I have a for loop that iterates over a fair few items.
At the start of each iteration I use the same var newObj = new ...
However, I'm wondering, is this completely replacing that object each time or should I be releasing it at the end of each iteration.
If I should release - how do I do release?
For example - say x is a large number (10.000 as a high example)
for (var x = 0; x<10000; x++) {
var newObj = new someThing(x, y, z)
newObj.dance()
newObj.andDanceMore()
//Should I do some kind of release here - or am I replacing it each time
}
Thanks.
in javascript you don't have to worry about "releasing" allocated memmory,
from the great MDN docs
High-level languages (such as JS) embed a piece of software called "garbage
collector" whose job is to track memory allocation and use in order to
find when a piece of allocated memory is not needed any longer in
which case, it will automatically free it. This process is an
approximation since the general problem of knowing whether some piece
of memory is needed is undecidable (can't be solved by an algorithm).
basically the general behavior of the G.C is when an object has zero references to it - It can be garbage collected.
in the case you reffering to, each time you assigning a new value to var newObj, the G.C detect that there are 0 reffernces to it and garbage collected it - releasing it -
Short answer: no.
Slightly longer answer: There is no manual memory management in JavaScript. It is a garbage collected language. Each time the GC is launched (and you have no control over when that happens), all objects that have no references will be automatically collected, effectively freeing the memory they occupied.
Further reading: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Memory_Management
Do you know what Garbage Collector is? If not, you could find very nice blogposts googling "node.js garbage collection".
In brief:
1) When you override the variable value (or local variable is being dropped with its scope), the object still remains in memory.
2) When Node.js decides to run garbage collector, your object will be wiped out from memory in the following cases:
2.1) There are no variables, containing it (actually pointing to it).
2.2) There are some variables/properties pointing to it, but the entire cluster of objects and closures with mutual links is identified as isolated.
3) You may force garbage collector run (How to request the Garbage Collector in node.js to run?), but in regular cases you do not need this.
4) Difference between let and var, mentioned in comments, does not affect your case. Yes, let is more local than var, but what? If Node.js drops let variable quitting the loop, or you override any variable (var, let, property - whatever) with new value, the result is the same.
var newObj
Will be executed only once.What the program did is that Constantly assigned to newObj in the loop, objects that are not referenced will be recycled by GC.
Related
Is there any way to control when Javascript performs garbage collection? I would like to enable it to perform garbage collection at certain times to ensure the smooth operation of my web site
Javascript doesn't have explicit memory management, it's the browser which decides when to clean it up. Sometimes it may happen that you experience un-smooth rendering of JavaScript due to a garbage collection pause.
There are many techniques that you can apply to overcome glitches caused by garbage collection (GC). More you apply more you explore. Suppose you have a game written in JavaScript , and every second you are creating a new object then its obvious that at after certain amount of time GC will occur to make further space for your application.
For real time application like games, which requires lot of space the simplest thing you can do is to reuse the same memory. It depends on you how you structure your code. If it generates lots of garbage then it might give choppy experience.
By using simple procedures: This is well know that new keyword indicates allocation. Wherever possible you can try to reuse the same object by each time by adding or modifying properties. This is also called recycling of object
In case of Arrays, assigning [] is often used to clear array, but you should keep in mind that it also creates a new array and garbages the old one. To reuse the same block you should use arr.length = 0 This has the same effect but it reuses the same array object instead of creating new one.
In case of functions: Sometimes our program needed to call a specific function more time or on certain intervals by using setInterval or setTimeout.
ex: setTimeout(function() { doSomething() }, 10);
You can optimize the above code by assigning the function to a permanent variable rather than spawning each time at regular intervals.
ex : var myfunc = function() { doSomething() }
setTimeout(myfunc, 10);
Other possible thing is that, the array slice() method returns a new array (based on a range in the original array,that can remain untouched), string's substr also returns a new string (based on a range of characters in the original string, that can remain untouched), and so on. Calling these functions creates garbage if not reutilized properly.
