Add random component to track query interval

Add per-query jitter to the track query interval, to avoid repeatedly
sending queries to all clients connected at the same time, which in
turn is to avoid load spikes.

Signed-off-by: Mattias Rönnblom <[email protected]>

paf/sd.py

@@ -4,6 +4,7 @@
 
 import contextlib
 import enum
+import random
 import time
 
 
@@ -289,6 +290,7 @@ def assure_not_connected(fun):
 
 
 WARNING_THRESHOLD = 0.5
+WARNING_JITTER = 0.1
 MIN_IDLE_TIME = 4
 
 
@@ -298,6 +300,11 @@ class IdleState(enum.Enum):
     TIMED_OUT = enum.auto()
 
 
+def jitter(base, max_jitter):
+    k = 1.0 + (random.random() - 0.5) * 2 * max_jitter
+    return k * base
+
+
 class Connection:
     def __init__(self, client, timer_manager, max_idle_time, idle_cb):
         self.client = client
@@ -393,7 +400,9 @@ def idle_time(self):
             return t
 
     def install_idle_warning_timer(self):
-        self.install_idle_timer(WARNING_THRESHOLD * self.idle_time())
+        warning_time = jitter(WARNING_THRESHOLD * self.idle_time(),
+                              WARNING_JITTER)
+        self.install_idle_timer(warning_time)
 
     def install_idle_timeout_timer(self):
         self.install_idle_timer((1 - WARNING_THRESHOLD) * self.idle_time())

test/test_paf.py

@@ -738,7 +738,7 @@ def test_server_tracking_client(impatient_server):
     wait(conn, criteria=lambda: track_recorder.count_notifications() > 0)
 
     latency = time.time() - start
-    assert latency > query_time
+    assert latency > (query_time - 0.5)
     assert latency < (query_time + 0.5)
 
     notifications = track_recorder.get_notifications()