mcvsubbu commented on issue #3454: what is the difference between mmap and heap?
URL: 
https://github.com/apache/incubator-pinot/issues/3454#issuecomment-467521349
 
 
   The time unit is not the cause of memory overflow. It allows you to utilize 
memory more efficiently while getting good query performance.
   
   We will be providing documentation with the next apache release on mmap vs 
heap, etc. Basic difference (as I have said so before) is that MMAP maps the 
segment file onto memory. So, it the pages are not resident, it will paged in 
the operating system. HEAP reads in the segment via Direct memory allocation.  
So, we are never dependent on IO during query serving. The performance of these 
is dependent on your use case, hardware and operating system settings. 

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