To avoid garbage completely in JavaScript is very difficult, you could say impossible. Its depends, how you reuse the objects and variables to avoid garbage. If your code is well structured and optimized you can minimize the overhead.
Unfortunately, there is no way to control WHEN the garbage collection takes place but with the proper formation of objects, you CAN control how quickly and cleanly it happens. Take a look at these documents on Mozilla Dev Net.
This algorithm assumes the knowledge of a set of objects called roots
(In JavaScript, the root is the global object). Periodically, the
garbage-collector will start from these roots, find all objects that
are referenced from these roots, then all objects referenced from
these, etc. Starting from the roots, the garbage collector will thus
find all reachable objects and collect all non-reachable objects.
This algorithm is better than the previous one since "an object has
zero reference" leads to this object being unreachable. The opposite
is not true as we have seen with cycles.
Why not keep references to all your objects until you want them to be GC'd?
var delayed_gc_objects = [];
function delayGC(obj) { // keeps reference alive
return delayed_gc_objects[delayed_gc_objects.length] = obj;
}
function resumeGC() { // kills references, letting them be GCd
delayed_gc_objects.length = 0;
}
you can perform some changes to improve your memory use, like:
don't set variables on loops
avoid using of global variables and functions. they will take a piece of memory until you get out
JavaScript is a garbage-collected language, meaning that the execution environment is responsible for managing the memory required during code execution.
The most popular form of garbage collection for JavaScript is called mark-and-sweep.
A second, less-popular type of garbage collection is reference counting. The idea is that every value keeps track of how many references are made to it
GC follows these algo, even if you manage to trigger the GC, it will not be guaranteed that it will run immediately, you are only marking it
garbage collection (GC) is a form of automatic memory management by removing the objects that no needed anymore.
any process deal with memory follow these steps:
1 - allocate your memory space you need
2 - do some processing
3 - free this memory space
there are two main algorithm used to detect which objects no needed anymore.
Reference-counting garbage collection: this algorithm reduces the definition of "an object is not needed anymore" to "an object has no other object referencing to it", the object will removed if no reference point to it
Mark-and-sweep algorithm: connect each objects to root source. any object doesn't connect to root or other object. this object will be removed.
currently most modern browsers using the second algorithm.
This question already has answers here:
About closure, LexicalEnvironment and GC
(3 answers)
Closed 3 years ago.
Consider the following example:
function makeFunction() {
let x = 3;
let s = "giant string, 100 MB in size";
return () => { console.log(x); };
}
// Are both x and s held in memory here
// or only x, because only x was referred to by the closure returned
// from makeFunction?
let made = makeFunction();
// Suppose there are no further usages of makeFunction after this point
// Let's assume there's a thorough GC run here
// Is s from makeFunction still around here, even though made doesn't use it?
made();
So if I close around just one variable from a parent lexical environment, is that variable kept around or is every sibling variable in its lexical environment also kept around?
Also, what if makeFunction was itself nested inside another outer function, would that outer lexical environment be retained even though neither makeFunction nor makeFunction's return value referred to anything in that outer lexical environment?
I'm asking for performance reasons - do closures keep a bunch of stuff around or only what they directly refer to? This impacts memory usage and also resource usage (e.g. open connections, handles, etc.).
This would be mostly in a NodeJS context, but could also apply in the browser.
V8 developer here. This is a bit complicated ;-)
The short answer is: closures only keep around what they need.
So in your example, after makeFunction has run, the string referred to by s will be eligible for garbage collection. Due to how garbage collection works, it's impossible to predict when exactly it'll be freed; "at the next garbage collection cycle". Whether makeFunction runs again doesn't matter; if it does run again, a new string will be allocated (assuming it was dynamically computed; if it's a literal in the source then it's cached). Whether made has already run or will run again doesn't matter either; what matters is that you have a variable referring to it so you could run it (again). Engines generally can't predict which functions will or won't be executed in the future.
