Is Scala functional programming slower than traditional coding?

Question

In one of my first attempts to create functional code, I ran into a performance issue.

I started with a common task - multiply the elements of two arrays and sum up

Answer 1

I did some variations of this with Scala 2.8. The loop version is as you write but the functional version is slightly different:

(xs, ys).zipped map (_ * _) reduceLeft(_ + _)

I ran with Double instead of Float, because currently specialization only kicks in for Double. I then tested with arrays and vectors as the carrier type. Furthermore, I tested Boxed variants which work on java.lang.Double's instead of primitive Doubles to measure the effect of primitive type boxing and unboxing. Here is what I got (running Java 1.6_10 server VM, Scala 2.8 RC1, 5 runs per test).

loopArray               461             437             436             437             435
reduceArray             6573            6544            6718            6828            6554
loopVector              5877            5773            5775            5791            5657
reduceVector            5064            4880            4844            4828            4926

loopArrayBoxed          2627            2551            2569            2537            2546
reduceArrayBoxed        4809            4434            4496            4434            4365
loopVectorBoxed         7577            7450            7456            7463            7432
reduceVectorBoxed       5116            4903            5006            4957            5122

The first thing to notice is that by far the biggest difference is between primitive array loops and primitive array functional reduce. It's about a factor of 15 instead of the 40 you have seen, which reflects improvements in Scala 2.8 over 2.7. Still, primitive array loops are the fastest of all tests whereas primitive array reduces are the slowest. The reason is that primitive Java arrays and generic operations are just not a very good fit. Accessing elements of primitive Java arrays from generic functions requires a lot of boxing/unboxing and sometimes even requires reflection. Future versions of Scala will specialize the Array class and then we should see some improvement. But right now that's what it is.

If you go from arrays to vectors, you notice several things. First, the reduce version is now faster than the imperative loop! This is because vector reduce can make use of efficient bulk operations. Second, vector reduce is faster than array reduce, which illustrates the inherent overhead that arrays of primitive types pose for generic higher-order functions.

If you eliminate the overhead of boxing/unboxing by working only with boxed java.lang.Double values, the picture changes. Now reduce over arrays is a bit less than 2 times slower than looping, instead of the 15 times difference before. That more closely approximates the inherent overhead of the three loops with intermediate data structures instead of the fused loop of the imperative version. Looping over vectors is now by far the slowest solution, whereas reducing over vectors is a little bit slower than reducing over arrays.

So the overall answer is: it depends. If you have tight loops over arrays of primitive values, nothing beats an imperative loop. And there's no problem writing the loops because they are neither longer nor less comprehensible than the functional versions. In all other situations, the FP solution looks competitive.