mcvsubbu commented on issue #3454: what is the difference between mmap and heap?
URL: 
https://github.com/apache/incubator-pinot/issues/3454#issuecomment-467091867
 
 
   Our documentation is still in progress. We expect to finish it by the time 
we make the first apache release (which should be real soon, once we verify the 
current candidate in production). Meanwhile, a few comments:
   1. You seem to be running out of direct memory while loading a completed 
segment. Changing HEAP to MMAP will help there (assuming you Operating system 
over-commits mmap memory). Your time column unit is in MILLISECONDS. This can 
cause dictionary to explode unless you are rounding off the time values. Please 
check if you are doing so.  So, if your millisecond value is 1551113826143 then 
round it off to (for example) the nearest 5 minute value 1551113710000. You 
will need to do this before you send your Kafka events, if you have control 
over it. We are thinking of providing an automatic switch in pinot to do this 
(i.e. round off milliseonds to nearest 5 mins or 15 mins or hour, whatever)
   2. You may want to try using auto-segment siizing. See 
https://cwiki.apache.org/confluence/display/PINOT/Consuming+and+Indexing+rows+in+Realtime
   

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