The longer answer is that there are some footnotes. For one thing, as comments already pointed out, if your closure uses eval, then everything has to be kept around, because whatever source snippet is eval'ed could refer to any variable. (What one comment mentioned about global variables that could be referring to eval is not true though; there is a semantic difference for "global eval", a.k.a. "indirect eval": it cannot see local variables. Which is usually considered an advantage for both performance and debuggability -- but even better is to not use eval at all.)
The other footnote is that somewhat unfortunately, the tracking is not as fine-grained as it could be: each closure will keep around what any closure needs. We have tried fixing this, but as it turns out finer-grained tracking causes more memory consumption (for metadata) and CPU consumption (for doing the work) and is therefore usually not worth it for real code (although it can have massive impact on artificial tests stressing precisely this scenario). To give an example:
function makeFunction() {
let x = 3;
let s = "giant string, 100 MB in size";
let short_lived = function() { console.log(s.length); }
// short_lived(); // Call this or don't, doesn't matter.
return function long_lived() { console.log(x); };
}
let long_lived = makeFunction();
With this modified example, even though long_lived only uses x, short_lived does use s (even if it's never called!), and there is only one bucket for "local variables from makeFunction that are needed by some closure", so that bucket keeps both x and s alive. But as I said earlier: real code rarely runs into this issue, so this is usually not something you have to worry about.
Side note:
and also resource usage (e.g. open connections, handles, etc.)
As a very general statement (i.e., in any language or runtime environment, regardless of closures or whatnot), it's usually advisable not to rely on garbage collection for resource management. I recommend to free your resources manually and explicitly as soon as it is appropriate to free them.
In the following JS code, will example1 or example2 be ever cleaned by the garbage collector? Will their memory be ever released by the GC? I am dealing with memory leaks and I have a few such instances in my code.
example1 = 'Outside a function'
function sample(x){
example2 = 'Inside a function'
}
sample(1)
From what I understand is neither of the variables will ever be cleaned by Garbage collector because they are treated as global variables. Might sound like a trivial question but finding memory leaks is like finding a needle in a haystack
The heap has two main segments, the New Space and the Old Space. The New Space is where new allocations are happening; it is fast to collect garbage here and has a size of ~1-8MBs. Objects living in the New Space are called Young Generation. The Old Space where the objects that survived the collector in the New Space are promoted into - they are called the Old Generation. Allocation in the Old Space is fast, however collection is expensive so it is infrequently performed .
You need understand few thinks, like:
1 - The garbage collector builds a list of "roots". Roots usually are global variables to which a reference is kept in code. In JavaScript, the "window" object is an example of a global variable that can act as a root. The window object is always present, so the garbage collector can consider it and all of its children to be always present (i.e. not garbage).
2 - All roots are inspected and marked as active (i.e. not garbage). All children are inspected recursively as well. Everything that can be reached from a root is not considered garbage.
3- All pieces of memory not marked as active can now be considered garbage. The collector can now free that memory and return it to the OS.
So about your questions:
From what I understand is neither of the variables will ever be cleaned by Garbage collector because they are treated as global variables.
Might sound like a trivial question but finding memory leaks is like finding a needle in a haystack
You are correct.
I'm learning Javascript, and in the various texts the authors will speak of javascript using a mark and sweep gc to deallocate objects from memory. They will also speak of how if you set the value a variable references to null it will delete the reference to that value, allowing the allocated space to be set for gc. This SO answer says that you can remove the allocated memory and the reference by setting the value the variable contains to null and then to undefined, effectively removing the allocated space from the heap (if I understood it correctly).
So my question is this: Is it possible to write javascript in such a way that you can eliminate gc?
