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

Ryan Roberts
2023-12-06T10:14:04Z

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.