runtime: shrink time histogram buckets

There are lots of useless buckets with too much precision. Introduce a
minimum level of precision with a minimum bucket bit. This cuts down on
the size of a time histogram dramatically (~3x). Also, pick a smaller
sub bucket count; we don't need 6% precision.

Also, rename super-buckets to buckets to more closely line up with HDR
histogram literature.

Change-Id: I199449650e4b34f2a6dca3cf1d8edb071c6655c0
Reviewed-on: https://go-review.googlesource.com/c/go/+/427615
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 13 insertions, 7 deletions

diff --git a/src/runtime/export_test.go b/src/runtime/export_test.go
index c29d64a885..93cae48211 100644
--- a/src/runtime/export_test.go
+++ b/src/runtime/export_test.go
@@ -1228,22 +1228,28 @@ func MSpanCountAlloc(ms *MSpan, bits []byte) int {
 }
 
 const (
-	TimeHistSubBucketBits   = timeHistSubBucketBits
-	TimeHistNumSubBuckets   = timeHistNumSubBuckets
-	TimeHistNumSuperBuckets = timeHistNumSuperBuckets
+	TimeHistSubBucketBits = timeHistSubBucketBits
+	TimeHistNumSubBuckets = timeHistNumSubBuckets
+	TimeHistNumBuckets    = timeHistNumBuckets
+	TimeHistMinBucketBits = timeHistMinBucketBits
+	TimeHistMaxBucketBits = timeHistMaxBucketBits
 )
 
 type TimeHistogram timeHistogram
 
 // Counts returns the counts for the given bucket, subBucket indices.
 // Returns true if the bucket was valid, otherwise returns the counts
-// for the underflow bucket and false.
-func (th *TimeHistogram) Count(bucket, subBucket uint) (uint64, bool) {
+// for the overflow bucket if bucket > 0 or the underflow bucket if
+// bucket < 0, and false.
+func (th *TimeHistogram) Count(bucket, subBucket int) (uint64, bool) {
 	t := (*timeHistogram)(th)
-	i := bucket*TimeHistNumSubBuckets + subBucket
-	if i >= uint(len(t.counts)) {
+	if bucket < 0 {
 		return t.underflow.Load(), false
 	}
+	i := bucket*TimeHistNumSubBuckets + subBucket
+	if i >= len(t.counts) {
+		return t.overflow.Load(), false
+	}
 	return t.counts[i].Load(), true
 }