can objects be marked not to be garbage collected? - javascript

I don't know too much about the JavaScript garbage collector, just that it attempts to manage references so that unreferenced objects can be periodically purged from memory. I was thinking about something that I thought might improve performance if it was feasible by the language implementers.
It would go something like this. In a file add a line:
"no gc";
This is similar to the use strict setting. It would mark everything defined in the file as not for garbage collection. I'm thinking this would be used in libraries like jQuery and underscore. All of the helper methods would be marked and stored in a separate area of memory that is not managed by the GC.
While I know this might end up keeping around stuff that is not ever used; it would at least isolate it from periodic GC process. So while we perhaps gobble up some extra memory, we at least lighten the load of GC processing.
I apologize for the naivety of this suggestion as I have never implemented GC. I am just wondering if this idea is feasible or if JavaScript somehow does this already.

if you want to keep them as cache then you have global scope.
In browser global scope is window,
hence consider if you dont want object X to never get garbage collected then simply you can write
window.nogc = X;
since window which is global scoped ,will be never garbage collected so its child references also wont be garbage colleted until we explicitly make it.

Garbage collection only runs when the thread is free. Nothing would be saved because GC only occurs when the system isn't busy.
So no, this isn't possible.

You can ensure that an object does not get collected by referencing it from a GC root, but not that it doesn't get processed by the GC.
The reason is that GC in JS VMs is usually implemented via Mark-and-Sweep, or a method that is functionally equivalent. The basic premise is that the GC goes through a cycle that goes like this:
The GC marks all objects in the heap as being "potentially available for release". This is usually done by a flag toggle which changes the interpretation of the existing marks on objects from meaning "need to keep" to "safe to release". So no actual iteration over the objects occurs at this stage. Telling the GC to "not mark" certain objects would actually require extra operations, not less.
The GC starts at the GC roots, and traverse through the reference tree, changing the mark of objects from "safe to release" to "need to keep". This is the "Mark" phase. GC roots can be global objects, the currently executing stack, pending callbacks on the event loop, etc. The tree-traversal itself can be done in various way as well, with DFS being the simplest.
The GC goes over all objects in the heap, and removes any that is still marked "safe to release". This is the "Sweep" phase. Many optimizations exist in this phase, which allows the GC to free memory used by a group of objects in a single operation. Nevertheless, this requires at least some level of iteration, over groups of objects, if not over objects themselves.
Now here's the issue with setting a "non-GC" arena: Suppose that an object in your "non-GC" arena was to reference a regular object. During the "Mark" phase, if that object is not marked, it would be released. That means that all of your "non-collectible" objects need to be GC roots in order to "keep" regular objects to which they refer. And being a GC root offers no performance advantage over being directly referenced by a GC root. Both must equally participate in the "Mark" phase.
The only alternative is for your non-collectible object to not be able to strongly reference collectible objects at all. This can be achieved by having your own binary heap, rather than using native JS objects. The binary data would not be interpreted as references, and the heap object would finish its marking phase in a single operation. This is what asm.js does: It preallocates a large array to act as its internal heap. To the VM, the entire array counts as one big object, and therefore no garbage collection is done on any data structures that are encoded within it. Using this method does have the flaw that you need to encode all of your objects and data structures into the heap's binary format, and decode them when you want to use them. When working with asm.js, this is handled by the compiler.

in JavaScript if you want to kill a object, you have to delete all the references.
let's say we have a object like this:
var myobj={ obj : {} };
and we want to GC the obj object, we have to do this:
myobj["obj"] = null;
delete myobj["obj"];
but if you define another variable referenced to that object, it won't be killed with those line. for instance if you do:
var obj = myobj.obj;
if you really want to delete the object, you have do the same for this variable:
obj = null;
delete obj;
then if there is no reference it would be killed.
in other word if you want to prevent your object from GC, create a reference some where and keep it private.

Related

Do we need to “unset” variables in JavaScript?

