From 07701117cd6f71a8f69de2ec57e65a5bf5adb7a1 Mon Sep 17 00:00:00 2001
From: Bruno Windels <brunow@matrix.org>
Date: Wed, 14 Oct 2020 12:43:35 +0200
Subject: [PATCH 1/2] reduce megolm sync cache size to 1 session

as this is kept around for every e2ee room, and we only have
limited olm memory
---
 src/matrix/e2ee/RoomEncryption.js                 |  2 +-
 src/matrix/e2ee/megolm/Decryption.js              |  4 ++--
 src/matrix/e2ee/megolm/decryption/SessionCache.js | 11 ++++++-----
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/src/matrix/e2ee/RoomEncryption.js b/src/matrix/e2ee/RoomEncryption.js
index 69a93e57..f47b92ed 100644
--- a/src/matrix/e2ee/RoomEncryption.js
+++ b/src/matrix/e2ee/RoomEncryption.js
@@ -41,7 +41,7 @@ export class RoomEncryption {
         this._encryptionParams = encryptionParams;
 
         this._megolmBackfillCache = this._megolmDecryption.createSessionCache();
-        this._megolmSyncCache = this._megolmDecryption.createSessionCache();
+        this._megolmSyncCache = this._megolmDecryption.createSessionCache(1);
         // session => event ids of messages we tried to decrypt and the session was missing
         this._missingSessions = new SessionToEventIdsMap();
         // sessions that may or may not be missing, but that while
diff --git a/src/matrix/e2ee/megolm/Decryption.js b/src/matrix/e2ee/megolm/Decryption.js
index 09662e96..b2221028 100644
--- a/src/matrix/e2ee/megolm/Decryption.js
+++ b/src/matrix/e2ee/megolm/Decryption.js
@@ -41,8 +41,8 @@ export class Decryption {
         this._olmWorker = olmWorker;
     }
 
-    createSessionCache(fallback) {
-        return new SessionCache(fallback);
+    createSessionCache(size) {
+        return new SessionCache(size);
     }
 
     /**
diff --git a/src/matrix/e2ee/megolm/decryption/SessionCache.js b/src/matrix/e2ee/megolm/decryption/SessionCache.js
index efb7ef54..d6482a01 100644
--- a/src/matrix/e2ee/megolm/decryption/SessionCache.js
+++ b/src/matrix/e2ee/megolm/decryption/SessionCache.js
@@ -14,13 +14,14 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-const CACHE_MAX_SIZE = 10;
+const DEFAULT_CACHE_SIZE = 10;
 
 /**
  * Cache of unpickled inbound megolm session.
  */
 export class SessionCache {
-    constructor() {
+    constructor(size) {
+        this._size = typeof size === "number" ? size : DEFAULT_CACHE_SIZE;
         this._sessions = [];
     }
 
@@ -51,12 +52,12 @@ export class SessionCache {
         sessionInfo.retain();
         // add new at top
         this._sessions.unshift(sessionInfo);
-        if (this._sessions.length > CACHE_MAX_SIZE) {
+        if (this._sessions.length > this._size) {
             // free sessions we're about to remove
-            for (let i = CACHE_MAX_SIZE; i < this._sessions.length; i += 1) {
+            for (let i = this._size; i < this._sessions.length; i += 1) {
                 this._sessions[i].release();
             }
-            this._sessions = this._sessions.slice(0, CACHE_MAX_SIZE);
+            this._sessions = this._sessions.slice(0, this._size);
         }
     }
 

From b00db1af3e82d70eb60b95d67a0a15b1e84f4e5c Mon Sep 17 00:00:00 2001
From: Bruno Windels <brunow@matrix.org>
Date: Wed, 14 Oct 2020 12:44:27 +0200
Subject: [PATCH 2/2] only keep 20 outbound olm sessions in memory at once to
 prevent OOM

---
 src/matrix/e2ee/olm/Encryption.js | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/src/matrix/e2ee/olm/Encryption.js b/src/matrix/e2ee/olm/Encryption.js
index ff5f1a8f..18bbc5fa 100644
--- a/src/matrix/e2ee/olm/Encryption.js
+++ b/src/matrix/e2ee/olm/Encryption.js
@@ -31,7 +31,9 @@ function findFirstSessionId(sessionIds) {
 const OTK_ALGORITHM = "signed_curve25519";
 // only encrypt this amount of olm messages at once otherwise we run out of wasm memory
 // with all the sessions loaded at the same time
-const MAX_BATCH_SIZE = 50;
+// See https://github.com/vector-im/hydrogen-web/issues/150 as well, which indicates the limit is 44,
+// but let's take a conservative limit as the megolm session cache also takes space
+const MAX_BATCH_SIZE = 20;
 
 export class Encryption {
     constructor({account, olm, olmUtil, ownUserId, storage, now, pickleKey, senderKeyLock}) {