fix: benchmark single operation in parallel benchmark not b.N (#6422)

Co-authored-by: Sam Kleinman <garen@tychoish.com>

mempool/bench_test.go

@@ -2,6 +2,7 @@ package mempool
 
 import (
 	"encoding/binary"
+	"sync/atomic"
 	"testing"
 
 	"github.com/tendermint/tendermint/abci/example/kvstore"
@@ -54,25 +55,21 @@ func BenchmarkParallelCheckTx(b *testing.B) {
 
 	mempool.config.Size = 100000000
 
-	txCt := 500000000
-	counter := make(chan int, txCt)
-	for i := 0; i < txCt; i++ {
-		counter <- i
+	var txcnt uint64
+	next := func() uint64 {
+		return atomic.AddUint64(&txcnt, 1) - 1
 	}
-	close(counter)
+
 	b.ResetTimer()
-	for i := 0; i < b.N; i++ {
-		b.RunParallel(func(pb *testing.PB) {
-			for pb.Next() {
-				tx := make([]byte, 8)
-				binary.BigEndian.PutUint64(tx, uint64(<-counter))
-				if err := mempool.CheckTx(tx, nil, TxInfo{}); err != nil {
-					b.Fatal(err)
-				}
+	b.RunParallel(func(pb *testing.PB) {
+		for pb.Next() {
+			tx := make([]byte, 8)
+			binary.BigEndian.PutUint64(tx, next())
+			if err := mempool.CheckTx(tx, nil, TxInfo{}); err != nil {
+				b.Fatal(err)
 			}
-		})
-
-	}
+		}
+	})
 }
 
 func BenchmarkCheckDuplicateTx(b *testing.B) {