Matthew Ball created SPARK-58177:
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Summary: Fix wrong operand in SVD++ aggregateMessages combiner
Key: SPARK-58177
URL: https://issues.apache.org/jira/browse/SPARK-58177
Project: Spark
Issue Type: Bug
Components: GraphX
Affects Versions: 5.0.0
Reporter: Matthew Ball
The Phase-2 aggregateMessages combiner in SVDPlusPlus.run merges two
(Array[Double], Array[Double], Double) messages destined for the same vertex. It
sums the _1 and _3 elements correctly but seeds the _2 accumulator from the
wrong
operand:
val out2 = g2._2.clone()
BLAS.nativeBLAS.daxpy(out2.length, 1.0, g2._2, 1, out2, 1)
Because daxpy computes y = x + y, this yields 2 * g2._2 and discards g1._2
entirely. Since aggregateMessages folds all messages arriving at a vertex
through
this function, the _2 term (the implicit-feedback / updateY accumulator)
collapses
to 2 * lastMessage for any vertex with two or more incident edges. The reduce
function is also non-commutative, so the result depends on the (undefined)
message
order.
The correct sibling merge in the same method (the outerJoinVertices step just
below) confirms the intended semantics: out2 = g1._2 + g2._2.
Fix: seed out2 from g1._2 instead of g2._2 (a one-character change).
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