F# – Speed test iter and map operations with array vs list

I have heard a few people argue that when it comes to performance critical code you should prefer arrays over other collections (such as F#’s lists) as it benefits from sequential reads (which is faster than seeks) and offers better memory locality.

To test that theory somewhat, I wanted to see if there is any difference in how fast you can iterate through an array versus a list in F#, and how much faster you can map over an array compared to a list:

The result is a little surprising, whilst I wasn’t expecting there to be a massive difference in the iterating through the two types of collections, I didn’t think mapping over a list would be quite as slow in comparison. I knew that constructing a list is much heavier than constructing an array, but I didn’t think it’d take 22x as long in this case.

What was even more surprising was how much slower the Seq.iter and Seq.map functions are compared to the Array and List module equivalents! This is, according to John Palmer:

Once you call in to Seq you lose the type information – moving to the next element in the list requires a call to IEnumerator.MoveNext. Compare to for Array you just increment an index and for List you can just dereference a pointer. Essentially, you are getting an extra function call for each element in the list.

The conversions back to List and Array also slow the code down for similar reasons

Update 2012/06/04:

As a work around, you COULD shadow the Seq module with iter and map functions that adds simple type checking and in the case of an array or list simply call the corresponding function in the Array or List module instead:

Whilst this approach will work to a certain extend, you should be careful with which functions you shadow. For instance, it’s not safe to shadow Seq.map because it can be used in conjunction with other functions such as Seq.takeWhile or Seq.take. In the base implementation, a line of code such as:

arr |> Seq.map incr |> Seq.take 3

will not map over every element in the source array.

With the shadowed version (see above) of Seq.map, however, this would first create a new array by applying the mapper function against every element in the source array before discarding all but the first three elements in the new array. This, as you can imagine, is far less efficient and requires much more memory space (for the new array) and defeats the purpose of using Seq module functions in most cases.

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