Re: [RFC PATCH 0/5] add mTHP support for anonymous share pages

[David Hildenbrand]

On 23.04.24 12:41, Ryan Roberts wrote:
> On 22/04/2024 08:02, Baolin Wang wrote:
> > Anonymous pages have already been supported for multi-size (mTHP) allocation
> > through commit 19eaf44954df, that can allow THP to be configured through the
> > sysfs interface located at '/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled'.
> >
> > However, the anonymous shared pages will ignore the anonymous mTHP rule
> > configured through the sysfs interface, and can only use the PMD-mapped
> > THP, that is not reasonable. Many implement anonymous page sharing through
> > mmap(MAP_SHARED | MAP_ANONYMOUS), especially in database usage scenarios,
> > therefore, users expect to apply an unified mTHP strategy for anonymous pages,
> > also including the anonymous shared pages, in order to enjoy the benefits of
> > mTHP. For example, lower latency than PMD-mapped THP, smaller memory bloat
> > than PMD-mapped THP, contiguous PTEs on ARM architecture to reduce TLB miss etc.
>
> This sounds like a very useful addition!
>
> Out of interest, can you point me at any workloads (and off-the-shelf benchmarks
> for those workloads) that predominantly use shared anon memory?
>
> > The primary strategy is that, the use of huge pages for anonymous shared pages
> > still follows the global control determined by the mount option "huge=" parameter
> > or the sysfs interface at '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
> > The utilization of mTHP is allowed only when the global 'huge' switch is enabled.
> > Subsequently, the mTHP sysfs interface (/sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled)
> > is checked to determine the mTHP size that can be used for large folio allocation
> > for these anonymous shared pages.
>
> I'm not sure about this proposed control mechanism; won't it break
> compatibility? I could be wrong, but I don't think shmem's use of THP used to
> depend upon the value of /sys/kernel/mm/transparent_hugepage/enabled? So it
> doesn't make sense to me that we now depend upon the
> /sys/kernel/mm/transparent_hugepage/hugepage-XXkb/enabled values (which by
> default disables all sizes except 2M, which is set to "inherit" from
> /sys/kernel/mm/transparent_hugepage/enabled).
>
> The other problem is that shmem_enabled has a different set of options
> (always/never/within_size/advise/deny/force) to enabled (always/madvise/never)
>
> Perhaps it would be cleaner to do the same trick we did for enabled; Introduce
> /mm/transparent_hugepage/hugepage-XXkb/shmem_enabled, which can have all the
> same values as the top-level /sys/kernel/mm/transparent_hugepage/shmem_enabled,
> plus the additional "inherit" option. By default all sizes will be set to
> "never" except 2M, which is set to "inherit".

Matches what I had in mind.

--
Cheers,

David / dhildenb