Stop setting core affinity on macos (#4676)

# Description of Changes

Explicitly disables core pinning on macos since it's not supported
anyway.

# API and ABI breaking changes

None

# Expected complexity level and risk

1

# Testing

N/A

Should have no behavior change since setting core affinity was basically
a no-op already.

Co-authored-by: clockwork-labs-bot <clockwork-labs-bot@users.noreply.github.com>

crates/core/src/startup.rs

@@ -14,6 +14,7 @@ use tracing_subscriber::{reload, EnvFilter};
 
 use crate::config::{ConfigFile, LogConfig};
 use crate::util::jobs::JobCores;
+use crate::util::thread_scheduling::apply_compute_thread_hint;
 
 pub use core_affinity::CoreId;
 
@@ -362,7 +363,7 @@ impl TokioCores {
             // so this ends up working fine
             builder.on_thread_start(move || {
                 if let Some(core) = cores_queue.pop() {
-                    core_affinity::set_for_current(core);
+                    apply_compute_thread_hint(Some(core));
                 } else {
                     #[cfg(target_os = "linux")]
                     if let Some(cpuset) = &self.blocking {
@@ -388,9 +389,8 @@ impl RayonCores {
             .spawn_handler(thread_spawn_handler(tokio_handle))
             .num_threads(self.0.as_ref().map_or(0, |cores| cores.len()))
             .start_handler(move |i| {
-                if let Some(cores) = &self.0 {
-                    core_affinity::set_for_current(cores[i]);
-                }
+                let core = self.0.as_ref().and_then(|cores| cores.get(i).copied());
+                apply_compute_thread_hint(core);
             })
             .build_global()
             .unwrap()

crates/core/src/util/jobs.rs

@@ -11,6 +11,8 @@ use tokio::runtime;
 use tokio::sync::{mpsc, oneshot, watch};
 use tracing::Instrument;
 
+use crate::util::thread_scheduling::apply_compute_thread_hint;
+
 /// A handle to a pool of Tokio executors for running database WASM code on.
 ///
 /// Each database has a [`SingleCoreExecutor`],
@@ -239,7 +241,7 @@ impl CorePinner {
     #[inline]
     fn do_pin(move_core_rx: &mut watch::Receiver<CoreId>) {
         let core_id = *move_core_rx.borrow_and_update();
-        core_affinity::set_for_current(core_id);
+        apply_compute_thread_hint(Some(core_id));
     }
 
     /// Pin the current thread to the appropriate core.

crates/core/src/util/mod.rs

@@ -8,6 +8,7 @@ pub mod prometheus_handle;
 
 pub mod jobs;
 pub mod notify_once;
+pub mod thread_scheduling;
 
 // TODO: use String::from_utf8_lossy_owned once stabilized
 pub(crate) fn string_from_utf8_lossy_owned(v: Vec<u8>) -> String {

crates/core/src/util/thread_scheduling.rs

@@ -0,0 +1,17 @@
+use core_affinity::CoreId;
+
+/// Apply the current platform's preferred scheduler hint for compute-heavy worker threads.
+///
+/// On Linux and other non-macOS platforms, this uses CPU affinity when a core is provided.
+/// On macOS, scheduler hints are intentionally disabled.
+pub(crate) fn apply_compute_thread_hint(core_id: Option<CoreId>) {
+    #[cfg(target_os = "macos")]
+    {
+        let _ = core_id;
+    }
+
+    #[cfg(not(target_os = "macos"))]
+    if let Some(core_id) = core_id {
+        core_affinity::set_for_current(core_id);
+    }
+}