diff --git a/shadowsocks-csharp/Controller/Service/AvailabilityStatistics.cs b/shadowsocks-csharp/Controller/Service/AvailabilityStatistics.cs
index a43c4780..a691d0e8 100644
--- a/shadowsocks-csharp/Controller/Service/AvailabilityStatistics.cs
+++ b/shadowsocks-csharp/Controller/Service/AvailabilityStatistics.cs
@@ -34,12 +34,77 @@ namespace Shadowsocks.Controller
         //records cache for current server in {_monitorInterval} minutes
         private readonly ConcurrentDictionary<string, List<int>> _latencyRecords = new ConcurrentDictionary<string, List<int>>();
         //speed in KiB/s
-        private readonly ConcurrentDictionary<string, long> _inboundCounter = new ConcurrentDictionary<string, long>();
-        private readonly ConcurrentDictionary<string, long> _lastInboundCounter = new ConcurrentDictionary<string, long>();
         private readonly ConcurrentDictionary<string, List<int>> _inboundSpeedRecords = new ConcurrentDictionary<string, List<int>>();
-        private readonly ConcurrentDictionary<string, long> _outboundCounter = new ConcurrentDictionary<string, long>();
-        private readonly ConcurrentDictionary<string, long> _lastOutboundCounter = new ConcurrentDictionary<string, long>();
         private readonly ConcurrentDictionary<string, List<int>> _outboundSpeedRecords = new ConcurrentDictionary<string, List<int>>();
+        private readonly ConcurrentDictionary<string, InOutBoundRecord> _inOutBoundRecords = new ConcurrentDictionary<string, InOutBoundRecord>();
+        private class InOutBoundRecord
+        {
+            private long _inbound;
+            private long _lastInbound;
+            private long _outbound;
+            private long _lastOutbound;
+
+            private SpinLock _lock = new SpinLock();
+
+            public void UpdateInbound(long delta)
+            {
+                bool lockTaken = false;
+                try
+                {
+                    _lock.Enter(ref lockTaken);
+                    Interlocked.Add(ref _inbound, delta);
+                }
+                finally
+                {
+                    if (lockTaken)
+                    {
+                        _lock.Exit(false);
+                    }
+                }
+            }
+
+            public void UpdateOutbound(long delta)
+            {
+                bool lockTaken = false;
+                try
+                {
+                    _lock.Enter(ref lockTaken);
+                    Interlocked.Add(ref _outbound, delta);
+                }
+                finally
+                {
+                    if (lockTaken)
+                    {
+                        _lock.Exit(false);
+                    }
+                }
+            }
+
+            public void GetDelta(out long inboundDelta, out long outboundDelta)
+            {
+                bool lockTaken = false;
+                try
+                {
+                    _lock.Enter(ref lockTaken);
+
+                    var i = Interlocked.Read(ref _inbound);
+                    var il = Interlocked.Exchange(ref _lastInbound, i);
+                    inboundDelta = i - il;
+
+
+                    var o = Interlocked.Read(ref _outbound);
+                    var ol = Interlocked.Exchange(ref _lastOutbound, o);
+                    outboundDelta = o - ol;
+                }
+                finally
+                {
+                    if (lockTaken)
+                    {
+                        _lock.Exit(false);
+                    }
+                }
+            }
+        }
 
         //tasks
         private readonly TimeSpan _delayBeforeStart = TimeSpan.FromSeconds(1);
@@ -98,36 +163,26 @@ namespace Shadowsocks.Controller
 
         private void UpdateSpeed(object _)
         {
-            foreach (var kv in _lastInboundCounter)
+            foreach (var kv in _inOutBoundRecords)
             {
                 var id = kv.Key;
+                var record = kv.Value;
 
-                var lastInbound = kv.Value;
-                var inbound = _inboundCounter[id];
-                var bytes = inbound - lastInbound;
-                _lastInboundCounter[id] = inbound;
-                var inboundSpeed = GetSpeedInKiBPerSecond(bytes, _monitorInterval.TotalSeconds);
-                _inboundSpeedRecords.GetOrAdd(id, (k) =>
-                {
-                    List<int> records = new List<int>();
-                    records.Add(inboundSpeed);
-                    return records;
-                });
+                long inboundDelta, outboundDelta;
 
-                var lastOutbound = _lastOutboundCounter[id];
-                var outbound = _outboundCounter[id];
-                bytes = outbound - lastOutbound;
-                _lastOutboundCounter[id] = outbound;
-                var outboundSpeed = GetSpeedInKiBPerSecond(bytes, _monitorInterval.TotalSeconds);
-                _outboundSpeedRecords.GetOrAdd(id, (k) =>
-                {
-                    List<int> records = new List<int>();
-                    records.Add(outboundSpeed);
-                    return records;
-                });
+                record.GetDelta(out inboundDelta, out outboundDelta);
+
+                var inboundSpeed = GetSpeedInKiBPerSecond(inboundDelta, _monitorInterval.TotalSeconds);
+                var outboundSpeed = GetSpeedInKiBPerSecond(outboundDelta, _monitorInterval.TotalSeconds);
+
+                var inR = _inboundSpeedRecords.GetOrAdd(id, (k) => new List<int>());
+                var outR = _outboundSpeedRecords.GetOrAdd(id, (k) => new List<int>());
+
+                inR.Add(inboundSpeed);
+                outR.Add(outboundSpeed);
 
                 Logging.Debug(
-                    $"{id}: current/max inbound {inboundSpeed}/{_inboundSpeedRecords[id].Max()} KiB/s, current/max outbound {outboundSpeed}/{_outboundSpeedRecords[id].Max()} KiB/s");
+                    $"{id}: current/max inbound {inboundSpeed}/{inR.Max()} KiB/s, current/max outbound {outboundSpeed}/{outR.Max()} KiB/s");
             }
         }
 
@@ -327,20 +382,32 @@ namespace Shadowsocks.Controller
 
         public void UpdateInboundCounter(Server server, long n)
         {
-            _inboundCounter.AddOrUpdate(server.Identifier(), (k) =>
+            _inOutBoundRecords.AddOrUpdate(server.Identifier(), (k) =>
             {
-                _lastInboundCounter.GetOrAdd(server.Identifier(), 0);
-                return n;
-            }, (k, v) => (v + n));
+                var r = new InOutBoundRecord();
+                r.UpdateInbound(n);
+
+                return r;
+            }, (k, v) =>
+            {
+                v.UpdateInbound(n);
+                return v;
+            });
         }
 
         public void UpdateOutboundCounter(Server server, long n)
         {
-            _outboundCounter.AddOrUpdate(server.Identifier(), (k) =>
+            _inOutBoundRecords.AddOrUpdate(server.Identifier(), (k) =>
             {
-                _lastOutboundCounter.GetOrAdd(server.Identifier(), 0);
-                return n;
-            }, (k, v) => (v + n));
+                var r = new InOutBoundRecord();
+                r.UpdateOutbound(n);
+
+                return r;
+            }, (k, v) =>
+            {
+                v.UpdateOutbound(n);
+                return v;
+            });
         }
 
         class UpdateRecordsState