Do we need to unset variables in JavaScript, and why?
For example in PHP it's recommended to unset all temporary variables in all kind of loops to keep memory free and avoid variables duplicates.
But anyways according to php - what's the benefit of unsetting variables? question:
Is it true that unsetting variables doesn't actually decrease the memory consumption during runtime?
-Yep.
So should we delete window.some_var;?
Or we should use some_var = null?
Or we should not unset variables to avoid additional CPU cycles?
UPD:
Related questions:
How to unset a Javascript variable?
How to remove a property from a javascript object
No, plus in some cases you can't delete variables in the global scope. Variables that are properly declared using the var keyword cannot be deleted (implied global variables can be deleted, on the other hand. Always use the var keyword to declare variables).
Also, javascript engines have this thing called garbage collector that automatically looks for variables that are no longer used or referenced somewhere when your code is 'running'. Once it finds one, it'll shove it off immediately into the Abyss (deletes the variable in the memory)
No, it is not necessary to delete variables when you’re done with them. If you have so many enormous, global variables that this is actually a problem, you need to rethink your design.
And yes, it’s also silly in PHP, and no, it has nothing to do with avoiding “additional CPU cycles”.
Specifically:
“Unsetting temporary variables in loops”
They’re temporary, and probably either a) integers or b) references that take up about as much space as an integer. Don’t bother.
“Avoiding duplicate variables”
Again – references. Setting things that are still referenced elsewhere to null is pointless.
If you feel like it’s clutter, that is a good sign that more things should be going out of scope (i.e. use more functions).
In most other cases that you haven’t mentioned, the engine is also smart enough to manage your stuff properly. Do not optimize prematurely.
David Flanagan answers this quite nicely in his book, JavaScript: The Definitive Guide:
The JavaScript interpreter performs automatic garbage collection for memory management. This means that a program can create objects as needed, and the programmer never needs to worry about destruction or deallocation of those objects. When an object is no longer reachable – when a program no longer has any way to refer to it – the interpreter knows it can never be used again and automatically reclaims the memory it was occupying.
This doesn't mean that memory leaks can't happen in JavaScript. Far from it. Circular (or cyclic) references are a frequent problem in browsers which implement reference counting, as documented here by IBM:
A circular reference is formed when two objects reference each other, giving each object a reference count of 1. In a purely garbage collected system, a circular reference is not a problem: If neither of the objects involved is referenced by any other object, then both are garbage collected. In a reference counting system, however, neither of the objects can be destroyed, because the reference count never reaches zero. In a hybrid system, where both garbage collection and reference counting are being used, leaks occur because the system fails to identify a circular reference. In this case, neither the DOM object nor the JavaScript object is destroyed. Listing 1 shows a circular reference between a JavaScript object and a DOM object.
If you're worried that your website contains a JavaScript memory leak, Google has a tool, aptly named "Leak Finder for JavaScript", which can help you find the cause.
Further reading: What is JavaScript garbage collection?

How do you clear memory in Javascript?

