Re: more details on jscript memory leak from DOM object references

"Mark D. Anderson" <mda (AT) discerning (DOT) com> wrote

About a month ago Richard Cornford did an interesting
analysis of a memory leak
in jscript (internet explorer) when there are "circular"
references between DOM objects and (real) jscript objects:

<snip URL of previous discussion via groups.google.com>

This message summarizes some testing I've done and their
results. These results somewhat contradict Cornford's
conclusions; I haven't analyzed his test page to
come to an explanation.

Below is an html test page so that anyone can (attempt
to) reproduce my results.

To test:
- Bring up task manager and a fresh IE process on this
web page.
- Click on one of the div links, refresh the page, repeat,
and watch the process size.
I have been doing 3 refresh/click sequences per div.

I am not going to have time to examine your test page in detail tonight
(and maybe not tomorrow either) to see if and why you think your results
are different to mine (if you are going to claim your results contradict
my conclusions it would have saved a bit of time if you had stated the
specific conclusions that you think are contradicted). I should be able
to find time to look at your page in detail over the weekend and let you
know what I think.

However, did you notice early in the previous thread Jim Ley implying
that just repeatedly refreshing the same page may give the impression of
a memory leak in IE when navigating away from the page would free the
memory. He specifically agreed with my suggestion that any page wishing
to demonstrate a real problem in IE would have to cycle between at least
two distinct pages.

While I would not take anyone's word as gospel, when it comes to
obscurer details of browser behaviour I would always pay close attention
to what Jim has to say. Accordingly my tests used two or three pages and
cycled between them. As a result I am certain that they do demonstrate
the memory leaks I described, though I may have concluded that the
problem was more general than it actually is. But I will be
incorporating your code into Multi-page examples for testing.

Giving your script a cursory glance, specifically the -
leak_test_global - function, I don't think that it demonstrates that
references to global objects do not produce leaks as the DIV element
does not have a reference to the global object and the closure formed by
assigning the event handling function does not contain a reference to
the global object. If you replace the event handling function with an
expando property that refers to the global object, so that the reference
is circular:-

function leak_test_global(node) {
test_start('global');
node.expando1 = myglobal;
myglobal.push(node); // make circular
}

- I think you will find that it does leak.

Richard.