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

[David Hildenbrand]

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 ...

> 
>>
>>> 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.

In comparison to CMA / ZONE_MOVABLE, a bad guess won't cause instabilities.

-- 
Thanks,

David / dhildenb