J.Doe J.Doe - 1 year ago 77
Swift Question

Using something other than a Swift array for mutable fixed-size thread-safe data passed to OpenGL buffer

I am trying to squeeze every bit of efficiency out of my application I am working on.

I have a couple arrays that follow the following conditions:

  1. They are NEVER appended to, I always calculate the index myself

  2. The are allocated once and never change size

  3. It would be nice if they were thread safe as long as it doesn't cost performance

  4. Some hold primitives like floats, or unsigned ints. One of them does hold a class.

  5. Most of these arrays at some point are passed into a glBuffer

  6. Never cleared just overwritten

  7. Some of the arrays individual elements are changed entirely by
    others are changed by

I currently am using swift native arrays and am allocating them like
var arr = [GLfloat](count: 999, repeatedValue: 0)
however I have been reading a lot of documentation and it sounds like Swift arrays are much more abstract then a traditional C-style array. I am not even sure if they are allocated in a block or more like a linked list with bits and pieces thrown all over the place. I believe by doing the code above you cause it to allocate in a continuous block but i'm not sure.

I worry that the abstract nature of Swift arrays is something that is wasting a lot of precious processing time. As you can see by my above conditions I dont need any of the fancy appending, or safety features of Swift arrays. I just need it simple and fast.

My question is: In this scenario should I be using some other form of array? NSArray, somehow get a C-style array going, create my own data type?

Im looking into thread safety, would a different array type that was more thread safe such as NSArray be any slower?

Answer Source

Note that your requirements are contradictory, particularly #2 and #7. You can't operate on them with += and also say they will never change size. "I always calculate the index myself" also doesn't make sense. What else would calculate it? The requirements for things you will hand to glBuffer are radically different than the requirements for things that will hold objects.

If you construct the Array the way you say, you'll get contiguous memory. If you want to be absolutely certain that you have contiguous memory, use a ContiguousArray (but in the vast majority of cases this will give you little to no benefit while costing you complexity; there appear to be some corner cases in the current compiler that give a small advantage to ContinguousArray, but you must benchmark before assuming that's true). It's not clear what kind of "abstractness" you have in mind, but there's no secrets about how Array works. All of stdlib is open source. Go look and see if it does things you want to avoid.

For certain kinds of operations, it is possible for other types of data structures to be faster. For instance, there are cases where a dispatch_data is better and cases where a regular Data would be better and cases where you should use a ManagedBuffer to gain more control. But in general, unless you deeply know what you're doing, you can easily make things dramatically worse. There is no "is always faster" data structure that works correctly for all the kinds of uses you describe. If there were, that would just be the implementation of Array.

None of this makes sense to pursue until you've built some code and started profiling it in optimized builds to understand what's going on. It is very likely that different uses would be optimized by different kinds of data structures.

It's very strange that you ask whether you should use NSArray, since that would be wildly (orders of magnitude) slower than Array for dealing with very large collections of numbers. You definitely need to experiment with these types a bit to get a sense of their characteristics. NSArray is brilliant and extremely fast for certain problems, but not for that one.

But again, write a little code. Profile it. Look at the generated assembler. See what's happening. Watch particularly for any undesired copying or retain counting. If you see that in a specific case, then you have something to think about changing data structures over. But there's no "use this to go fast." All the trade-offs to achieve that in the general case are already in Array.

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