add utils to detect device io characteristics

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/utils/sys/compute.rs

@@ -0,0 +1,74 @@
+//! System utilities related to compute/processing
+
+use std::{cell::Cell, sync::LazyLock};
+
+use crate::is_equal_to;
+
+/// The list of cores available to the process (at startup)
+static CORES_AVAILABLE: LazyLock<u128> = LazyLock::new(|| {
+	core_affinity::get_core_ids()
+		.unwrap_or_default()
+		.into_iter()
+		.map(|core_id| core_id.id)
+		.inspect(|&id| assert!(id < 128, "Core ID must be < 128 at least for now"))
+		.fold(0_u128, |mask, id| mask | (1 << id))
+});
+
+thread_local! {
+	/// Tracks the affinity for this thread. This is updated when affinities
+	/// are set via our set_affinity() interface.
+	static CORE_AFFINITY: Cell<u128> = Cell::default();
+}
+
+/// Set the core affinity for this thread. The ID should be listed in
+/// CORES_AVAILABLE. Empty input is a no-op; prior affinity unchanged.
+pub fn set_affinity<I>(ids: I)
+where
+	I: Iterator<Item = usize>,
+{
+	use core_affinity::{set_for_current, CoreId};
+
+	let mask: u128 = ids.fold(0, |mask, id| {
+		debug_assert!(is_core_available(id), "setting affinity to unavailable core");
+		set_for_current(CoreId { id });
+		mask | (1 << id)
+	});
+
+	if mask.count_ones() > 0 {
+		CORE_AFFINITY.replace(mask);
+	}
+}
+
+/// Get the core affinity for this thread.
+pub fn get_affinity() -> impl Iterator<Item = usize> {
+	(0..128).filter(|&i| ((CORE_AFFINITY.get() & (1 << i)) != 0))
+}
+
+/// Gets the ID of the nth core available. This bijects our sequence of cores to
+/// actual ID's which may have gaps for cores which are not available.
+#[inline]
+#[must_use]
+pub fn get_core_available(i: usize) -> Option<usize> { cores_available().nth(i) }
+
+/// Determine if core (by id) is available to the process.
+#[inline]
+#[must_use]
+pub fn is_core_available(id: usize) -> bool { cores_available().any(is_equal_to!(id)) }
+
+/// Get the list of cores available. The values were recorded at program start.
+#[inline]
+pub fn cores_available() -> impl Iterator<Item = usize> {
+	(0..128).filter(|&i| ((*CORES_AVAILABLE & (1 << i)) != 0))
+}
+
+/// Get the number of threads which could execute in parallel based on the
+/// hardware and administrative constraints of this system. This value should be
+/// used to hint the size of thread-pools and divide-and-conquer algorithms.
+///
+/// * <https://doc.rust-lang.org/std/thread/fn.available_parallelism.html>
+#[must_use]
+pub fn parallelism() -> usize {
+	std::thread::available_parallelism()
+		.expect("Unable to query for available parallelism.")
+		.get()
+}