var Obj = function(){}; var X = new Obj();
will X = null properly clear memory?
Also would this be equivalent?
var Obj = function(){};
var X = {};
X.obj = new Obj();
delete(X.obj);
EDIT
It would seem that although deleting X.obj would NOT immediately clear memory, it would help the garbage collection. If I don't delete X.obj, there would still be a pointer to an object and so the GC may not clean it up.
Although I'm picking #delnan's answer, if you're reading this you should def also catch Benubird's article.
I also notice I accidentally wrote delete(X) originally instead of delete(X.obj) - sorry.
The short answer is that you don't. delete simply removes a reference (and not in the way you try to use it, see the above link - delete is one of those language features few people actually understand), nothing more. The implementation clears memory for you, but it's not your business when (and even if, strictly speaking - this is why one shouldn't rely on finalizers in GC'd languages that offer them) it does. Note though:
Only objects that can be proven to be unreachable (i.e. no way to access it) to all code can be removed. What keeps references to whom is usually fairly obvious, as least conceptually. You just need to watch out when dealing with lots of closures, as they may capture more variables than you think. Also note that circular references are cleaned up properly.
There's a bug in old (but sadly still used) IE versions involving garbage collection of JS event handlers and DOM elements. Google (perhaps even SO) should have better material on my memory.
On the plus side, that means you won't get dangling pointer bugs or (save of course the aforementioned pitfalls) memory leaks.
No, that will not clear memory.
Read this:
http://perfectionkills.com/understanding-delete/
No - Javascript runs GC when it feels like it.
The Delete method only deletes the reference - not the object. Any other references would be left out in the open waiting for the garbage collector.
JavaScript has its own GC, and it will run around and clean things up when nothing refers to them anymore.
I still think it's a good practice to null objects.
Deleteing an object also helps the GC because it will see something dangling, and say "I'm going to eat you because you're all alone (and now some cynical laugh)".
You should look at Deleting Objects in JavaScript
Even though there's a GC, you still want to ensure your script is optimized for performance as peoples computers, browsers, and fricken toolbars (and the number of them), will vary.
Generally speaking, memory management in Javascript is user-agent-specific. The basics of the garbage collector are through reference-counting. So, by setting a reference to null (using the delete keyword or by explicit assignment), you can assure yourself that a reference will be cleaned up, IF the object does not have any references that will live outside of its creation scope. That being the case, the GC will have already cleaned up any objects or variables whose scope has ended without your explicitly setting it to null.
There are some things to take care of, though - circular references are easy to create in JS, especially between a DOM element and an object. Care must be taken to clear (or not create in the first place) references to and/or from DOM elements within objects. If you do create a to/from reference related to DOM, be sure to explicitly clean them up by setting the references to null - both on your object and on the DOM element. Simply setting a parent object to null is not sufficient if there are child objects with references to/from DOM or localStorage because those references will live on, and if there was any reference from the child to the parent, then the parent will live on in memory because of that reference.
Web pages can actually leak trash in your memory this way - after you navigate away, the circular references keep objects and DOM elements in memory until you've restarted the browser!
An article on the subject: http://docstore.mik.ua/orelly/webprog/jscript/ch11_03.htm, and another detailed look: http://blogs.msdn.com/b/ericlippert/archive/2003/09/17/53038.aspx
JavaScript memory is generally handled similarly to Java - I mean there is (or there should be) a garbage collector which would delete the object if there is no references to it. So yes, simply "nullifying " the reference is the only way you should "handle" freeing memory, and the real freeing is the JS host part.

Is it possible to retrieve an anonymous JavaScript object?

For example, I have this code:
{a: 42}
After this line was executed, I think the object is stored somehow in the memory, I'm wondering how can I get it, in some tricky way?
No.
You can't do this.
Any decent js interpreter will destroy it with the garbage collector.
No: once you lose all references to an object, you cannot recover it and the GC will collect it.
No.
Anonymous objects are intended to work this way. If you need to retrieve an object later on, you should simply name it. (I assume you are asking this question out of curiosity and not out of neccessity).
As soon as an object has no existing references to it, the garbage collector should destroy the object, as is confirmed by this page:
ECMAScript uses automatic garbage
collection. The specification does not
define the details, leaving that to
the implementers to sort out, and some
implementations are known to give a
very low priority to their garbage
collection operations. But the general
idea is that if an object becomes
un-referable (by having no remaining
references to it left accessible to
executing code) it becomes available
for garbage collection and will at
some future point be destroyed and any
resources it is consuming freed and
returned to the system for re-use.
This would normally be the case upon
exiting an execution context. The
scope chain structure, the
Activation/Variable object and any
objects created within the execution
context, including function objects,
would no longer be accessible and so
would become available for garbage
collection.

Javascript variable reuse?

Is it OK to reuse variables for different data types in terms of performance and memory usage ?
What happens to old data, is it garbage collected immediately after type casting ?
It's OK to reuse variables, although unless you're doing some crazy things (say so in the question) with the amount of variables you're using, you probably should not reuse them too liberally in this way. It's considered good coding practice in general to have a variable declared to point to a specific thing, and use a different variable when you want to refer to something else.
"Variables" in Javascript are just references. They're not inherently expensive-- they don't take up more space than their text in the code and a few bytes in memory pointing to somewhere else. If you reuse a variable name by setting the reference to something else (or null/undefined), then the GC will know that that original reference is detached and know that it can be collected.
The GC in whatever browser or environment you're using will choose when to actually run the collector based on lots of factors.
Full disclosure: I have no knowledge of the internals of any particular JavaScript engines. I'm going from general principles of VMs and interpreters.
Usually, variable names just refer to other memory locations. So, whether you remove an old variable (which happens when it goes out of scope) and introduce a new one, or replace the current contents with a new object, doesn't matter much in terms of memory allocation.
Garbage collection might be different in each implementation. Immediate garbage collection is difficult; the only way I can think of doing it involves reference counters, and it's tough to make that work even for cyclic data structures. So, most garbage collectors in the wild do non-immediate collection cycles where, each time, a whole bunch of data gets removed. The cycles might, for example, be run automatically when memory use goes above a certain threshold within the engine (but it'll usually be more refined than that).
JavaScript is a loosely-typed language, and can store any datatype in any variable (even reused ones).
If you are combining types, though, you should check them periodically using the typeof keyword to ensure they are the type you think they are (for instance, trying to perform a mathematical operation on a string will concatenate or break, depending on the situation).
Furthermore, JavaScript variables stick around as long as they are within scope. Once a scope is left, the variables within it are destroyed (eventually - it's automatic and transparent). As far as garbage collection on reassigned variables, the old value is destroyed as soon as the new value is assigned.

