Github user srowen commented on the issue: https://github.com/apache/spark/pull/16038 (I am in the UK, but let's keep the discussion online here) OK, I had assumed that `Vectors.zeros` makes a sparse vector. It does not; it makes a dense vector. If it were sparse, then it would be tiny to serialize. Hence I didn't understand this to be the issue, and though you were saying that another copy of a dense representation was accidentally in the closure. However, this means there's a more direct solution without changing or adding to the public API: pass a sparse vector (of zeroes) as the zero value. It looks like this should work, even be a little faster on the first pass when the gradient is 0. If for some reason something doesn't like the sparse vector, it's easy enough to call `.toDense` such that it's only expanded once and not on the driver. In that case, I would not make the change in this PR, and change https://github.com/apache/spark/pull/16037 to make the initial vector sparse only.
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