Re: [PATCH v8 00/10] Multi-size THP for anonymous memory

[Ryan Roberts]

On 06/12/2023 10:22, David Hildenbrand wrote:
> On 06.12.23 11:13, Ryan Roberts wrote:
>> On 05/12/2023 17:21, David Hildenbrand wrote:
>>> On 04.12.23 11:20, Ryan Roberts wrote:
>>>> Hi All,
>>>>
>>>> A new week, a new version, a new name... This is v8 of a series to implement
>>>> multi-size THP (mTHP) for anonymous memory (previously called "small-sized THP"
>>>> and "large anonymous folios"). Matthew objected to "small huge" so hopefully
>>>> this fares better.
>>>>
>>>> The objective of this is to improve performance by allocating larger chunks of
>>>> memory during anonymous page faults:
>>>>
>>>> 1) Since SW (the kernel) is dealing with larger chunks of memory than base
>>>>      pages, there are efficiency savings to be had; fewer page faults,
>>>> batched PTE
>>>>      and RMAP manipulation, reduced lru list, etc. In short, we reduce kernel
>>>>      overhead. This should benefit all architectures.
>>>> 2) Since we are now mapping physically contiguous chunks of memory, we can take
>>>>      advantage of HW TLB compression techniques. A reduction in TLB pressure
>>>>      speeds up kernel and user space. arm64 systems have 2 mechanisms to
>>>> coalesce
>>>>      TLB entries; "the contiguous bit" (architectural) and HPA (uarch).
>>>>
>>>> This version changes the name and tidies up some of the kernel code and test
>>>> code, based on feedback against v7 (see change log for details).
>>>>
>>>> By default, the existing behaviour (and performance) is maintained. The user
>>>> must explicitly enable multi-size THP to see the performance benefit. This is
>>>> done via a new sysfs interface (as recommended by David Hildenbrand - thanks to
>>>> David for the suggestion)! This interface is inspired by the existing
>>>> per-hugepage-size sysfs interface used by hugetlb, provides full backwards
>>>> compatibility with the existing PMD-size THP interface, and provides a base for
>>>> future extensibility. See [8] for detailed discussion of the interface.
>>>>
>>>> This series is based on mm-unstable (715b67adf4c8).
>>>
>>> I took a look at the core pieces. Some things might want some smaller tweaks,
>>> but nothing that should stop this from having fun in mm-unstable, and replacing
>>> the smaller things as we move forward.
>>>
>>
>> Thanks! I'll address your comments and see if I can post another (final??)
>> version next week.
> 
> It's always possible to do incremental changes on top that Andrew will squash in
> the end. I even recall that he prefers that way once a series has been in
> mm-unstable for a bit, so one can better observe the diff and which effects they
> have.
> 

I've responded to all your comments. There are a bunch of changes that I agree
would be good to make (and some which I disagree with - would be good if you get
a chance to respond).

I think I can get all the changes done and tested by Friday. So perhaps it's
simplest to keep this out of mm-unstable until then, and put the new version in
on Friday? Then if there are any more small changes to do, I can do those as diffs?

Thanks,
Ryan