Re: [PATCH v4 00/12] Optimize anonymous large folio unmapping

["David Hildenbrand (Arm)" <david@kernel.org>]

On 5/26/26 08:36, Dev Jain wrote:
> Speed up unmapping of anonymous large folios by clearing the ptes, and
> setting swap ptes, in one go.
> 
> The following benchmark (stolen from Barry at [1]) is used to measure the
> time taken to swapout 256M worth of memory backed by 64K large folios:
> 
>  #define _GNU_SOURCE
>  #include <stdio.h>
>  #include <stdlib.h>
>  #include <sys/mman.h>
>  #include <string.h>
>  #include <time.h>
>  #include <unistd.h>
>  #include <errno.h>
> 
>  #define SIZE_MB 256
>  #define SIZE_BYTES (SIZE_MB * 1024 * 1024)
> 
>  int main() {
>      void *addr = mmap(NULL, SIZE_BYTES, PROT_READ | PROT_WRITE,
>                        MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
>      if (addr == MAP_FAILED) {
>          perror("mmap failed");
>          return 1;
>      }
> 
>      memset(addr, 0, SIZE_BYTES);
> 
>      struct timespec start, end;
>      clock_gettime(CLOCK_MONOTONIC, &start);
> 
>      if (madvise(addr, SIZE_BYTES, MADV_PAGEOUT) != 0) {
>          perror("madvise(MADV_PAGEOUT) failed");
>          munmap(addr, SIZE_BYTES);
>          return 1;
>      }
> 
>      clock_gettime(CLOCK_MONOTONIC, &end);
> 
>      long duration_ns = (end.tv_sec - start.tv_sec) * 1e9 +
>                         (end.tv_nsec - start.tv_nsec);
>      printf("madvise(MADV_PAGEOUT) took %ld ns (%.3f ms)\n",
>             duration_ns, duration_ns / 1e6);
> 
>      munmap(addr, SIZE_BYTES);
>      return 0;
>  }
> 
> Performance as measured on a Linux VM on Apple M3 (arm64):
> 
> Vanilla - Mean: 37401913 ns, std dev: 12%
> Patched - Mean: 17420282 ns, std dev: 11%

-53%

> 
> No regression observed on 4K folios.
> 
> Performance as measured on bare metal x86:
> 
> Vanilla - mean: 54986286 ns, std dev: 1.5%
> Patched - mean: 51930795 ns, std dev: 3%

That's still -5.5% (with a stddev of 3%).

What happens on larger folio sizes? (e.g., 1 MiB)

-- 
Cheers,

David