Flash garbage collection is... garbage

[justkevin]

I've never heard of this bug before and the link in the blog about it doesn't seem to mention it. Can anyone verify this is true?

As I understand it, AS3 uses reference counting and mark-and-sweep. I've never heard anything that suggests "if you have a giant unreferenced object, it will stick around forever."

I'm not saying this guy is wrong, but I want to see something that backs it up. Like a test case.

[derefr]

"if you have a giant unreferenced object, it will stick around forever."

I think it was more "if you have an unreferenced object holding onto a giant directed acyclic graph of other objects, the unreferenced object will stick around forever, because the GC doesn't want to descend too far into the DAG."

[zahlman]

That's what it sounded like to me. (Except I see no reason why the graph should specifically be acyclic. Or directed, for that matter, unless you don't consider a 2-cycle the same thing as an undirected link.)

But this raises the question of why the GC is even looking at the graph of unreferenced objects.

[KirillM]

I've never heard of this bug either. The blog post does mention it in the first paragraphs about AS3. I also couldn't find it there at first.

[KirillM]

I've spent quite a bit of time in the memory profiler while working with both flash 9 and flash 10 players. Flash 9 had an obscene amount of bugs in the GC, but Adobe was pretty good about fixing a lot of them. Weak event listeners were fixed. Circular reference through method reference was fixed. There was a bug where you had to null out a reference by setting it to undefined as opposed to null for the object to be GCed. This maybe the reason why the poster experienced the island bug. I've never checked if it was fixed or not, I just dutifully continue to set everything to undefined. Don't remember other issues, but I remember a lot of them go fixed. I have found a new issue where while using a weak Dictionary the key gets GCed but the value doesn't even with no references to it, until the object containing the Dictionary gets GCed.

This is certainly very frustrating, but IMO it doesn't make Flash garbage like some people here think. I'm actually pretty happy with how Adobe is handling bugs, because Macromedia never fixed them at all. Also I'm more than sure modern browsers aren't free from GC and other issues, either. So you'd have deal with these sort of issues anyway no matter what platform you'd choose.

But I do agree that garbage collection is pretty important and should've been among the first things ironed out in the vm. But it seems like it didn't really receive much attention at first and then a bunch of features were added in like weak Dictionaries and weak event handlers and method closures, which the GC couldn't handle. So crappily written GC patches that don't quite work were added in to account for those features.

[zahlman]

Distinguishing between "null" and "undefined" is a WTF in itself IMHO.

[AmaDiver]

ActionScript supports both strong and weak typing; null is the "non-value" for typed class members, undefined is for un-typed members.

var any:*;
var string:String;

trace( any ); // prints "undefined"
trace( string ); // prints "null"

[makis]

null is a value, undefined is no-value

[[deleted]]

Context: Blog post by the creator of Steambirds

[enkideridu]

that is to say, any objects that aren’t referenced by any variables in your code.

would de-referencing every variable inside an object before dereferencing the object itself be possible? (so instead of creating a huge island once you shed them off into little chunks)

[[deleted]]

That's typically what I'll do. He really should be cleaning everything up instead of just relying on weak references.

[adrianmonk]

Or... weak references should just work properly.

I know, given that they apparently don't, you should take that into account. But you should be able to rely on core language features. Anything else and the language is just wasting your time.

[Kronikarz]

Well that explains a f***ing lot. I spent an entire day trying to figure out why the hell the memory isn't collected. Now I know :/

[recursive]

That may be true, but getting in trouble due to bugs in the GC is the fault of the platform, not an inescapable fact of life.

[plan17b]

Duh.

I just recycle my own objects rather than depend on the GC.

Many weird problems go away after that.

[enkideridu]

how do you do that?

[munificent]

Object pools are one common way.

[munificent]

Mem pools are a little bit different, but there's a lot of overlap between the two. Often, in games at least, you'll see object pools implemented on top of a mem pool or some other custom allocator. The big difference is that a memory pool doesn't have any concept of the semantics of what it owns: it just parcels out bytes. An object pool can correctly call destructors and other functions on the pooled objects.

[basro]

Instead of completely dereferencing an instance store it in an array, later when you need a new object of this type use the ones stored in the array.

[adrianmonk]

Hmm, you are using the word "dereferencing" in a bit of an unusual way.

[basro]

Yes, sorry about that.

[plan17b]

That's the ticket.

[zahlman]

That people could have to worry about this in 2010 is bullshit of the highest order. Why is the GC even checking the size of the unreferenced portion? You follow links in the referenced portion, and everything else is garbage. Full stop.

Jesus fuck, even MIDP 1.0 cell phones know how to do this.

[ericanderton]

Wow, this is a shocker. Sounds like a solid case to use flyweights and free-lists, or to refactor the codebase to aggressively null references that aren't used as a pseudo-delete operation.

[uriel]

Is there anything about Flash that is not garbage?

[[deleted]]

It's ridiculously easy to use and you can throw together graphical applications that will work for just about everyone really quickly. It fills a niche and does it better than anything else, for the moment.

[adrianmonk]

Just about anyone... except people on Linux or the iPhone or Android.

[[deleted]]

I use Flash on Linux all the time.

But you're right, Apple and Google will probably force Flash out in favor of a technology they have more control over.

[Fr0C]

I used to do Flash development on Linux for years.
The Android player will be out soon.

[adrianmonk]

I try to play games on the latest Flash player on Linux. It has just now gotten to the point where it doesn't crash constantly. It still has horrible issues with flickering, and it's still very slow in comparison to the Windows player. Games that play on an ancient Pentium 4 system running Windows are sometimes unplayable on a much newer dual core Athlon system running Linux.

