Github user AnthonyTruchet commented on the issue:

    https://github.com/apache/spark/pull/16038
  
    @mridulm we don't know how to monitor the size of the serialize task. Sure 
it would not 10MB due to all those zeros. But we nonetheless measure a 
significant increase in performance and (more importantly) in stability when 
using the workaround and our custom TreeAggregatorWithZeroGenerator 
    
    @srowen when the density of a Sparse Vector increases it became *very* 
inefficient, not only because it is using almost twice the size of memory, but 
mainly because you fragment memory access on overload the GC :( 
    
    See #16078 for a generic wrapper around treeAggregate with uses Option[U] 
do denote the zero of U by None and generate a full representation by need when 
calling seqOp or comboOp. 


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