1 files changed, 68 insertions, 0 deletions
diff --git a/src/encoding/json/bench_test.go b/src/encoding/json/bench_test.go
index 42439eb705..bd322db2e6 100644
--- a/src/encoding/json/bench_test.go
+++ b/src/encoding/json/bench_test.go
@@ -13,9 +13,14 @@ package json
 import (
 	"bytes"
 	"compress/gzip"
+	"fmt"
+	"internal/testenv"
 	"io/ioutil"
 	"os"
+	"reflect"
+	"runtime"
 	"strings"
+	"sync"
 	"testing"
 )
 
@@ -265,3 +270,66 @@ func BenchmarkUnmapped(b *testing.B) {
 		}
 	})
 }
+
+func BenchmarkTypeFieldsCache(b *testing.B) {
+	var maxTypes int = 1e6
+	if testenv.Builder() != "" {
+		maxTypes = 1e3 // restrict cache sizes on builders
+	}
+
+	// Dynamically generate many new types.
+	types := make([]reflect.Type, maxTypes)
+	fs := []reflect.StructField{{
+		Type:  reflect.TypeOf(""),
+		Index: []int{0},
+	}}
+	for i := range types {
+		fs[0].Name = fmt.Sprintf("TypeFieldsCache%d", i)
+		types[i] = reflect.StructOf(fs)
+	}
+
+	// clearClear clears the cache. Other JSON operations, must not be running.
+	clearCache := func() {
+		fieldCache = sync.Map{}
+	}
+
+	// MissTypes tests the performance of repeated cache misses.
+	// This measures the time to rebuild a cache of size nt.
+	for nt := 1; nt <= maxTypes; nt *= 10 {
+		ts := types[:nt]
+		b.Run(fmt.Sprintf("MissTypes%d", nt), func(b *testing.B) {
+			nc := runtime.GOMAXPROCS(0)
+			for i := 0; i < b.N; i++ {
+				clearCache()
+				var wg sync.WaitGroup
+				for j := 0; j < nc; j++ {
+					wg.Add(1)
+					go func(j int) {
+						for _, t := range ts[(j*len(ts))/nc : ((j+1)*len(ts))/nc] {
+							cachedTypeFields(t)
+						}
+						wg.Done()
+					}(j)
+				}
+				wg.Wait()
+			}
+		})
+	}
+
+	// HitTypes tests the performance of repeated cache hits.
+	// This measures the average time of each cache lookup.
+	for nt := 1; nt <= maxTypes; nt *= 10 {
+		// Pre-warm a cache of size nt.
+		clearCache()
+		for _, t := range types[:nt] {
+			cachedTypeFields(t)
+		}
+		b.Run(fmt.Sprintf("HitTypes%d", nt), func(b *testing.B) {
+			b.RunParallel(func(pb *testing.PB) {
+				for pb.Next() {
+					cachedTypeFields(types[0])
+				}
+			})
+		})
+	}
+}