(If it is implementation specific I would like to know if it is possible on v8, though if this is possible on rhino or other js implementations that would be of immense use too)
Judging by projects like LLJS my request isn't too unreasonable, but I'm not entirely sure how the memory module does it.
I've always found it helpful if I explain why I'm asking so here it goes. I really like compilers, and I wanted to write a compile-to-js language that leveraged a static inferred typing system similar to SML. The reason why I wanted to write my own was because I wanted to utilize region inference to determine exactly when objects and variables come out of scope (as much as possible) and upon leaving scope remove it from the heap, thereby eliminating as much gc as possible. This is mostly a research project (read: because I can) so any resources on memory optimization in javascript would also be greatly appreciated!
EDIT: I guess another way to phrase it would be "Is it possible to write js in such a way that the gc will deterministically never run (as much as possible)? If so what techniques would be involved?"
I'm not looking per se for delete because that marks the element for deletion thereby invoking what I wanted to (try to) avoid, I was curious if the implementation's gc would run if I removed all references (and the value) associated with the variable.
Alternatively, paraphrasing from the referenced SO Answer:
x = foo;
x = null;
x;
Is x still on the heap?
It's not entirely clear what you're looking for.
The standard Javascript implementations have NO way of manually deallocating memory. You can remove a property with the delete operator, but that just removes the property from the object. It doesn't even free any contents that the property points to (that is left for garbage collection if there are no other references to that data).
Javascript was designed from the ground up to be a garbage collected language. It frees things from physical memory only when the garbage collector runs and that garbage collector finds objects that are unreachable (e.g. there are no references to those objects still in use). The language does not contain commands to free memory.
It is possible (in some JS implementations) to call the GC yourself rather than wait for the JS engine to run it, but it's still using GC to decide what to free.
Responding to the additional things you added to your answer.
To the best I know, javascript only cleans things up when the GC runs. Until then objects are marked such that the GC can see that there are no references to them anywhere, but they aren't actually freed until the GC checks and notices this. Further, local variables in a function scope are themselves a type of object and those are not freed until the GC runs and notices that there are no references to the function scope (in JS, a closure can maintain a reference to a function scope even after the function has completed).
So, in your code example:
x = foo; x = null; x;
x is still alive and occupying some space because it's still in scope and code could still reach it. It's contents will be null which presumably takes no extra space beyond the variable itself, but the space for the variable itself won't be freed until the function context it is in is found to be reference free by the garbage collector.
JS is a garbage collected language. That's when things are actually freed from the heap. There are no instructions in the language to free things anytime sooner.
The delete keyword will trigger garbage collection by the browser. Be aware that it deletes entire chains of objects unless you nullify object references.
var o = {...};
delete o;
If I create an object without assigning it to anything, when will Javascript garbage collect this object? Here is an example:
alert(new Date().getTime());
If no such garbage collection is done, will this cause a memory leak?
for (var i = 0; i < 99999999; i++) {
console.info(new Date().getTime());
}
If nobody in scope is referencing the anonymous objects, they'll get garbage collected the next time the GC runs.
So, after Console.info finishes running, they're up for garbage collecting. If you set any in-scope variables to refer to them, then they won't.
The beauty of garbage collection is that you dont know when the memory will be reclaimed, nor should you care (unless it's happening far too frequently).
In this situation, the runtime should eventually reclaim those Date instances, but nothing you do is really going to change how fast it does that. Also, this does NOT cause a memory leak.
JavaScript runs GC (garbage collector) automatically to reclaim the memory occupied by strings, objects, arrays, and functions that are no longer in use.
A key feature of garbage collection is that the garbage collector must
be able to determine when it is safe to reclaim memory. Obviously, it
must never reclaim values that are still in use and should collect
only values that are no longer reachable; that is, values that cannot
be referred to through any of the variables, object properties, or
array elements in the program.
Reference: Here.
This is a nice reference about JS garbare collection: http://www.scirra.com/blog/76/how-to-write-low-garbage-real-time-javascript