Setting the environment variable WOSIM_CLIENT_CHURN=0 turns off client churning

That way, we can measure the effect of client churn on relay bandwidth

simulator.py

@@ -3,6 +3,7 @@
 import random # For simulation, not cryptography!
 import math
 import sys
+import os
 import logging
 import resource
 
@@ -52,9 +53,18 @@ class Simulator:
         # we treat them as "new") over several days in late Dec 2019,
         # newmean is about 16% of all clients, and newstddev is about 4%
         # of all clients.
-        self.client_newmean = 0.16 * self.clienttarget
-        self.client_newstddev = 0.04 * self.clienttarget
-        self.client_oldprob = 0.16
+
+        # if the environment variable WOSIM_CLIENT_CHURN is set to 0,
+        # don't churn clients at all.  This allows us to see the effect
+        # of client churn on relay bandwidth.
+        if os.getenv('WOSIM_CLIENT_CHURN', '1') == '0':
+            self.client_newmean = 0
+            self.client_newstddev = 0
+            self.client_oldprob = 0
+        else:
+            self.client_newmean = 0.16 * self.clienttarget
+            self.client_newstddev = 0.04 * self.clienttarget
+            self.client_oldprob = 0.16
 
         # Start some dirauths
         self.dirauthaddrs = []
@@ -246,35 +256,36 @@ class Simulator:
                 bw = int(200000-(200000-25000)/3*math.log10(x))
                 self.relays.append(relay.Relay(self.dirauthaddrs, bw, 0))
 
-        # TODO: churn clients
-
-        # Stop some of the clients
-        clients_remaining = []
-        numclients = len(self.clients)
-        numclientsterminated = 0
-        lastpercent = 0
-        logging.info("Terminating some clients")
-        for i, c in enumerate(self.clients):
-            percent = int(100*(i+1)/numclients)
-            if random.random() < self.client_oldprob:
-                c.terminate()
-                numclientsterminated += 1
-            else:
-                # Keep this client
-                clients_remaining.append(c)
-            if percent != lastpercent:
-                lastpercent = percent
-                logging.info("%d%% clients considered, %d terminated",
-                        percent, numclientsterminated)
-        self.clients = clients_remaining
-
-        # Start some new clients
-        clients_new = int(random.normalvariate(self.client_newmean,
-                self.client_newstddev))
-        logging.info("Starting %d new clients", clients_new)
-        if clients_new > 0:
-            for i in range(clients_new):
-                self.clients.append(client.Client(self.dirauthaddrs))
+        # churn clients
+
+        if self.client_oldprob > 0:
+            # Stop some of the clients
+            clients_remaining = []
+            numclients = len(self.clients)
+            numclientsterminated = 0
+            lastpercent = 0
+            logging.info("Terminating some clients")
+            for i, c in enumerate(self.clients):
+                percent = int(100*(i+1)/numclients)
+                if random.random() < self.client_oldprob:
+                    c.terminate()
+                    numclientsterminated += 1
+                else:
+                    # Keep this client
+                    clients_remaining.append(c)
+                if percent != lastpercent:
+                    lastpercent = percent
+                    logging.info("%d%% clients considered, %d terminated",
+                            percent, numclientsterminated)
+            self.clients = clients_remaining
+
+            # Start some new clients
+            clients_new = int(random.normalvariate(self.client_newmean,
+                    self.client_newstddev))
+            logging.info("Starting %d new clients", clients_new)
+            if clients_new > 0:
+                for i in range(clients_new):
+                    self.clients.append(client.Client(self.dirauthaddrs))
 
         # Tick the epoch
         network.thenetwork.nextepoch()