Nd4j: using multiple threads is slower than single-thread

Question

This is my processor: 2.3 GHz Intel Core i7 (so 4 cores with hyper threading) on MacOs Sierra

This is my program:

package nn

import org.nd4j.linalg.api.ndarray.INDArray
import org.nd4j.linalg.factory.Nd4j.randn
import org.nd4j.linalg.ops.transforms.Transforms._
import org.nd4s.Implicits._

object PerfTest extends App {

  val topology = List(784, 30, 10)
  val biases: List[INDArray] =
    topology.tail.map(size => randn(size, 1))

  val weights: List[INDArray] =
    topology.sliding(2).map(t => randn(t(1), t.head)) toList

  (1 to 100000).foreach { i =>
    val x = randn(784, 1)
    biases.zip(weights).foldLeft(List(x)) {
      case (as, (b, w)) =>
        val z = (w dot as.last) + b
        val a = sigmoid(z)
        as :+ a
    }
  }
}

When I run above program with the default threading (for nd4j and this processor this will be 4), it takes around 28 seconds.

When I run it on 1 core (export OMP_NUM_THREADS=1), then it takes 18 seconds.

Any idea why this is ? Thank you.

Answer 1

I wasn't able to find a clarification for this. So I migrated to Breeze, which is like 6 times faster, without much ado.