[R] R matrix multiplication slowdown

ken eagle eaglek2011 at gmail.com
Thu Jan 26 23:21:07 CET 2017

Hi all.  A question about performance of matrix multiplication.

I have two relatively large matrices:

A is 100x3072, all integers 0-255, not sparse

B is 1016x3072, all integers 0-255, not sparse

The command z<-B %*% t(A) works fine and takes roughly 0.2 seconds .  If I
add one row to B, the same command takes 2.4 seconds; at 1050 rows in B,
the command is up to almost 4 seconds.  Just trying to understand why the
big slowdown is occurring, especially since the matrices I actually want to
multiply have 1000 and 5000 rows, not 100 and 1017.



(Macbook Pro running 10.11.6, 16Gb, R v 3.3.1)

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