Question on segments in memory


Recently we experienced that some slow query and finally lots of the
queries timeouted. We found that on some data nodes the heap usage is
nearly > 90% and GCs can not free much space.
As we already marked many fields as doc values so at this time, the
fielddata cache size is small (~ 100MB) but we found that there's lots of
segments in memory and GCs seems not able to free them.

More precisely on data nodes we have 4GB heap and nearly 3GB is occupied by
in memory segments. In the same time we checked that only a few (~5)
segments are not committed.

Our questions are

  • why we have these in memory segments in memory when they are all
  • why when there's pressure on the heap, GC can not free them?
  • what brings these segments in memory? search, aggregation and maybe
    recovery process?


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