Gradually evict oldest cache entries (#1768)

* Gradually evict oldest cache entries

* Keep the remaining 10% of cached entries

internal/caching/impl_inmemorylru.go

@@ -2,6 +2,7 @@ package caching
 
 import (
 	"fmt"
+	"time"
 
 	lru "github.com/hashicorp/golang-lru"
 	"github.com/prometheus/client_golang/prometheus"
@@ -72,6 +73,11 @@ func NewInMemoryLRUCache(enablePrometheus bool) (*Caches, error) {
 	if err != nil {
 		return nil, err
 	}
+	go cacheCleaner(
+		roomVersions, serverKeys, roomServerStateKeyNIDs,
+		roomServerEventTypeNIDs, roomServerRoomIDs,
+		roomInfos, federationEvents,
+	)
 	return &Caches{
 		RoomVersions:            roomVersions,
 		ServerKeys:              serverKeys,
@@ -83,6 +89,20 @@ func NewInMemoryLRUCache(enablePrometheus bool) (*Caches, error) {
 	}, nil
 }
 
+func cacheCleaner(caches ...*InMemoryLRUCachePartition) {
+	for {
+		time.Sleep(time.Minute)
+		for _, cache := range caches {
+			// Hold onto the last 10% of the cache entries, since
+			// otherwise a quiet period might cause us to evict all
+			// cache entries entirely.
+			if cache.lru.Len() > cache.maxEntries/10 {
+				cache.lru.RemoveOldest()
+			}
+		}
+	}
+}
+
 type InMemoryLRUCachePartition struct {
 	name       string
 	mutable    bool