From 2386e0c7af5b29b3d45373db812204cd720f7f88 Mon Sep 17 00:00:00 2001
From: Neil Alexander <neilalexander@users.noreply.github.com>
Date: Wed, 17 Feb 2021 13:50:27 +0000
Subject: [PATCH] Gradually evict oldest cache entries (#1768)

* Gradually evict oldest cache entries

* Keep the remaining 10% of cached entries
---
 internal/caching/impl_inmemorylru.go | 20 ++++++++++++++++++++
 1 file changed, 20 insertions(+)

diff --git a/internal/caching/impl_inmemorylru.go b/internal/caching/impl_inmemorylru.go
index cf05a8b55..f0915d7ca 100644
--- a/internal/caching/impl_inmemorylru.go
+++ b/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