Github user wangli1426 commented on the pull request:
https://github.com/apache/storm/pull/716#issuecomment-138027523
Hi @HeartSaVioR,
I am glad to explain the mechanism of RateTracker as follows.
In general, RateTracker provides a way to monitor the rate of certain
event. We notify RateTracker upon the occurrence of the event by calling
```notify()``` method, and call ```reportRate``` to retrieve the instantaneous
rate of the event.
The rate is calculated as ```counts of occurrence``` / ```time```. As
RateTracker reports instantaneous rate, it only maintains the counts of
occurrences within a given period of time as specified by
```validTimeWindowInMils```. This is implemented by dividing the
```validTimeWindowInMils``` into ```numOfSlides``` time slides and employing a
timer to periodically (every ```validTimeWindowInMils```/ ```numOfSlides```
milliseconds) clean the count in the oldest time slide.
Finally, RateTracker is very efficient. First, ```notify()``` only does
addition on a count and thus can be finished within few CPU cycles. Second, the
update of the counts is also light-weight and only happens periodically, e.g.,
every 1 or 2 seconds. Third, all the instances of RateTracker share the same
timer, avoiding using too many threads.
If you have further question, please let me know.
Thanks.
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