diff --git a/cached_ops.go b/cached_ops.go
index 4ccad1a..009e573 100644
--- a/cached_ops.go
+++ b/cached_ops.go
@@ -251,7 +251,9 @@ func generateArticle(key cache.Key) (cache.Cacheable, error) {
 	r := key.(*ArticleRequest)
 	ctx := context.TODO()
 	ctx = context.WithValue(ctx, CtxKeyBoardname, r)
-	ctx = extcache.WithExtCache(ctx, extCache)
+	if config.Experiments.ExtCache.Enabled(fastStrHash64(r.Filename)) {
+		ctx = extcache.WithExtCache(ctx, extCache)
+	}
 
 	p, err := r.Select(pttbbs.SelectHead, 0, HeadSize)
 	if err != nil {
@@ -332,7 +334,9 @@ func generateArticlePart(key cache.Key) (cache.Cacheable, error) {
 	r := key.(*ArticlePartRequest)
 	ctx := context.TODO()
 	ctx = context.WithValue(ctx, CtxKeyBoardname, r)
-	ctx = extcache.WithExtCache(ctx, extCache)
+	if config.Experiments.ExtCache.Enabled(fastStrHash64(r.Filename)) {
+		ctx = extcache.WithExtCache(ctx, extCache)
+	}
 
 	p, err := ptt.GetArticleSelect(r.Brd.Ref(), pttbbs.SelectHead, r.Filename, r.CacheKey, r.Offset, -1)
 	if err == pttbbs.ErrNotFound {
diff --git a/config.go b/config.go
index d972b63..3f1b823 100644
--- a/config.go
+++ b/config.go
@@ -4,6 +4,7 @@ import (
 	"errors"
 
 	"github.com/ptt/pttweb/captcha"
+	"github.com/ptt/pttweb/experiment"
 	"github.com/ptt/pttweb/extcache"
 )
 
@@ -42,6 +43,12 @@ type PttwebConfig struct {
 	CaptchaRedisConfig  *captcha.RedisConfig
 
 	ExtCacheConfig extcache.Config
+
+	Experiments Experiments
+}
+
+type Experiments struct {
+	ExtCache experiment.OptIn
 }
 
 const (
diff --git a/experiment/struct.go b/experiment/struct.go
new file mode 100644
index 0000000..823118d
--- /dev/null
+++ b/experiment/struct.go
@@ -0,0 +1,32 @@
+package experiment
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+const percentBase = uint64(10000)
+
+type Percent struct {
+	threshold uint64
+}
+
+func (p *Percent) UnmarshalJSON(b []byte) error {
+	var pct float64
+	if err := json.Unmarshal(b, &pct); err != nil {
+		return err
+	}
+	if pct < 0 {
+		return fmt.Errorf("percent is negative: %v", pct)
+	}
+	p.threshold = uint64(pct * float64(percentBase) / 100)
+	return nil
+}
+
+type OptIn struct {
+	OptIn Percent
+}
+
+func (o *OptIn) Enabled(val uint64) bool {
+	return val%percentBase < o.OptIn.threshold
+}
diff --git a/pttweb.go b/pttweb.go
index 0e86fb8..841371a 100644
--- a/pttweb.go
+++ b/pttweb.go
@@ -6,6 +6,7 @@ import (
 	"errors"
 	"flag"
 	"fmt"
+	"hash/fnv"
 	"html/template"
 	"log"
 	"net"
@@ -954,3 +955,9 @@ func manSelectType(m pttbbs.SelectMethod) manpb.ArticleRequest_SelectType {
 		panic("unknown select type")
 	}
 }
+
+func fastStrHash64(s string) uint64 {
+	h := fnv.New64()
+	_, _ = h.Write([]byte(s))
+	return h.Sum64()
+}