Update default configuration for revoked HTLC clean-up (#3212)

The default configuration for removing old rows containing revoked HTLC
data after a channel close was too slow and inadequate for most nodes.
We update it to use smaller batches without a much shorter interval.

Thanks @DerEwige for providing data about their experiments.

Fixes #3211

Author: t-bast

eclair-core/src/main/resources/reference.conf

@@ -586,9 +586,10 @@ eclair {
     // down the node, we delete those rows in batches at regular intervals.
     revoked-htlc-info-cleaner {
       // Number of rows to delete per batch: a higher value will clean up the DB faster, but may have a higher impact on performance.
-      batch-size = 50000
+      batch-size = 500
       // Frequency at which batches of rows are deleted: a lower value will clean up the DB faster, but may have a higher impact on performance.
-      interval = 15 minutes
+      // If you are frequently closing or splicing channels, you may want to increase the default interval if you're seeing IO performance issues.
+      interval = 1 seconds
     }
   }
 

eclair-core/src/main/scala/fr/acinq/eclair/db/RevokedHtlcInfoCleaner.scala

@@ -43,13 +43,16 @@ object RevokedHtlcInfoCleaner {
     Behaviors.setup { context =>
       context.system.eventStream ! EventStream.Subscribe(context.self)
       Behaviors.withTimers { timers =>
-        timers.startTimerWithFixedDelay(DeleteBatch, config.interval)
+        timers.startSingleTimer(DeleteBatch, config.interval)
         Behaviors.receiveMessage {
           case ForgetHtlcInfos(channelId, beforeCommitIndex) =>
             db.markHtlcInfosForRemoval(channelId, beforeCommitIndex)
             Behaviors.same
           case DeleteBatch =>
             db.removeHtlcInfos(config.batchSize)
+            // We restart a new timer after each batch, instead of using a timer at fixed intervals.
+            // This ensures that we don't have DeleteBatch messages queuing up in the mailbox when the DB is busy.
+            timers.startSingleTimer(DeleteBatch, config.interval)
             Behaviors.same
         }
       }