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//! Platform-independent abstractions over [`memmap2::Mmap::advise`]/[`memmap2::MmapMut::advise`]
//! and [`memmap2::Advice`].
use std::hint::black_box;
use std::io;
use std::num::Wrapping;
use serde::Deserialize;
/// Global [`Advice`] value, to trivially set [`Advice`] value
/// used by all memmaps created by the `segment` crate.
///
/// See [`set_global`] and [`get_global`].
static ADVICE: parking_lot::RwLock<Advice> = parking_lot::RwLock::new(Advice::Random);
/// Set global [`Advice`] value.
///
/// When the `segment` crate creates [`memmap2::Mmap`] or [`memmap2::MmapMut`]
/// _for a memory-mapped, on-disk HNSW index or vector storage access_
/// it will "advise" the created memmap with the current global [`Advice`] value
/// (obtained with [`get_global`]).
///
/// It is recommended to set the desired [`Advice`] value before calling any other function
/// from the `segment` crate and not to change it afterwards.
///
/// The `segment` crate itself does not modify the global [`Advice`] value.
///
/// The default global [`Advice`] value is [`Advice::Random`].
pub fn set_global(advice: Advice) {
*ADVICE.write() = advice;
}
/// Get current global [`Advice`] value.
pub fn get_global() -> Advice {
*ADVICE.read()
}
/// Platform-independent version of [`memmap2::Advice`].
/// See [`memmap2::Advice`] and [`madvise(2)`] man page.
///
/// [`madvise(2)`]: https://man7.org/linux/man-pages/man2/madvise.2.html
#[derive(Copy, Clone, Debug, Deserialize)]
#[serde(rename_all = "snake_case")]
pub enum Advice {
/// See [`memmap2::Advice::Normal`].
Normal,
/// See [`memmap2::Advice::Random`].
Random,
/// See [`memmap2::Advice::Sequential`].
Sequential,
}
#[cfg(unix)]
impl From<Advice> for memmap2::Advice {
fn from(advice: Advice) -> Self {
match advice {
Advice::Normal => memmap2::Advice::Normal,
Advice::Random => memmap2::Advice::Random,
Advice::Sequential => memmap2::Advice::Sequential,
}
}
}
/// Either the global [`Advice`] value or a specific [`Advice`] value.
#[derive(Copy, Clone, Debug)]
pub enum AdviceSetting {
/// Use the global [`Advice`] value (see [`set_global`] and [`get_global`]).
Global,
/// Use the specific [`Advice`] value.
Advice(Advice),
}
impl From<Advice> for AdviceSetting {
fn from(advice: Advice) -> Self {
AdviceSetting::Advice(advice)
}
}
impl AdviceSetting {
/// Get the specific [`Advice`] value.
pub fn resolve(self) -> Advice {
match self {
AdviceSetting::Global => get_global(),
AdviceSetting::Advice(advice) => advice,
}
}
}
/// Advise OS how given memory map will be accessed. On non-Unix platforms this is a no-op.
pub fn madvise(madviseable: &impl Madviseable, advice: Advice) -> io::Result<()> {
madviseable.madvise(advice)
}
/// Generic, platform-independent abstraction
/// over [`memmap2::Mmap::advise`] and [`memmap2::MmapMut::advise`].
pub trait Madviseable {
/// Advise OS how given memory map will be accessed. On non-Unix platforms this is a no-op.
fn madvise(&self, advice: Advice) -> io::Result<()>;
fn populate(&self);
}
impl Madviseable for memmap2::Mmap {
fn madvise(&self, advice: Advice) -> io::Result<()> {
#[cfg(unix)]
self.advise(advice.into())?;
#[cfg(not(unix))]
log::debug!("Ignore {advice:?} on this platform");
Ok(())
}
fn populate(&self) {
#[cfg(target_os = "linux")]
if *POPULATE_READ_IS_SUPPORTED {
match self.advise(memmap2::Advice::PopulateRead) {
Ok(()) => return,
Err(err) => log::warn!(
"Failed to populate with MADV_POPULATE_READ: {err}. \
Falling back to naive approach."
),
}
}
populate_simple(self);
}
}
impl Madviseable for memmap2::MmapMut {
fn madvise(&self, advice: Advice) -> io::Result<()> {
#[cfg(unix)]
self.advise(advice.into())?;
#[cfg(not(unix))]
log::debug!("Ignore {advice:?} on this platform");
Ok(())
}
fn populate(&self) {
#[cfg(target_os = "linux")]
if *POPULATE_READ_IS_SUPPORTED {
match self.advise(memmap2::Advice::PopulateRead) {
Ok(()) => return,
Err(err) => log::warn!(
"Failed to populate with MADV_POPULATE_READ: {err}. \
Falling back to naive approach."
),
}
}
populate_simple(self);
}
}
/// True if `MADV_POPULATE_READ` is supported (added in Linux 5.14).
#[cfg(target_os = "linux")]
static POPULATE_READ_IS_SUPPORTED: std::sync::LazyLock<bool> =
std::sync::LazyLock::new(|| memmap2::Advice::PopulateRead.is_supported());
/// On older Linuxes and non-Unix platforms, we just read every 512th byte to
/// populate the page cache. This is not as efficient as `madvise(2)` with
/// `MADV_POPULATE_READ` but it's better than nothing.
fn populate_simple(slice: &[u8]) {
black_box(
slice
.iter()
.copied()
.map(Wrapping)
.step_by(512)
.sum::<Wrapping<u8>>(),
);
}
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