Re: [PATCH 0/2] support batched checks of the references for large folios

[Baolin Wang <baolin.wang@linux.alibaba.com>]

On 2025/12/2 00:23, David Hildenbrand (Red Hat) wrote:
> On 11/25/25 01:56, Baolin Wang wrote:
>> Currently, folio_referenced_one() always checks the young flag for 
>> each PTE
>> sequentially, which is inefficient for large folios. This inefficiency is
>> especially noticeable when reclaiming clean file-backed large folios, 
>> where
>> folio_referenced() is observed as a significant performance hotspot.
>>
>> Moreover, on Arm architecture, which supports contiguous PTEs, there 
>> is already
>> an optimization to clear the young flags for PTEs within a contiguous 
>> range.
>> However, this is not sufficient. We can extend this to perform batched 
>> operations
>> for the entire large folio (which might exceed the contiguous range: 
>> CONT_PTE_SIZE).
>>
>> By supporting batched checking of the young flags and flushing TLB 
>> entries,
>> I observed a 33% performance improvement in my file-backed folios 
>> reclaim tests.
> 
> Can you point at the benchmark or briefly explain what it does? What 
> exactly are we measuring that improves by 33%?

Sorry for not being clear. I've described the performance test in patch 
2, and I should have copied it to the cover letter:

"
Performance testing:
Allocate 10G clean file-backed folios by mmap() in a memory cgroup, and 
try to reclaim 8G file-backed folios via the memory.reclaim interface. I 
can observe 33% performance improvement on my Arm64 32-core server (and 
10%+ improvement on my X86 machine). Meanwhile, the hotspot 
folio_check_references() dropped from approximately 35% to around 5%.

W/o patchset:
real	0m1.518s
user	0m0.000s
sys	0m1.518s

W/ patchset:
real	0m1.018s
user	0m0.000s
sys	0m1.018s
"