qbt: tweak

configuration.nix

@@ -132,6 +132,31 @@
       compressor = "zstd";
       supportedFilesystems = [ "f2fs" ];
     };
+
+    # BBR congestion control handles variable-latency VPN connections much
+    # better than CUBIC by probing bandwidth continuously rather than
+    # reacting to packet loss.
+    kernelModules = [ "tcp_bbr" ];
+
+    kernel.sysctl = {
+      # Use BBR + fair queuing for smooth throughput through the WireGuard VPN
+      "net.core.default_qdisc" = "fq";
+      "net.ipv4.tcp_congestion_control" = "bbr";
+
+      # Disable slow-start after idle: prevents TCP from resetting window
+      # size on each burst cycle (the primary cause of the 0 -> 40 MB/s spikes)
+      "net.ipv4.tcp_slow_start_after_idle" = 0;
+
+      # Larger socket buffers to accommodate the VPN bandwidth-delay product
+      # (22ms RTT * target throughput). Current 2.5MB max is too small.
+      "net.core.rmem_max" = 16777216;
+      "net.core.wmem_max" = 16777216;
+      "net.ipv4.tcp_rmem" = "4096 87380 16777216";
+      "net.ipv4.tcp_wmem" = "4096 65536 16777216";
+
+      # Higher backlog for the large number of concurrent torrent connections
+      "net.core.netdev_max_backlog" = 5000;
+    };
   };
 
   environment.etc = {

services/qbittorrent.nix

@@ -48,7 +48,7 @@
 
     serverConfig.BitTorrent = {
       Session = {
-        MaxConnectionsPerTorrent = 100;
+        MaxConnectionsPerTorrent = 50;
         MaxUploadsPerTorrent = 10;
         MaxConnections = -1;
         MaxUploads = -1;
@@ -57,7 +57,7 @@
 
         # queueing
         QueueingSystemEnabled = true;
-        MaxActiveDownloads = 8; # num of torrents that can download at the same time
+        MaxActiveDownloads = 5; # keep focused: fewer torrents, each gets more bandwidth
         MaxActiveUploads = -1;
         MaxActiveTorrents = -1;
         IgnoreSlowTorrentsForQueueing = true;
@@ -84,8 +84,9 @@
         inherit (config.services.qbittorrent.serverConfig.Preferences.Downloads) TempPath;
         TempPathEnabled = true;
 
-        # how many connections per sec
-        ConnectionSpeed = 300;
+        # Reduced from 300: that rate floods the VPN tunnel with SYN packets,
+        # starving actual data transfer and causing the 0->40MB/s spike pattern.
+        ConnectionSpeed = 20;
 
         # Automatic Torrent Management: use category save paths for new torrents
         DisableAutoTMMByDefault = false;