atomically increment exp histograms

Author: dashpole

sdk/metric/internal/aggregate/exponential_histogram.go

@@ -98,8 +98,9 @@ func (p *expoHistogramDataPoint[N]) record(v N) {
 	// If the new bin would make the counts larger than maxScale, we need to
 	// downscale current measurements.
 	scaleDelta := p.scaleChange(bin, bucket.startBin, len(bucket.counts))
-	if scaleDelta <= 0 {
-		bucket.record(bin)
+	if scaleDelta <= 0 && !bucket.needsResize(bin) {
+		// Fast path without requiring the full lock
+		bucket.recordWithoutResize(bin)
 		p.scaleMux.RUnlock()
 		return
 	}
@@ -110,18 +111,19 @@ func (p *expoHistogramDataPoint[N]) record(v N) {
 		p.scaleMux.RUnlock()
 		return
 	}
-	// Switch to a full Lock for downscaling
+	// Switch to a full Lock for downscaling or resizing
 	p.scaleMux.RUnlock()
 	p.scaleMux.Lock()
 	defer p.scaleMux.Unlock()
-	// recompute the scaleDelta now that we hold the exclusive lock
+	// recompute the scaleDelta now that we hold the full lock
 	scaleDelta = p.scaleChange(bin, bucket.startBin, len(bucket.counts))
 	// Downscale
-	p.scale -= scaleDelta
-	p.posBuckets.downscale(scaleDelta)
-	p.negBuckets.downscale(scaleDelta)
+	if scaleDelta > 0 {
+		p.scale -= scaleDelta
+		p.posBuckets.downscale(scaleDelta)
+		p.negBuckets.downscale(scaleDelta)
+	}
 
-	// TODO: is it worth switching back to a read-lock?
 	bucket.record(p.getBin(absV))
 }
 
@@ -202,6 +204,15 @@ type expoBuckets struct {
 	counts   []uint64
 }
 
+func (b *expoBuckets) needsResize(bin int32) bool {
+	endBin := int(b.startBin) + len(b.counts) - 1
+	return len(b.counts) == 0 || bin < b.startBin || int(bin) > endBin
+}
+
+func (b *expoBuckets) recordWithoutResize(bin int32) {
+	atomic.AddUint64(&b.counts[bin-b.startBin], 1)
+}
+
 // record increments the count for the given bin, and expands the buckets if needed.
 // Size changes must be done before calling this function.
 func (b *expoBuckets) record(bin int32) {
@@ -215,6 +226,7 @@ func (b *expoBuckets) record(bin int32) {
 
 	// if the new bin is inside the current range
 	if bin >= b.startBin && int(bin) <= endBin {
+		// No need to use atomics since we hold the RW lock.
 		b.counts[bin-b.startBin]++
 		return
 	}