asim: add TestExploreTDTV

This can be used to understand how the tdtv approaches
its final value as it is repeatedly evaluated over longer
prefixes of the underlying metric.

Author: tbg

pkg/kv/kvserver/asim/history/history.go

@@ -67,28 +67,28 @@ func (h *History) ThrashingForStat(stat string) ThrashingSlice {
 
 	ths := make(ThrashingSlice, numStores)
 	for storeIdx := range vsByStore {
-		sl := vsByStore[storeIdx]
-		// HACK: instead of the slice directly, we measure the thrashing of a slice
-		// that has all leading zeroes removed. This works around the fact that some
-		// timeseries only show sensible values after an initial period of
-		// inactivity. For example, CPU usage is zero until the first stats tick.
-		// Without this hack, the large initial jump from zero to the first value
-		// would be interpreted as variation.
-		noLeadingZeroes := sl
-		for i := 0; i < len(sl); i++ {
-			if sl[i] == 0 {
-				noLeadingZeroes = sl[i+1:]
-				continue
-			}
-			break
-		}
-		th := computeThrashing(noLeadingZeroes)
+		// HACK: we remove leading zeroes before computing the thrashin This works
+		// around the fact that some timeseries only show sensible values after an
+		// initial period of inactivity. For example, CPU usage is zero until the
+		// first stats tick. Without this hack, the large initial jump from zero to
+		// the first value would be interpreted as variation.
+		th := computeThrashing(stripLeaderingZeroes(vsByStore[storeIdx]))
 		ths[storeIdx] = th
 	}
 	ths.normalize()
 	return ths
 }
 
+func stripLeaderingZeroes(vs []float64) []float64 {
+	for i := range vs {
+		if vs[i] == 0 {
+			continue
+		}
+		return vs[i:]
+	}
+	return nil
+}
+
 // Thrashing returns a string representation of the thrashing for the given
 // stat.
 func (h *History) Thrashing(stat string) string {

pkg/kv/kvserver/asim/history/trashing_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 	"testing/quick"
 
+	"github.com/cockroachdb/cockroach/pkg/testutils"
 	"github.com/cockroachdb/cockroach/pkg/testutils/datapathutils"
 	"github.com/cockroachdb/cockroach/pkg/testutils/echotest"
 	"github.com/cockroachdb/cockroach/pkg/util/randutil"
@@ -169,3 +170,23 @@ func TestTDTV(t *testing.T) {
 	_, _ = fmt.Fprint(&buf, asciigraph.Plot(sl, asciigraph.Width(2*N), asciigraph.Height(N)))
 	echotest.Require(t, buf.String(), datapathutils.TestDataPath(t, t.Name()+".txt"))
 }
+
+func TestExploreTDTV(t *testing.T) {
+	// Not a real test, more a helper.
+	sl := []float64{
+		// You can paste a time series copied from viewer.html here to explore
+		// the variation of its prefixes.
+		0,
+		0,
+	}
+	testutils.RunTrueAndFalse(t, "stripped", func(t *testing.T, stripped bool) {
+		sl := sl
+		if stripped {
+			sl = stripLeaderingZeroes(sl)
+		}
+		for i := len(sl) / 100; i < len(sl); i += len(sl) / 100 {
+			t.Logf("sl[:%d]: %+v", i, computeThrashing(sl[:i]))
+		}
+
+	})
+}