perf: avoid formatter allocations in YCSB hash keys

client/cmd/ycsb/ycsb.go

@@ -10,6 +10,7 @@ import (
 	"io"
 	"math/rand"
 	"slices"
+	"strconv"
 	"strings"
 	"sync"
 	"time"
@@ -135,7 +136,9 @@ func hashKey(keyNum int) string {
 		h *= prime64
 		n >>= 8
 	}
-	return fmt.Sprintf("user%x", h)
+	// "user" plus at most 16 hexadecimal digits fits in this stack buffer.
+	key := [20]byte{'u', 's', 'e', 'r'}
+	return string(strconv.AppendUint(key[:4], h, 16))
 }
 
 // casKey names the CAS-guarded twin of key number keyNum. CAS keys live in

client/cmd/ycsb/ycsb_test.go

@@ -7,6 +7,7 @@ import (
 	"bytes"
 	"context"
 	"errors"
+	"fmt"
 	"math/rand"
 	"net"
 	"strings"
@@ -23,6 +24,43 @@ import (
 	"google.golang.org/grpc/status"
 )
 
+func TestHashKeyMatchesFormatter(t *testing.T) {
+	maxInt := int(^uint(0) >> 1)
+	tests := []struct {
+		name   string
+		keyNum int
+	}{
+		{name: "zero", keyNum: 0},
+		{name: "positive", keyNum: 42},
+		{name: "negative", keyNum: -42},
+		{name: "max int", keyNum: maxInt},
+		{name: "min int", keyNum: -maxInt - 1},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got, want := hashKey(tt.keyNum), formatterHashKey(tt.keyNum); got != want {
+				t.Errorf("hashKey(%d) = %q, want %q", tt.keyNum, got, want)
+			}
+		})
+	}
+}
+
+func formatterHashKey(keyNum int) string {
+	const (
+		offset64 = 14695981039346656037
+		prime64  = 1099511628211
+	)
+	h := uint64(offset64)
+	n := uint64(keyNum)
+	for range 8 {
+		h ^= n & 0xff
+		h *= prime64
+		n >>= 8
+	}
+	return fmt.Sprintf("user%x", h)
+}
+
 const (
 	ycsbReadWorkers    = 4
 	ycsbReadKeys       = 1