CODEWITHSUNDEEP
javascriptarraysmemory-management
Ted Hopp
Ted Hopp

Reputation: 234795

Are there ideal array sizes in JavaScript?

I've seen little utility routines in various languages that, for a desired array capacity, will compute an "ideal size" for the array. These routines are typically used when it's okay for the allocated array to be larger than the capacity. They usually work by computing an array length such that the allocated block size (in bytes) plus a memory allocation overhead is the smallest exact power of 2 needed for a given capacity. Depending on the memory management scheme, this can significantly reduce memory fragmentation as memory blocks are allocated and then freed.

JavaScript allows one to construct arrays with predefined length. So does the concept of "ideal size" apply? I can think of four arguments against it (in no particular order):

On the other hand, perhaps all of those arguments are wrong and a little utility routine would actually be effective (as in: make a measurable difference in script performance).

So: Can one write an effective "ideal size" routine for JavaScript arrays?

Upvotes: 16

Views: 503

Answers (3)

oookoook
oookoook

Reputation: 1

I know this question and the answer was about memory usage. BUT although there might be no difference in the allocated memory size between calling the two constructors (with and without the size parameter), there is a difference in performance when filling the array. Chrome engine obviously performs some pre-allocation, as suggested by this code run in the Chrome profiler:

<html>

<body>
<script>
    function preAlloc() {
        var a = new Array(100000);
        
        for(var i = 0; i < a.length; i++) {
            a[i] = i;
        }
    }

    function noAlloc() {
        var a = [];
        var length = 100000;
        for(var i = 0; i < length; i++) {
            a[i] = i;
        }
    }

    function repeat(func, count) {
        var i = 0;
        while (i++ < count) {
            func();
        }
    }
</script>
</body>
Array performance test
<script>
    // 2413 ms scripting
    repeat(noAlloc, 10000);
    repeat(preAlloc, 10000);
    
    
</script>
</html>

The profiler shows that the function with no size parameter took 28 s to allocate and fill 100,000 items array for 1000 times and the function with the size parameter in the array constructor took under 7 seconds.

Upvotes: 0

Travis J
Travis J

Reputation: 82267

Arrays in javascript are at their core objects. They merely act like arrays through an api. Initializing an array with an argument merely sets the length property with that value.

If the only argument passed to the Array constructor is an integer between 0 and 232-1 (inclusive), this returns a new JavaScript array with length set to that number. -Array MDN

Also, there is no array "Type". An array is an Object type. It is thus an Array Object ecma 5.1.

As a result, there will be no difference in memory usage between using

var one = new Array();
var two = new Array(1000);

aside from the length property. When tested in a loop using chrome's memory timeline, this checks out as well. Creating 1000 of each of those both result in roughly 2.2MB of allocation on my machine.

one

two enter image description here

Upvotes: 8

pid
pid

Reputation: 11597

You'll have to measure performance because there are too many moving parts. The VM and the engine and browser. Then, the virtual memory (the platform windows/linux, the physical available memory and mass storage devices HD/SSD). And, obviously, the current load (presence of other web pages or if server-side, other applications).

I see little use in such an effort. Any ideal size for performance may just not be ideal anymore when another tab loads in the browser or the page is loaded on another machine.

Best thing I see here to improve is development time, write less and be quicker on deploying your website.

Upvotes: 1

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