Suneel Marthi created MAHOUT-1780:
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Summary: Multi-threaded Matrix Multiplication is slower than
Single-thread variant
Key: MAHOUT-1780
URL: https://issues.apache.org/jira/browse/MAHOUT-1780
Project: Mahout
Issue Type: Bug
Components: Math
Affects Versions: 0.11.0, 0.10.2, 0.10.1, 0.10.0
Reporter: Suneel Marthi
Assignee: Dmitriy Lyubimov
Priority: Critical
Fix For: 0.12.0, 0.13.0
Capturing here the Conversation on this subject:
{quote}
Turns out that matrix view traversal (of dense matrices, anyway) is 4 times
slower than regular matrix traversal in the same direction. I.e.
Ad %*% Bd: (106.33333333333333,85.0)
Ad(r,::) %*% Bd: (356.0,328.0)
where r=0 until Ad.nrow.
Investigated MatrixView, it reports correct matrix flavor (as the owner's) and
correct algorithm is selected (the same as for the row above). Sure, MatrixView
gives an indirection(sometimes even double indirection) but 4x?? It should not
be that much different from transpose view overhead, and transpose view
overhead is very small in the tests (compared to the rest of the cost)
The main difference seems to be that the algorithm over matrices ends up doing
a dot over DenseVector and a DenseVector (even that the wrapper object is
created inside the row iterations) whereas the inefficient algorithm does the
same over VectorView wrappers. I wonder if VectorView has not been equipped to
pass on the flavors of its backing vector to the vector-vector optimization.
Apparently the dot algorithm on vector view goes to the in-core vector-vector
optimization framework (calls aggregate()) but denseVector applies custom
iteration. Hence it may boil down to experiments of avec dot bvec vs. avec(::)
dot bvec(::).
{quote}
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