mboehm7 commented on pull request #1362:
URL: https://github.com/apache/systemds/pull/1362#issuecomment-898735466


   LGTM - great catch @ywcb00 and thanks for fixing this performance issue. For 
local CP and distributed Spark operations we actually decide based on estimated 
sparsity if we do a sparse hash aggregation or an accumulation into a dense 
matrix. Likely the federated ctable operations was implemented for correctness 
so far. In your experiments, however, please test both sparse ctable scenario 
(like creating permutation matrices) and dense scenarios (like confusion 
matrices with moderate number of classes). In any case, the revised change is 
much better, because the previous commit affected the defaults, partially 
without considering the value, and unintended sparse accumulation can be vastly 
slower due to binary search and potential shifting. But you're right 
reevaluated these defaults (e.g., with our revived perftest suite). This 
reminds me, if feel it's beneficial, please add your federated algorithm tests 
to this perftest so we can automatically run them before releases.


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