runtime,runtime/metrics: use explicit histogram boundaries

This change modifies the semantics of
runtime/metrics.Float64Histogram.Buckets to remove implicit buckets to
that extend to positive and negative infinity and instead defines all
bucket boundaries as explicitly listed.

Bucket boundaries remain the same as before except
/gc/heap/allocs-by-size:objects and /gc/heap/frees-by-size:objects no
longer have a bucket that extends to negative infinity.

This change simplifies the Float64Histogram API, making it both easier
to understand and easier to use.

Also, add a test for allocs-by-size and frees-by-size that checks them
against MemStats.

Fixes #43443.

Change-Id: I5620f15bd084562dadf288f733c4a8cace21910c
Reviewed-on: https://go-review.googlesource.com/c/go/+/281238
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Trust: Michael Knyszek <mknyszek@google.com>

1 files changed, 28 insertions, 4 deletions

diff --git a/src/runtime/histogram.go b/src/runtime/histogram.go
index d48e856cd0..42baa6c5e2 100644
--- a/src/runtime/histogram.go
+++ b/src/runtime/histogram.go
@@ -7,6 +7,7 @@ package runtime
 import (
 	"runtime/internal/atomic"
 	"runtime/internal/sys"
+	"unsafe"
 )
 
 const (
@@ -69,7 +70,13 @@ const (
 // for concurrent use. It is also safe to read all the values
 // atomically.
 type timeHistogram struct {
-	counts    [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
+	counts [timeHistNumSuperBuckets * timeHistNumSubBuckets]uint64
+
+	// underflow counts all the times we got a negative duration
+	// sample. Because of how time works on some platforms, it's
+	// possible to measure negative durations. We could ignore them,
+	// but we record them anyway because it's better to have some
+	// signal that it's happening than just missing samples.
 	underflow uint64
 }
 
@@ -107,14 +114,30 @@ func (h *timeHistogram) record(duration int64) {
 	atomic.Xadd64(&h.counts[superBucket*timeHistNumSubBuckets+subBucket], 1)
 }
 
+const (
+	fInf    = 0x7FF0000000000000
+	fNegInf = 0xFFF0000000000000
+)
+
+func float64Inf() float64 {
+	inf := uint64(fInf)
+	return *(*float64)(unsafe.Pointer(&inf))
+}
+
+func float64NegInf() float64 {
+	inf := uint64(fNegInf)
+	return *(*float64)(unsafe.Pointer(&inf))
+}
+
 // timeHistogramMetricsBuckets generates a slice of boundaries for
 // the timeHistogram. These boundaries are represented in seconds,
 // not nanoseconds like the timeHistogram represents durations.
 func timeHistogramMetricsBuckets() []float64 {
-	b := make([]float64, timeHistTotalBuckets-1)
+	b := make([]float64, timeHistTotalBuckets+1)
+	b[0] = float64NegInf()
 	for i := 0; i < timeHistNumSuperBuckets; i++ {
 		superBucketMin := uint64(0)
-		// The (inclusive) minimum for the first bucket is 0.
+		// The (inclusive) minimum for the first non-negative bucket is 0.
 		if i > 0 {
 			// The minimum for the second bucket will be
 			// 1 << timeHistSubBucketBits, indicating that all
@@ -141,8 +164,9 @@ func timeHistogramMetricsBuckets() []float64 {
 
 			// Convert the subBucketMin which is in nanoseconds to a float64 seconds value.
 			// These values will all be exactly representable by a float64.
-			b[i*timeHistNumSubBuckets+j] = float64(subBucketMin) / 1e9
+			b[i*timeHistNumSubBuckets+j+1] = float64(subBucketMin) / 1e9
 		}
 	}
+	b[len(b)-1] = float64Inf()
 	return b
 }