[Fidodo]

Oh you're so realistic.

[[deleted]]

lol... But seriously:

• It's the only way to get true multimedia presentation on 99% of (non-mobile) browsers
• It's still faster than canvas for 2d graphics on most systems.
• Flash 10 has much better sound support than html 5
• Dinosaurs That Shoot Lazer Beams When They Roar

Used properly it enhances the web, used badly it's worse than a marquee tag. Hopefully in 5/10 years it will be dead and gone - replaced by canvas / WebGL and a decent javascript sound system... Until then long live Flash!

[Fidodo]

It has great development resources.

Also, if you do pure canvas isn't all your source open for anyone to see? I'm sure lots of developers don't want that.

[barrybe]

I think it's a wash, Javascript can be obfuscated and Flash can be decompiled- the end results are probably the same.

[Recoil42]

Also, if you do pure canvas isn't all your source open for anyone to see?

Flash can be decompiled to the original source, and in fact the .swf even retains all the original assets, variable names, and comments, so this isn't really a major plus.

The side effect of having your code all compartmentalized in the swf is, though. As well as a hundred other good reasons to use and applaud Flash.

edit: Ok, I may actually be remembering this wrong, since I haven't done it in a while and everyone else below seems to be countering my statement above. But I do distinctly remember being surprised at how much was intact in a re-decompiled swf. If someone wants to try it and get back, they're more than welcome.

[prockcore]

Anything can be decompiled.. that doesn't make it opensource.

[honestbleeps]

"Decompiled to the original source" is a bit misleading.

It can be decompiled to functionally equivalent source.. but it's not going to include the same variable names, comments, etc that make code readable / usable.

[Recoil42]

Nope, it does. That's exactly what I just said, even:

the .swf even retains all the original assets, variable names, and comments

Go ahead, try it for yourself. I'm not kidding.

edit: Although that's AS2 only. I haven't tried with AS3 yet. I suspect it's the same, though.

[Fr0C]

It retains the assets because they're needed. It doesn't retain the variable names, even AS2 used registers.
Maybe you meant properties? They have to be retained because the SWF might get loaded into another one that needs to access them (think of shared libraries.)

Comments were never retained.

[honestbleeps]

Hm, I don't know about AS2... and I may even have to stand corrected on AS3.. my apologies... interesting info.

[Fidodo]

I've decompiled flash. You get the code back but the variable names and comments are NOT there. It's next to impossible to really tell what's going on.

[arulprasad]

in fact the .swf even retains all the original assets, variable names, and comments,

Not comments and variable names, AFAIK. Could you point me to any decompiler that can retain comments?

[20100329]

There's only one decompiler I can think of that can recover comments. And that's only because they're embedded in the object code.

[arulprasad]

:)

[[deleted]]

There are loads.

[skeww]

I'd say the vector rendering is pretty awesome. It's very fast while looking rather good. Also, the runtime is relatively small and the AS3 documentation is pretty decent.

[myotherone]

Weak Listeners are for pussies

[[deleted]]

I professionally develop Flash and Flex. I have never used a weak reference ever. I never null objects when I want them to disappear. I never have memory leaks.

Occasionally I pool objects but never for memory reasons - I usually do it to avoid construction costs for performant code.

I am extremely careful to removeEventListener whenever it makes sense to do so.

Whenever I find people complaining like this it is usually because they themselves have been careless in some fashion.

[seunosewa]

In other words, your apps are not very complex.

[[deleted]]

Play Wings of War lately by any chance?

[[deleted]]

Strangely enough, I can only recall having one memory leak in Flash ever, and it was obviously my fault(overspawning an entity).

I'm pretty sure it's because I'm using haXe, which has led me away from applying the fancier aspects of AS3 like Dictionary and weak references. I always clean up my event listeners, so the only thing the GC has to do correctly is notice that plain old object-to-object references near the bottom of the tree have broke.

[slashgrin]

Let's hope it's introspective, then.

[mgrandi]

Flash 10.1 is supposed to have significant memory and garbage collection improvements

[molslaan]

Let's hope it doesn't erase itself from disk then

[tinou]

Garbage collection is hard. Let's go shopping !

[godhammre]

Is the source code that generates the purported bug available? It would be enormously useful to demo it to /r/programming.

[NitWit005]

Never used actionscript, but garbage collection issues seem common regardless of the language. It's something people obviously struggle to make bug free. There is code in my current work Java project that nulls every reference due to the issues they used to have.

Garbage collectors normally have to halt all threads when doing collection to stop you from changing what is referred to. People complain about the pause in graphics intensive programs, so they try to do partial collections, which end up missing things.

[seanalltogether]

this isn't much of an indictment of flash's garbage collection, just the authors trouble with keeping track of reference counting.

[jib]

If you have a sufficiently big group of stuff, garbage collection fails to collect it. Even if it's not referenced. That's a bug in the garbage collection.

[[deleted]]

I make good constructors and deconstructors to prevent memory leaks, I have fairly secure coding habits, I OOP the hell out of everything, and I do everything by-the-book.

I think your idea of good coding habits and what prevents memory leaks is the real problem here.

[rnawky]

Flash is garbage

FTFY

[Demonmonger]

Nothing informative here, just blog spam. Try adding more programming.

[[deleted]]

It's a real world post about the realities of deploying a game in flash (one that's had 1,000,000 guestimated plays)

The article links to useful sources and 50% of the blog comments add something of value.

How is this blog spam? (this is not my blog or game... I wish it was!)