- /* 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;
+ }