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enhancement(observability)!: Expand internal histogram precision #24497

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Merged
bruceg merged 1 commit into from
Jan 15, 2026
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enhancement(observability)!: Expand internal histogram precision #24497

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5 changes: 5 additions & 0 deletions changelog.d/expand-internal-histogram-precision.breaking.md
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Original file line number Diff line number Diff line change
@@ -0,0 +1,5 @@
Increased the number of buckets in internal histograms to reduce the smallest
bucket down to approximately 0.000244 (2.0^-12). Since this shifts all the
bucket values out, it may break VRL scripts that rely on the previous values.

authors: bruceg
52 changes: 29 additions & 23 deletions lib/vector-core/src/metrics/storage.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -31,15 +31,15 @@ impl<K> Storage<K> for VectorStorage {

#[derive(Debug)]
pub(super) struct Histogram {
buckets: Box<[(f64, AtomicU32); 20]>,
buckets: Box<[(f64, AtomicU32); 26]>,
count: AtomicU64,
sum: AtomicF64,
}

impl Histogram {
const MIN_BUCKET: f64 = 0.015_625; // (-6_f64).exp2() is not const yet
const MIN_BUCKET_EXP: f64 = -6.0;
const BUCKETS: usize = 20;
const MIN_BUCKET: f64 = 1.0 / (1 << 12) as f64; // f64::powi() is not const yet
const MIN_BUCKET_EXP: f64 = -12.0;
const BUCKETS: usize = 26;

pub(crate) fn new() -> Self {
// Box to avoid having this large array inline to the structure, blowing
Expand All @@ -52,25 +52,31 @@ impl Histogram {
// long-tail. This also lets us find the right bucket to record into using simple
// constant-time math operations instead of a loop-and-compare construct.
let buckets = Box::new([
((-6_f64).exp2(), AtomicU32::new(0)),
((-5_f64).exp2(), AtomicU32::new(0)),
((-4_f64).exp2(), AtomicU32::new(0)),
((-3_f64).exp2(), AtomicU32::new(0)),
((-2_f64).exp2(), AtomicU32::new(0)),
((-1_f64).exp2(), AtomicU32::new(0)),
(0_f64.exp2(), AtomicU32::new(0)),
(1_f64.exp2(), AtomicU32::new(0)),
(2_f64.exp2(), AtomicU32::new(0)),
(3_f64.exp2(), AtomicU32::new(0)),
(4_f64.exp2(), AtomicU32::new(0)),
(5_f64.exp2(), AtomicU32::new(0)),
(6_f64.exp2(), AtomicU32::new(0)),
(7_f64.exp2(), AtomicU32::new(0)),
(8_f64.exp2(), AtomicU32::new(0)),
(9_f64.exp2(), AtomicU32::new(0)),
(10_f64.exp2(), AtomicU32::new(0)),
(11_f64.exp2(), AtomicU32::new(0)),
(12_f64.exp2(), AtomicU32::new(0)),
(2.0f64.powi(-12), AtomicU32::new(0)),
(2.0f64.powi(-11), AtomicU32::new(0)),
(2.0f64.powi(-10), AtomicU32::new(0)),
(2.0f64.powi(-9), AtomicU32::new(0)),
(2.0f64.powi(-8), AtomicU32::new(0)),
(2.0f64.powi(-7), AtomicU32::new(0)),
(2.0f64.powi(-6), AtomicU32::new(0)),
(2.0f64.powi(-5), AtomicU32::new(0)),
(2.0f64.powi(-4), AtomicU32::new(0)),
(2.0f64.powi(-3), AtomicU32::new(0)),
(2.0f64.powi(-2), AtomicU32::new(0)),
(2.0f64.powi(-1), AtomicU32::new(0)),
(2.0f64.powi(0), AtomicU32::new(0)),
(2.0f64.powi(1), AtomicU32::new(0)),
(2.0f64.powi(2), AtomicU32::new(0)),
(2.0f64.powi(3), AtomicU32::new(0)),
(2.0f64.powi(4), AtomicU32::new(0)),
(2.0f64.powi(5), AtomicU32::new(0)),
(2.0f64.powi(6), AtomicU32::new(0)),
(2.0f64.powi(7), AtomicU32::new(0)),
(2.0f64.powi(8), AtomicU32::new(0)),
(2.0f64.powi(9), AtomicU32::new(0)),
(2.0f64.powi(10), AtomicU32::new(0)),
(2.0f64.powi(11), AtomicU32::new(0)),
(2.0f64.powi(12), AtomicU32::new(0)),
(f64::INFINITY, AtomicU32::new(0)),
]);
Self {
Expand Down
6 changes: 3 additions & 3 deletions src/sources/internal_metrics.rs
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Expand Up @@ -256,7 +256,7 @@ mod tests {
// [`metrics::handle::Histogram::new`] are hard-coded. If this
// check fails you might look there and see if we've allowed
// users to set their own bucket widths.
assert_eq!(buckets[9].count, 2);
assert_eq!(buckets[15].count, 2);
assert_eq!(*count, 2);
assert_eq!(*sum, 11.0);
}
Expand All @@ -273,8 +273,8 @@ mod tests {
// [`metrics::handle::Histogram::new`] are hard-coded. If this
// check fails you might look there and see if we've allowed
// users to set their own bucket widths.
assert_eq!(buckets[9].count, 1);
assert_eq!(buckets[10].count, 1);
assert_eq!(buckets[15].count, 1);
assert_eq!(buckets[16].count, 1);
assert_eq!(*count, 2);
assert_eq!(*sum, 16.1);
}
Expand Down
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