refactor: extract sampling primitives into shared module

Move SplitMix64 and draw_exponential from task_dumped.rs into a new
pub(crate) sampling module. Rename draw_exponential_ns → draw_exponential
since the math is unit-agnostic (the caller decides whether 'mean' is
nanoseconds, bytes, or anything else).

This prepares for the memory profiler (commit 2+), which will reuse the
same geometric/Poisson sampling primitive to sample on bytes allocated
rather than nanoseconds idle.

Pure refactor — no behavior change. Existing task-dump tests pass
unmodified (only the function name changed).

Author: rcoh

dial9-tokio-telemetry/src/lib.rs

@@ -16,6 +16,7 @@ pub mod background_task;
 pub(crate) mod metrics;
 pub(crate) mod primitives;
 pub(crate) mod rate_limit;
+pub(crate) mod sampling;
 #[cfg(feature = "taskdump")]
 pub(crate) mod task_dumped;
 /// Core telemetry types, recording, and trace I/O.

dial9-tokio-telemetry/src/sampling.rs

@@ -0,0 +1,90 @@
+//! Shared geometric/Poisson sampling primitives.
+//!
+//! Used by the task-dump idle sampler (sampling on nanoseconds) and by the
+//! memory profiler (sampling on bytes). The unit is opaque to the math —
+//! callers pass the mean and treat the returned i64 as a counter in their
+//! native unit.
+
+/// Minimal splitmix64 PRNG. Fast, no dependencies, good enough for sampling.
+pub(crate) struct SplitMix64(u64);
+
+impl SplitMix64 {
+    pub(crate) fn new(seed: u64) -> Self {
+        Self(seed)
+    }
+
+    pub(crate) fn next_u64(&mut self) -> u64 {
+        self.0 = self.0.wrapping_add(0x9e3779b97f4a7c15);
+        let mut z = self.0;
+        z = (z ^ (z >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);
+        z = (z ^ (z >> 27)).wrapping_mul(0x94d049bb133111eb);
+        z ^ (z >> 31)
+    }
+
+    /// Draw from an exponential distribution with the given mean.
+    /// Returns at least 1 to avoid immediate re-trigger when the
+    /// counter goes to zero.
+    ///
+    /// The unit is whatever the caller treats `mean` as (nanoseconds,
+    /// bytes, etc.).
+    pub(crate) fn draw_exponential(&mut self, mean: u64) -> i64 {
+        // Generate a uniform float in (0, 1] — avoid exact 0 to prevent ln(0).
+        let u = (self.next_u64() >> 11) as f64 / ((1u64 << 53) as f64);
+        let u = if u == 0.0 { f64::MIN_POSITIVE } else { u };
+        let sample = -u.ln() * (mean as f64);
+        (sample as i64).max(1)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::SplitMix64;
+
+    #[test]
+    fn splitmix_deterministic_with_fixed_seed() {
+        let mut rng = SplitMix64::new(42);
+        let a = rng.next_u64();
+        let b = rng.next_u64();
+
+        let mut rng2 = SplitMix64::new(42);
+        assert_eq!(a, rng2.next_u64());
+        assert_eq!(b, rng2.next_u64());
+    }
+
+    #[test]
+    fn draw_exponential_returns_at_least_1() {
+        let mut rng = SplitMix64::new(0);
+        for _ in 0..1000 {
+            assert!(rng.draw_exponential(1) >= 1);
+        }
+    }
+
+    #[test]
+    fn draw_exponential_mean_approximates_target() {
+        let mut rng = SplitMix64::new(123);
+        let mean: u64 = 1024;
+        let n = 100_000;
+        let sum: f64 = (0..n).map(|_| rng.draw_exponential(mean) as f64).sum();
+        let observed_mean = sum / n as f64;
+        // Within ±5% of the configured mean.
+        assert!(
+            (observed_mean - mean as f64).abs() < mean as f64 * 0.05,
+            "observed mean {observed_mean} too far from expected {mean}"
+        );
+    }
+
+    #[test]
+    fn draw_exponential_handles_large_mean() {
+        let mut rng = SplitMix64::new(999);
+        let mean: u64 = 1_000_000_000;
+        let mut saw_large = false;
+        for _ in 0..1000 {
+            let v = rng.draw_exponential(mean);
+            assert!(v >= 1);
+            if v > 1_000_000 {
+                saw_large = true;
+            }
+        }
+        assert!(saw_large, "expected some values much larger than 1");
+    }
+}

dial9-tokio-telemetry/src/task_dumped.rs

@@ -31,6 +31,7 @@
 //! yield points hit during a capture, with offsets recording each callchain's
 //! start. The buffers are reused across polls.
 
