runtime: add timeHistogram type

This change adds a concurrent HDR time histogram to the runtime with
tests. It also adds a function to generate boundaries for use by the
metrics package.

For #37112.

Change-Id: Ifbef8ddce8e3a965a0dcd58ccd4915c282ae2098
Reviewed-on: https://go-review.googlesource.com/c/go/+/247046
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>

1 files changed, 58 insertions, 0 deletions

diff --git a/src/runtime/histogram_test.go b/src/runtime/histogram_test.go
new file mode 100644
index 0000000000..5f5b28f784
--- /dev/null
+++ b/src/runtime/histogram_test.go
@@ -0,0 +1,58 @@
+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	. "runtime"
+	"testing"
+)
+
+var dummyTimeHistogram TimeHistogram
+
+func TestTimeHistogram(t *testing.T) {
+	// We need to use a global dummy because this
+	// could get stack-allocated with a non-8-byte alignment.
+	// The result of this bad alignment is a segfault on
+	// 32-bit platforms when calling Record.
+	h := &dummyTimeHistogram
+
+	// Record exactly one sample in each bucket.
+	for i := 0; i < TimeHistNumSuperBuckets; i++ {
+		var base int64
+		if i > 0 {
+			base = int64(1) << (i + TimeHistSubBucketBits - 1)
+		}
+		for j := 0; j < TimeHistNumSubBuckets; j++ {
+			v := int64(j)
+			if i > 0 {
+				v <<= i - 1
+			}
+			h.Record(base + v)
+		}
+	}
+	// Hit the overflow bucket.
+	h.Record(int64(^uint64(0) >> 1))
+
+	// Check to make sure there's exactly one count in each
+	// bucket.
+	for i := uint(0); i < TimeHistNumSuperBuckets; i++ {
+		for j := uint(0); j < TimeHistNumSubBuckets; j++ {
+			c, ok := h.Count(i, j)
+			if !ok {
+				t.Errorf("hit overflow bucket unexpectedly: (%d, %d)", i, j)
+			} else if c != 1 {
+				t.Errorf("bucket (%d, %d) has count that is not 1: %d", i, j, c)
+			}
+		}
+	}
+	c, ok := h.Count(TimeHistNumSuperBuckets, 0)
+	if ok {
+		t.Errorf("expected to hit overflow bucket: (%d, %d)", TimeHistNumSuperBuckets, 0)
+	}
+	if c != 1 {
+		t.Errorf("overflow bucket has count that is not 1: %d", c)
+	}
+	dummyTimeHistogram = TimeHistogram{}
+}