Hi Joran,
Here are some insights:
0. do not rely on Activity Monitor. Some memory regions could be
committed but not used. There is --trace-gc flag which outputs heap
usage statistics.
1. Flat string of length L would occupy L + 12 bytes.
2. When you concatenate strings you get cons string as a result, not a
flat string. Cons string ultimately is a pair of pointers to strings
that were concatenated. It occupies 12 + 2*4 bytes. Try rewritting
set[id + length] = id + length;
to
var s = id + length; s.charCodeAt(0); // forces flattening
set[s] = s;
and measure memory consumption.
3. Dictionary with n properties will occupy ~
3*4*2*ClosestPowerOfTwo(n) bytes, where ClosestPowerOfTwo(n) = min { p
| n < 2^p }
4. result of Math.random is pure double and will occupy 4 + 8 bytes,
but you have to keep in mind that in JS all properties are strings. V8
optimizes dictionaries for continuous int32 indexes but Math.random
certainly does not fall into this category.
Hope that helps.
--
Vyacheslav Egorov
On Mon, Nov 22, 2010 at 10:09 AM, Joran Greef <[email protected]> wrote:
> I came across this trying to use a Javascript object to provide O(1)
> indexing for sets and sorted sets.
>
> I tried the following in Node.js:
>
> var set = {};
> var testMemoryUsage = function() {
> var length = 1000000;
> var id = 'testing';
> while (length--) {
> set[id + length] = id + length;
> }
> };
>
> I ran the function and then watched memory usage in Activity Monitor.
> On each of several attempts, the process would start out with 20mb and
> then grow to more than 170mb and never come down again, even after
> several minutes.
>
> The same thing happens in Chrome, but it goes up to 122mb.
>
> Of course the set variable is never dereferenced, but the data stored
> therein is in the order of 25mb. So one would expect the process to
> sit at (20mb + 25mb = 45mb). Give or take a few mb for V8
> implementation. But 170mb is surprising. Worse, this appears to scale
> with the size of the dictionary.
>
> Anyone have any idea what this could be?
>
> --
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> [email protected]
> http://groups.google.com/group/v8-users
>
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