Add note about pow-of-10 precision issue

beorn7 · beorn7 · commit a9d0066408f6 · 2020-04-13T16:43:23.000+02:00
Signed-off-by: beorn7 &lt;beorn@grafana.com&gt;
diff --git a/prometheus/histogram.go b/prometheus/histogram.go
@@ -531,6 +531,11 @@ func (h *histogram) observe(v float64, bucket int) {
 		case v < -h.sparseThreshold:
 			whichSparse = -1
 		}
+		// TODO(beorn7): This sometimes gives inaccurate results for
+		// floats that are actual powers of 10, e.g. math.Log10(0.1) is
+		// calculated as -0.9999999999999999 rather than -1 and thus
+		// yields a key unexpectedly one off. Maybe special-case precise
+		// powers of 10.
 		sparseKey = int(math.Ceil(math.Log10(math.Abs(v)) * float64(h.sparseResolution)))
 	}
 	// We increment h.countAndHotIdx so that the counter in the lower

Original file line number	Diff line number	Diff line change
`@@ -531,6 +531,11 @@ func (h *histogram) observe(v float64, bucket int) {`
`531`	`531`	`case v < -h.sparseThreshold:`
`532`	`532`	`whichSparse = -1`
`533`	`533`	`}`
	`534`	`+ // TODO(beorn7): This sometimes gives inaccurate results for`
	`535`	`+ // floats that are actual powers of 10, e.g. math.Log10(0.1) is`
	`536`	`+ // calculated as -0.9999999999999999 rather than -1 and thus`
	`537`	`+ // yields a key unexpectedly one off. Maybe special-case precise`
	`538`	`+ // powers of 10.`
`534`	`539`	`sparseKey = int(math.Ceil(math.Log10(math.Abs(v)) * float64(h.sparseResolution)))`
`535`	`540`	`}`
`536`	`541`	`// We increment h.countAndHotIdx so that the counter in the lower`