Allow sFlowCpInterval=0 to disable counter samples, and fix bug in sampler removal.

[sliver-openvswitch.git] / lib / sflow_poller.c

diff --git a/lib/sflow_poller.c b/lib/sflow_poller.c
index ffd09d3..e7dc2b1 100644 (file)
--- a/lib/sflow_poller.c
+++ b/lib/sflow_poller.c
@@ -68,10 +68,29 @@ u_int32_t sfl_poller_get_sFlowCpInterval(SFLPoller *poller) {
 
 void sfl_poller_set_sFlowCpInterval(SFLPoller *poller, u_int32_t sFlowCpInterval) {
     poller->sFlowCpInterval = sFlowCpInterval;
-    /* Set the countersCountdown to be a randomly selected value between 1 and
-       sFlowCpInterval. That way the counter polling would be desynchronised
-       (on a 200-port switch, polling all the counters in one second could be harmful). */
-    poller->countersCountdown = 1 + (random() % sFlowCpInterval);
+    if(sFlowCpInterval) {
+        /* Set the countersCountdown to be a randomly selected value between 1 and
+          sFlowCpInterval. That way the counter polling will be desynchronised
+          (on a 200-port switch, polling all the counters in one second could be harmful).
+          In a large network, even this might not be ideal if time-synchroniziation
+          between devices is close and counters are always polled on second boundaries. If
+          1000 different devices all send an sFlow datagram on the same second boundary
+          it could result in an antisocial burst.
+          However when counter-samples are packed into the export datagram they do not
+          always result in that datagram being sent immediately. It is more likely that
+          a subsequent packet-sample will be the one that triggers the datagram to be sent.
+          The packet-sample events are not sychronized to any clock, so that results in
+          excellent desynchronization (http://blog.sflow.com/2009/05/measurement-traffic.html).
+          Another smoothing factor is that the tick() function called here is usually
+          driven from a fairly "soft" polling loop rather than a hard real-time event.
+       */
+        poller->countersCountdown = 1 + (random() % sFlowCpInterval);
+    }
+    else {
+        /* Setting sFlowCpInterval to 0 disables counter polling altogether.  Thanks to
+          Andy Kitchingman for spotting this ommission. */
+        poller->countersCountdown = 0;
+    }
 }
 
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