other_cost: tweak costing

Based on suggestion of SomberNight

electrumx/server/session.py

@@ -83,9 +83,16 @@ def assert_tx_hash(value):
 @attr.s(slots=True)
 class SessionGroup(object):
     name = attr.ib()
+    weight = attr.ib()
     sessions = attr.ib()
     retained_cost = attr.ib()
 
+    def session_cost(self):
+        return sum(session.cost for session in self.sessions)
+
+    def cost(self):
+        return self.retained_cost + self.session_cost()
+
 
 class SessionManager(object):
     '''Holds global state about all sessions.'''
@@ -314,7 +321,7 @@ class SessionManager(object):
             sessions = group.sessions
             result.append([name,
                            len(sessions),
-                           sum(s.cost for s in sessions),
+                           group.session_cost(),
                            group.retained_cost,
                            sum(s.unanswered_request_count() for s in sessions),
                            sum(s.txs_sent for s in sessions),
@@ -506,17 +513,13 @@ class SessionManager(object):
                     await group.spawn(session.close(force_after=1))
 
     def extra_cost(self, session):
-        # Add 3% of the cost of other sessions in its groups
         # Note there is no guarantee that session is still in self.sessions.  Example traceback:
         # notify_sessions->notify->address_status->bump_cost->recalc_concurrency->extra_cost
         # during which there are many places the sesssion could be removed
         groups = self.sessions.get(session)
         if groups is None:
             return 0
-        other_sessions_cost = (sum(other.cost for group in groups for other in group.sessions)
-                               - session.cost * len(groups)) * 0.03
-        retained_cost = sum(group.retained_cost / len(group.sessions) for group in groups)
-        return other_sessions_cost + retained_cost
+        return sum((group.cost() - session.cost) * group.weight for group in groups)
 
     def session_count(self):
         '''The number of connections that we've sent something to.'''
@@ -586,10 +589,10 @@ class SessionManager(object):
     def _timeslice_name(self, session):
         return f't{int(session.start_time - self.start_time) // 300}'
 
-    def _session_group(self, name):
+    def _session_group(self, name, weight):
         group = self.session_groups.get(name)
         if not group:
-            group = SessionGroup(name, set(), 0)
+            group = SessionGroup(name, weight, set(), 0)
             self.session_groups[name] = group
         return group
 
@@ -597,8 +600,8 @@ class SessionManager(object):
         self.session_event.set()
         # Return the session groups
         groups = (
-            self._session_group(self._timeslice_name(session)),
-            self._session_group(self._ip_addr_group_name(session)),
+            self._session_group(self._timeslice_name(session), 0.03),
+            self._session_group(self._ip_addr_group_name(session), 1.0),
         )
         self.sessions[session] = groups
         for group in groups: