Re: [PATCH 0/8] add mTHP support for anonymous shmem

[David Hildenbrand <david@redhat.com>]

On 08.05.24 13:39, Daniel Gomez wrote:
> On Mon, May 06, 2024 at 04:46:24PM +0800, 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.
>>
>> The primary strategy is similar to supporting anonymous mTHP. Introduce
>> a new interface '/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', with adding a new
>> additional "inherit" option. By default all sizes will be set to "never"
>> except PMD size, which is set to "inherit". This ensures backward compatibility
>> with the shmem enabled of the top level, meanwhile also allows independent
>> control of shmem enabled for each mTHP.
> 
> I'm trying to understand the adoption of mTHP and how it fits into the adoption
> of (large) folios that the kernel is moving towards. Can you, or anyone involved
> here, explain this? How much do they overlap, and can we benefit from having
> both? Is there any argument against the adoption of large folios here that I
> might have missed?

mTHP are implemented using large folios, just like traditional PMD-sized 
THP are. (you really should explore the history of mTHP and how it all 
works internally)

The biggest challenge with memory that cannot be evicted on memory 
pressure to be reclaimed (in contrast to your ordinary files in the 
pagecache) is memory waste, well, and placement of large chunks of 
memory in general, during page faults.

In the worst case (no swap), you allocate a large chunk of memory once 
and it will stick around until freed: no reclaim of that memory.

That's the reason why THP for anonymous memory and SHMEM have toggles to 
manually enable and configure them, in contrast to the pagecache. The 
same was done for mTHP for anonymous memory, and now (anon) shmem follows.

There are plans to have, at some point, have it all working 
automatically, but a lot for that for anonymous memory (and shmem 
similarly) is still missing and unclear.

-- 
Cheers,

David / dhildenb