Re: [RFC PATCH v2 00/30] 1GB PUD THP support on x86_64

[Roman Gushchin]

On Mon, Oct 05, 2020 at 08:33:44PM +0200, David Hildenbrand wrote:
> On 05.10.20 20:25, Roman Gushchin wrote:
> > On Mon, Oct 05, 2020 at 07:27:47PM +0200, David Hildenbrand wrote:
> >> On 05.10.20 19:16, Roman Gushchin wrote:
> >>> On Mon, Oct 05, 2020 at 11:03:56AM -0400, Zi Yan wrote:
> >>>> On 2 Oct 2020, at 4:30, David Hildenbrand wrote:
> >>>>
> >>>>> On 02.10.20 10:10, Michal Hocko wrote:
> >>>>>> On Fri 02-10-20 09:50:02, David Hildenbrand wrote:
> >>>>>>>>>> - huge page sizes controllable by the userspace?
> >>>>>>>>>
> >>>>>>>>> It might be good to allow advanced users to choose the page sizes, so they
> >>>>>>>>> have better control of their applications.
> >>>>>>>>
> >>>>>>>> Could you elaborate more? Those advanced users can use hugetlb, right?
> >>>>>>>> They get a very good control over page size and pool preallocation etc.
> >>>>>>>> So they can get what they need - assuming there is enough memory.
> >>>>>>>>
> >>>>>>>
> >>>>>>> I am still not convinced that 1G THP (TGP :) ) are really what we want
> >>>>>>> to support. I can understand that there are some use cases that might
> >>>>>>> benefit from it, especially:
> >>>>>>
> >>>>>> Well, I would say that internal support for larger huge pages (e.g. 1GB)
> >>>>>> that can transparently split under memory pressure is a useful
> >>>>>> funtionality. I cannot really judge how complex that would be
> >>>>>
> >>>>> Right, but that's then something different than serving (scarce,
> >>>>> unmovable) gigantic pages from CMA / reserved hugetlbfs pool. Nothing
> >>>>> wrong about *real* THP support, meaning, e.g., grouping consecutive
> >>>>> pages and converting them back and forth on demand. (E.g., 1GB ->
> >>>>> multiple 2MB -> multiple single pages), for example, when having to
> >>>>> migrate such a gigantic page. But that's very different from our
> >>>>> existing gigantic page code as far as I can tell.
> >>>>
> >>>> Serving 1GB PUD THPs from CMA is a compromise, since we do not want to
> >>>> bump MAX_ORDER to 20 to enable 1GB page allocation in buddy allocator,
> >>>> which needs section size increase. In addition, unmoveable pages cannot
> >>>> be allocated in CMA, so allocating 1GB pages has much higher chance from
> >>>> it than from ZONE_NORMAL.
> >>>
> >>> s/higher chances/non-zero chances
> >>
> >> Well, the longer the system runs (and consumes a significant amount of
> >> available main memory), the less likely it is.
> >>
> >>>
> >>> Currently we have nothing that prevents the fragmentation of the memory
> >>> with unmovable pages on the 1GB scale. It means that in a common case
> >>> it's highly unlikely to find a continuous GB without any unmovable page.
> >>> As now CMA seems to be the only working option.
> >>>
> >>
> >> And I completely dislike the use of CMA in this context (for example,
> >> allocating via CMA and freeing via the buddy by patching CMA when
> >> splitting up PUDs ...).
> >>
> >>> However it seems there are other use cases for the allocation of continuous
> >>> 1GB pages: e.g. secretfd ( https://urldefense.proofpoint.com/v2/url?u=https-3A__lwn.net_Articles_831628_&d=DwIDaQ&c=5VD0RTtNlTh3ycd41b3MUw&r=jJYgtDM7QT-W-Fz_d29HYQ&m=mdcwiGna7gQ4-RC_9XdaxFZ271PEQ09M0YtCcRoCkf8&s=4KlK2p0AVh1QdL8XDVeWyXPz4F63pdbbSCoxQlkNaa4&e=  ), where using
> >>> 1GB pages can reduce the fragmentation of the direct mapping.
> >>
> >> Yes, see RFC v1 where I already cced Mike.
> >>
> >>>
> >>> So I wonder if we need a new mechanism to avoid fragmentation on 1GB/PUD scale.
> >>> E.g. something like a second level of pageblocks. That would allow to group
> >>> all unmovable memory in few 1GB blocks and have more 1GB regions available for
> >>> gigantic THPs and other use cases. I'm looking now into how it can be done.
> >>
> >> Anything bigger than sections is somewhat problematic: you have to track
> >> that data somewhere. It cannot be the section (in contrast to pageblocks)
> > 
> > Well, it's not a large amount of data: the number of 1GB regions is not that
> > high even on very large machines.
> 
> Yes, but then you can have very sparse systems. And some use cases would
> actually want to avoid fragmentation on smaller levels (e.g., 128MB) -
> optimizing memory efficiency by turning off banks and such ...

It's a definitely a good question.

> > 
> >>
> >>> If anybody has any ideas here, I'll appreciate a lot.
> >>
> >> I already brought up the idea of ZONE_PREFER_MOVABLE (see RFC v1). That
> >> somewhat mimics what CMA does (when sized reasonably), works well with
> >> memory hot(un)plug, and is immune to misconfiguration. Within such a
> >> zone, we can try to optimize the placement of larger blocks.
> > 
> > Thank you for pointing at it!
> > 
> > The main problem with it is the same as with ZONE_MOVABLE: it does require
> > a boot-time educated guess on a good size. I admit that the CMA does too.
> 
> "Educated guess" of ratios like 1:1. 1:2, and even 1:4 (known from
> highmem times) ares usually perfectly fine. And if you mess up - in
> comparison to CMA - you won't shoot yourself in the foot, you get less
> gigantic pages - which is usually better than before. I consider that a
> clear win. Perfect? No. Can we be perfect? unlikely.

I'm not necessarily opposing your idea, I just think it will be tricky
to not introduce an additional overhead if the ratio is not perfectly
chosen. And there is simple a cost of adding a zone.

But fundamentally we're speaking about the same thing: grouping pages
by their movability on a smaller scale. With a new zone we'll split
pages into two parts with a fixed border, with new pageblock layer
in 1GB blocks.

I think the agreement is that we need such functionality.

Thanks!