeah, I'd argue that this case is a lot easier to handle. When the buddy
is involved, things get more complicated.

-- 
Thanks,

David / dhildenb
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