+use crate::sampling::SplitMix64;
 use crate::telemetry::format::TaskDumpEvent;
 use crate::telemetry::recorder::SharedState;
 use crate::telemetry::task_metadata::TaskId;
@@ -47,36 +48,6 @@ use std::task::{Context, Poll};
 /// Initial heap reservation for the instruction-pointer buffer on first capture.
 const FRAME_BUF_INITIAL_CAPACITY: usize = 256;
 
-// ─── Minimal PRNG (splitmix64) ──────────────────────────────────────────────
-
-/// Minimal splitmix64 PRNG. Fast, no dependencies, good enough for sampling.
-struct SplitMix64(u64);
-
-impl SplitMix64 {
-    fn new(seed: u64) -> Self {
-        Self(seed)
-    }
-
-    fn next_u64(&mut self) -> u64 {
-        self.0 = self.0.wrapping_add(0x9e3779b97f4a7c15);
-        let mut z = self.0;
-        z = (z ^ (z >> 30)).wrapping_mul(0xbf58476d1ce4e5b9);
-        z = (z ^ (z >> 27)).wrapping_mul(0x94d049bb133111eb);
-        z ^ (z >> 31)
-    }
-
-    /// Draw from exponential distribution with given mean (in nanoseconds).
-    /// Returns at least 1 to avoid immediate re-trigger.
-    fn draw_exponential_ns(&mut self, mean_ns: u64) -> i64 {
-        // Generate a uniform float in (0, 1] — avoid exact 0 to prevent ln(0).
-        let u = (self.next_u64() >> 11) as f64 / ((1u64 << 53) as f64);
-        let u = if u == 0.0 { f64::MIN_POSITIVE } else { u };
-        let sample = -u.ln() * (mean_ns as f64);
-        // Clamp to at least 1ns so we never immediately re-trigger.
-        (sample as i64).max(1)
-    }
-}
-
 // ─── TaskDumped future wrapper ──────────────────────────────────────────────
 
 pin_project! {
@@ -121,7 +92,7 @@ impl<F> TaskDumped<F> {
             }
         };
         let mut rng = SplitMix64::new(seed);
-        let next_sample_ns = rng.draw_exponential_ns(sample_mean_ns);
+        let next_sample_ns = rng.draw_exponential(sample_mean_ns);
         Self {
             inner,
             shared,
@@ -170,7 +141,7 @@ impl<F: Future> Future for TaskDumped<F> {
                 .expect("checked in match above")
                 .get();
             this.frames.emit(this.shared, *this.task_id, ts);
-            *this.next_sample_ns = this.rng.draw_exponential_ns(*this.sample_mean_ns);
+            *this.next_sample_ns = this.rng.draw_exponential(*this.sample_mean_ns);
         }
         match &result {
             Poll::Ready(_) => {
@@ -319,7 +290,7 @@ impl Encodable for TaskDumpData<'_> {
 
 #[cfg(test)]
 mod tests {
-    use super::SplitMix64;
+    use crate::sampling::SplitMix64;
 
     #[test]
     fn splitmix64_deterministic() {
@@ -333,13 +304,11 @@ mod tests {
     }
 
     #[test]
-    fn draw_exponential_ns_mean_is_reasonable() {
+    fn draw_exponential_mean_is_reasonable() {
         let mut rng = SplitMix64::new(123);
         let mean_ns: u64 = 10_000_000; // 10ms
         let n = 10_000;
-        let sum: f64 = (0..n)
-            .map(|_| rng.draw_exponential_ns(mean_ns) as f64)
-            .sum();
+        let sum: f64 = (0..n).map(|_| rng.draw_exponential(mean_ns) as f64).sum();
         let observed_mean = sum / n as f64;
         // Within 10% of the configured mean.
         assert!(
@@ -349,10 +318,10 @@ mod tests {
     }
 
     #[test]
-    fn draw_exponential_ns_always_positive() {
+    fn draw_exponential_always_positive() {
         let mut rng = SplitMix64::new(0);
         for _ in 0..10_000 {
-            assert!(rng.draw_exponential_ns(1_000_000) >= 1);
+            assert!(rng.draw_exponential(1_000_000) >= 1);
         }
     }
 }