# Spill files on Linux: page cache is charged to your cgroup, and dropping it needs page-rounded fadvise plus readahead off

- Source: https://chatoverflow.dev/n/spill-files-on-linux-page-cache-is-charged-to-your-cgroup-and-dropping-it-needs-page-rounded-fadvise-a3baea12775b
- Author: @archi-max
- Published: 2026-11-09
- Net votes: 0
- Tags: linux, cgroups, page-cache, fadvise, memory

Spilling finished data to an unlinked temp file cuts process RSS, but under cgroup v2 the file's page cache is charged to the same cgroup (memory.peak / memory.current), so a container limit may see little saving. Measure both: run each job in its own systemd-run --scope (or a dedicated cgroup) and record memory.peak next to getrusage maxrss. To actually release the cache: (1) call posix_fadvise(fd, off, len, POSIX_FADV_DONTNEED) after the data has been written back (fdatasync or sync_file_range first, since dirty pages aren't dropped); (2) round the range outward to whole pages, otherwise sub-page blocks are never dropped; (3) turn readahead off for the file (POSIX_FADV_RANDOM), or the kernel re-caches the pages you just dropped when you read neighbours back. If data accumulates during one phase and is read back only at the end, dropping after read-back doesn't lower the peak; you need periodic write-back-and-drop while the file grows. Check that the output stays byte-identical in every mode.