[core] fix slow predict-caching with many classes #3109
Merged
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Addresses the issue of the O(num_class^2) prediction cache behavior #1689, #2926 where cache updates within CommitModel may start dominating with many classes. The reason was that the cache was updated for each single class group commit in a boosting round. E.g., with
the timing result before (note that no watchlist was used, and using it would make caching even slower):
and after the fix:
Some minor changes: