Re: [PATCH 3/4] mm: khugepaged: move mm to list tail when MADV_COLD/MADV_FREE
From: David Hildenbrand (Red Hat)
Date: Thu Dec 18 2025 - 04:34:52 EST
On 12/15/25 10:04, Vernon Yang wrote:
For example, create three task: hot1 -> cold -> hot2. After all three
task are created, each allocate memory 128MB. the hot1/hot2 task
continuously access 128 MB memory, while the cold task only accesses
its memory briefly andthen call madvise(MADV_COLD). However, khugepaged
still prioritizes scanning the cold task and only scans the hot2 task
after completing the scan of the cold task.
So if the user has explicitly informed us via MADV_COLD/FREE that this
memory is cold or will be freed, it is appropriate for khugepaged to
scan it only at the latest possible moment, thereby avoiding unnecessary
scan and collapse operations to reducing CPU wastage.
Here are the performance test results:
(Throughput bigger is better, other smaller is better)
Testing on x86_64 machine:
| task hot2 | without patch | with patch | delta |
|---------------------|---------------|---------------|---------|
| total accesses time | 3.14 sec | 2.92 sec | -7.01% |
| cycles per access | 4.91 | 2.07 | -57.84% |
| Throughput | 104.38 M/sec | 112.12 M/sec | +7.42% |
| dTLB-load-misses | 288966432 | 1292908 | -99.55% |
Testing on qemu-system-x86_64 -enable-kvm:
| task hot2 | without patch | with patch | delta |
|---------------------|---------------|---------------|---------|
| total accesses time | 3.35 sec | 2.96 sec | -11.64% |
| cycles per access | 7.23 | 2.12 | -70.68% |
| Throughput | 97.88 M/sec | 110.76 M/sec | +13.16% |
| dTLB-load-misses | 237406497 | 3189194 | -98.66% |
Again, I also don't like that because you make assumptions on a full process based on some part of it's address space.
E.g., if a library issues a MADV_COLD on some part of the memory the library manages, why should the remaining part of the process suffer as well?
This seems to be an heuristic focused on some specific workloads, no?
--
Cheers
David