urna_runtime/
mmap_cold.rs

1//! OS-level mmap hints, carved out of `mmap_file.rs`, which opens and
2//! validates the file.
3
4use crate::mmap_file::MmapUrnaFile;
5
6impl MmapUrnaFile {
7    /// Hint to the OS that the mmap pages won't be needed soon. The
8    /// next read will fault them back in from disk.
9    ///
10    /// **Caveat:** this is `posix_madvise(MADV_DONTNEED)` - an
11    /// approximation of cold cache, NOT a guarantee. The OS may
12    /// ignore the hint, keep pages around for prefetch, or return
13    /// them from the kernel's page cache anyway. Use it for
14    /// "madvise-cold" benchmarks (a useful upper bound on the warm
15    /// case) but don't claim it's equivalent to a fresh boot. Real
16    /// cold-cache benchmarks need `purge` (macOS) or `echo 3 >
17    /// /proc/sys/vm/drop_caches` (Linux), both of which require
18    /// privileged operations.
19    #[cfg(unix)]
20    pub fn madvise_cold(&self) {
21        use std::ffi::c_void;
22        // SAFETY: passing a valid mmap pointer + length. POSIX_MADV_DONTNEED
23        // does not invalidate or move the mapping; we still hold the Mmap
24        // and can read from it as before.
25        unsafe {
26            libc::posix_madvise(
27                self._mmap.as_ptr() as *mut c_void,
28                self._mmap.len(),
29                libc::POSIX_MADV_DONTNEED,
30            );
31        }
32    }
33
34    /// No-op on non-Unix platforms.
35    #[cfg(not(unix))]
36    pub fn madvise_cold(&self) {}
37}
urna_runtime/mmap_cold.rs

urna_runtime/
mmap_cold.rs
