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

[Dan Williams <dan.j.williams@intel.com>]

On Tue, Dec 8, 2020 at 9:32 AM Joao Martins <joao.m.martins@oracle.com> 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/

Clever!

>
> 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)

Nice!

>
> 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. IOW no
> freeing of pages of already initialized vmemmap like the case for hugetlbfs,
> which simplifies the logic (besides not being arch-specific). After these,
> there's quite visible region bootstrap of pmem memmap given that we would
> initialize fewer struct pages depending on the page size.
>
>     NVDIMM namespace bootstrap improves from ~750ms to ~190ms/<=1ms on emulated NVDIMMs
>     with 2M and 1G respectivally. The net gain in improvement is similarly observed
>     in proportion when running on actual NVDIMMs.

I
>
> Patch 6 - 8: Optimize grabbing/release a page refcount changes given that we
> are working with compound pages i.e. we do 1 increment/decrement to the head
> page for a given set of N subpages compared as opposed to N individual writes.
> {get,pin}_user_pages_fast() for zone_device with compound pagemap consequently
> improves considerably, and unpin_user_pages() improves as well when passed a
> set of consecutive pages:
>
>                                            before          after
>     (get_user_pages_fast 1G;2M page size) ~75k  us -> ~3.2k ; ~5.2k us
>     (pin_user_pages_fast 1G;2M page size) ~125k us -> ~3.4k ; ~5.5k us

Compelling!

>
> The RDMA patch (patch 8/9) is to demonstrate the improvement for an existing
> user. For unpin_user_pages() we have an additional test to demonstrate the
> improvement.  The test performs MR reg/unreg continuously and measuring its
> rate for a given period. So essentially ib_mem_get and ib_mem_release being
> stress tested which at the end of day means: pin_user_pages_longterm() and
> unpin_user_pages() for a scatterlist:
>
>     Before:
>     159 rounds in 5.027 sec: 31617.923 usec / round (device-dax)
>     466 rounds in 5.009 sec: 10748.456 usec / round (hugetlbfs)
>
>     After:
>     305 rounds in 5.010 sec: 16426.047 usec / round (device-dax)
>     1073 rounds in 5.004 sec: 4663.622 usec / round (hugetlbfs)

Why does hugetlbfs get faster for a ZONE_DEVICE change? Might answer
that question myself when I get to patch 8.

>
> Patch 9: Improves {pin,get}_user_pages() and its longterm counterpart. It
> is very experimental, and I imported most of follow_hugetlb_page(), except
> that we do the same trick as gup-fast. In doing the patch I feel this batching
> should live in follow_page_mask() and having that being changed to return a set
> of pages/something-else when walking over PMD/PUDs for THP / devmap pages. This
> patch then brings the previous test of mr reg/unreg (above) on parity
> between device-dax and hugetlbfs.
>
> Some of the patches are a little fresh/WIP (specially patch 3 and 9) and we are
> still running tests. Hence the RFC, asking for comments and general direction
> of the work before continuing.

Will go look at the code, but I don't see anything scary conceptually
here. The fact that pfn_to_page() does not need to change is among the
most compelling features of this approach.
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