make test hit the lock harder

Discovered test was running slightly strange and spending an unreasonable
amount of time on rand.Intn(), possibly because we weren't caching the
value used as the loop condition. Tweaked that, also made the pool a
bit different. Now it takes ~50 seconds for benchtime 100x, and produces
a profile with a TON of time spent waiting on sleeps (expected) and
the condition variable for waiting on checkpoints (the thing we want
to measure, really).

rbf/db_test.go

@@ -337,8 +337,11 @@ func TestDB_MultiTx(t *testing.T) {
 	}
 }
 
+// premake pool of random values
+const randPool = (1 << 18)
+
 // benchmarkOneCheckpoint
-func benchmarkOneCheckpoint(b *testing.B) {
+func benchmarkOneCheckpoint(b *testing.B, randInts []int) {
 	cfg := rbfcfg.NewDefaultConfig()
 	// extremely low to force checkpointing
 	cfg.MinWALCheckpointSize = rbf.PageSize * 16
@@ -350,7 +353,8 @@ func benchmarkOneCheckpoint(b *testing.B) {
 	// Run multiple readers in separate goroutines.
 	ctx, cancel := context.WithCancel(context.Background())
 	g, ctx := errgroup.WithContext(ctx)
-	for i := 0; i < 4; i++ {
+	for i := 0; i < 8; i++ {
+		i := i
 		g.Go(func() error {
 			for {
 				if ctx.Err() != nil {
@@ -362,10 +366,11 @@ func benchmarkOneCheckpoint(b *testing.B) {
 					}
 					defer tx.Rollback()
 
-					// time.Sleep(time.Duration(rand.Intn(int(3 * time.Millisecond))))
+					time.Sleep(time.Duration(rand.Intn(int(3 * time.Millisecond))))
 
-					for i := 0; i < rand.Intn(1000); i++ {
-						v := rand.Intn(1 << 20)
+					times := rand.Intn(1000) + 1
+					for j := 0; j < times; j++ {
+						v := randInts[((i<<10)+j)%(randPool-1)]
 						if _, err := tx.Contains("x", uint64(v)); err != nil {
 							return err
 						}
@@ -374,13 +379,13 @@ func benchmarkOneCheckpoint(b *testing.B) {
 				}(); err != nil {
 					return err
 				}
-
 				// time.Sleep(time.Duration(rand.Intn(int(3 * time.Millisecond))))
 			}
 		})
 	}
 
 	// Continuously set/clear bits while readers are executing.
+	next := 0
 	for i := 0; i < 1000; i++ {
 		func() {
 			tx, err := db.Begin(true)
@@ -389,14 +394,22 @@ func benchmarkOneCheckpoint(b *testing.B) {
 			}
 			defer tx.Rollback()
 
-			for j := 0; j < rand.Intn(100); j++ {
-				v := rand.Intn(1 << 20)
-				if _, err := tx.Add("x", uint64(v)); err != nil {
-					b.Fatal(err)
+			times := rand.Intn(100)
+			for j := 0; j < times; j++ {
+				v := randInts[next]
+				next = (next + 1) % (randPool - 1)
+				if j&7 == 0 {
+					// some removes but they're less frequent
+					if _, err := tx.Remove("x", uint64(v)); err != nil {
+						b.Fatal(err)
+					}
+				} else {
+					if _, err := tx.Add("x", uint64(v)); err != nil {
+						b.Fatal(err)
+					}
 				}
 
 			}
-
 			if err := tx.Commit(); err != nil {
 				b.Fatal(err)
 			}
@@ -416,9 +429,24 @@ func BenchmarkDbCheckpoint(b *testing.B) {
 		b.Fatalf("creating log file: %v", err)
 	}
 	done := fgprof.Start(out, fgprof.FormatPprof)
-	for i := 0; i < b.N; i++ {
-		benchmarkOneCheckpoint(b)
+	b.StopTimer()
+	// premake these because otherwise it's >5% of CPU in the reads
+	randInts := make([]int, randPool)
+	for i := range randInts {
+		v1, v2 := rand.Intn(1<<24), rand.Intn(1<<24)
+		// minimum gives us a skewed distribution which makes lower values more
+		// likely than higher values, so we get a mix of container types
+		if v1 < v2 {
+			randInts[i] = v1
+		} else {
+			randInts[i] = v2
+		}
 	}
+	b.StartTimer()
+	for i := 0; i < b.N; i++ {
+		benchmarkOneCheckpoint(b, randInts)
+	}
+	b.StopTimer()
 	done()
 }