Does JavaScript have a memory heap?

In other words, what options do I have to allocate memory in JavaScript?
I know you can allocate memory either globally, or inside function scope. Can I allocate memory dynamically? What does the new operator really mean?
Edit: here's a specific example. How would you implement reading an integer value from the user - n, and then read n integers into an array?
you can't allocate memory. you can create objects. that's what new does.
now, javascript is a queer creature: functions are also objects in javascript. So this mean that you can instantiate prettymuch everything using new.
So, the new operator means that a new object is being created.
Javascript also garbage-collects these variables, just like it happens in java. So if you know java, it should be easy for you to draw parallels.
cheers,
jrh
PS: when you allocate objects, you really are allocating memory. Only, you are not doing that explicitly. You can allocate an array, and make it behave like a memory buffer, but that will degrade javascript performance drastically: javascript arrays are not in-memory buffers, they are also objects (like everything else).
JavaScript has garbage collection and handles this for you.
However, you can help it by using the delete operator where appropriate.
From the Apple JavaScript Coding Guidelines:
Just as you used the new operator to
create an object, you should delete
objects when you are finished with
them, like this:
delete myObjectVariable;
The JavaScript runtime automatically
garbage collects objects when their
value is set to null. However, setting
an object to null doesn’t remove the
variable that references the object
from memory. Using delete ensures that
this memory is reclaimed in addition
to the memory used by the object
itself. (It is also easier to see
places where your allocations and
deallocations are unbalanced if you
explicitly call delete.)
Steve
Hmmm sounds to me like you are coming from the memory focused language and trying to shoe horn that logic into JS. Yes JS uses memory (of course), but we have garbage collection to take care of cleaning it all up.
If you are after specifics about the guts of memory allocation then you will have to hunt around for that. But as a rule thumb, when you use var, new or declaring a new function (or closure) you are gobbling up memory. You can get vars to null to flag them for garbage collection and you can use the delete keyword too although few do either of these unless they work Server-side (like myself with ASP JScript) where its important.
Javascript is really, really friendly — really, too friendly by half!
If you have an array of 3 elements, and you want to add a fourth, you can just act as if that array location already exists:
var arr = ['zero', 'one', 'two'];
// Now you have arr[0], arr[1] and arr[2].
// arr.length is equal to 3.
.
// to add arr[8]:
arr[8] = 'eight';
// Now you have arr[0] through arr[8]. arr.length is equal to 9.
// and arr[3] through arr[7] exist, and
// are initialized to undefined. (If I remember right.)
So being really specific with memory allocation is unnecessary when adding elelments.
No, you don’t need to and can’t allocate memory. The JavaScript interpreter does that automatically.
To answer the title of the question, if you are to trust in MDN, most JavaScript implementations have a heap:
Heap
Objects are allocated in a heap which is just a name to denote a large
mostly unstructured region of memory.
Several Runtimes Communicating Together
A web worker or a cross-origin iframe has its own stack, heap, and
message queue. Two distinct runtimes can only communicate through
sending messages via the postMessage method. This method adds a
message to the other runtime if the latter listens to message events.
For a deeper dive into memory management, there is also an article here although much of this is implementation specific.
You do not need to manually manage memory in Javascript. Both heap and stacks are used under the hood to manage memory and it depends on the implementation. Usually, local variables are on the stack and objects are on the heap.

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