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.
Related
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.
I have a fairly complex Javascript app, which has a main loop that is called 60 times per second. There seems to be a lot of garbage collection going on (based on the 'sawtooth' output from the Memory timeline in the Chrome dev tools) - and this often impacts the performance of the application.
So, I'm trying to research best practices for reducing the amount of work that the garbage collector has to do. (Most of the information I've been able to find on the web regards avoiding memory leaks, which is a slightly different question - my memory is getting freed up, it's just that there's too much garbage collection going on.) I'm assuming that this mostly comes down to reusing objects as much as possible, but of course the devil is in the details.
The app is structured in 'classes' along the lines of John Resig's Simple JavaScript Inheritance.
I think one issue is that some functions can be called thousands of times per second (as they are used hundreds of times during each iteration of the main loop), and perhaps the local working variables in these functions (strings, arrays, etc.) might be the issue.
I'm aware of object pooling for larger/heavier objects (and we use this to a degree), but I'm looking for techniques that can be applied across the board, especially relating to functions that are called very many times in tight loops.
What techniques can I use to reduce the amount of work that the garbage collector must do?
And, perhaps also - what techniques can be employed to identify which objects are being garbage collected the most? (It's a farly large codebase, so comparing snapshots of the heap has not been very fruitful)
A lot of the things you need to do to minimize GC churn go against what is considered idiomatic JS in most other scenarios, so please keep in mind the context when judging the advice I give.
Allocation happens in modern interpreters in several places:
When you create an object via new or via literal syntax [...], or {}.
When you concatenate strings.
When you enter a scope that contains function declarations.
When you perform an action that triggers an exception.
When you evaluate a function expression: (function (...) { ... }).
When you perform an operation that coerces to Object like Object(myNumber) or Number.prototype.toString.call(42)
When you call a builtin that does any of these under the hood, like Array.prototype.slice.
When you use arguments to reflect over the parameter list.
When you split a string or match with a regular expression.
Avoid doing those, and pool and reuse objects where possible.
Specifically, look out for opportunities to:
Pull inner functions that have no or few dependencies on closed-over state out into a higher, longer-lived scope. (Some code minifiers like Closure compiler can inline inner functions and might improve your GC performance.)
Avoid using strings to represent structured data or for dynamic addressing. Especially avoid repeatedly parsing using split or regular expression matches since each requires multiple object allocations. This frequently happens with keys into lookup tables and dynamic DOM node IDs. For example, lookupTable['foo-' + x] and document.getElementById('foo-' + x) both involve an allocation since there is a string concatenation. Often you can attach keys to long-lived objects instead of re-concatenating. Depending on the browsers you need to support, you might be able to use Map to use objects as keys directly.
Avoid catching exceptions on normal code-paths. Instead of try { op(x) } catch (e) { ... }, do if (!opCouldFailOn(x)) { op(x); } else { ... }.
When you can't avoid creating strings, e.g. to pass a message to a server, use a builtin like JSON.stringify which uses an internal native buffer to accumulate content instead of allocating multiple objects.
Avoid using callbacks for high-frequency events, and where you can, pass as a callback a long-lived function (see 1) that recreates state from the message content.
Avoid using arguments since functions that use that have to create an array-like object when called.
I suggested using JSON.stringify to create outgoing network messages. Parsing input messages using JSON.parse obviously involves allocation, and lots of it for large messages. If you can represent your incoming messages as arrays of primitives, then you can save a lot of allocations. The only other builtin around which you can build a parser that does not allocate is String.prototype.charCodeAt. A parser for a complex format that only uses that is going to be hellish to read though.
The Chrome developer tools have a very nice feature for tracing memory allocation. It's called the Memory Timeline. This article describes some details. I suppose this is what you're talking about re the "sawtooth"? This is normal behavior for most GC'ed runtimes. Allocation proceeds until a usage threshold is reached triggering a collection. Normally there are different kinds of collections at different thresholds.
Garbage collections are included in the event list associated with the trace along with their duration. On my rather old notebook, ephemeral collections are occurring at about 4Mb and take 30ms. This is 2 of your 60Hz loop iterations. If this is an animation, 30ms collections are probably causing stutter. You should start here to see what's going on in your environment: where the collection threshold is and how long your collections are taking. This gives you a reference point to assess optimizations. But you probably won't do better than to decrease the frequency of the stutter by slowing the allocation rate, lengthening the interval between collections.
The next step is to use the Profiles | Record Heap Allocations feature to generate a catalog of allocations by record type. This will quickly show which object types are consuming the most memory during the trace period, which is equivalent to allocation rate. Focus on these in descending order of rate.
The techniques are not rocket science. Avoid boxed objects when you can do with an unboxed one. Use global variables to hold and reuse single boxed objects rather than allocating fresh ones in each iteration. Pool common object types in free lists rather than abandoning them. Cache string concatenation results that are likely reusable in future iterations. Avoid allocation just to return function results by setting variables in an enclosing scope instead. You will have to consider each object type in its own context to find the best strategy. If you need help with specifics, post an edit describing details of the challenge you're looking at.
I advise against perverting your normal coding style throughout an application in a shotgun attempt to produce less garbage. This is for the same reason you should not optimize for speed prematurely. Most of your effort plus much of the added complexity and obscurity of code will be meaningless.
As a general principle you'd want to cache as much as possible and do as little creating and destroying for each run of your loop.
The first thing that pops in my head is to reduce the use of anonymous functions (if you have any) inside your main loop. Also it'd be easy to fall into the trap of creating and destroying objects that are passed into other functions. I'm by no means a javascript expert, but I would imagine that this:
var options = {var1: value1, var2: value2, ChangingVariable: value3};
function loopfunc()
{
//do something
}
while(true)
{
$.each(listofthings, loopfunc);
options.ChangingVariable = newvalue;
someOtherFunction(options);
}
would run much faster than this:
while(true)
{
$.each(listofthings, function(){
//do something on the list
});
someOtherFunction({
var1: value1,
var2: value2,
ChangingVariable: newvalue
});
}
Is there ever any downtime for your program? Maybe you need it to run smoothly for a second or two (e.g. for an animation) and then it has more time to process? If this is the case I could see taking objects that would normally be garbage collected throughout the animation and keeping a reference to them in some global object. Then when the animation ends you can clear all the references and let the garbage collector do it's work.
Sorry if this is all a bit trivial compared to what you've already tried and thought of.
I'd make one or few objects in the global scope (where I'm sure garbage collector is not allowed to touch them), then I'd try to refactor my solution to use those objects to get the job done, instead of using local variables.
Of course it couldn't be done everywhere in the code, but generally that's my way to avoid garbage collector.
P.S. It might make that specific part of code a little bit less maintainable.
My node/javascript program receives lots of messages.
Each time a message is received I create a new object, passing it message content.
Within the new objects constructor, the object does a bunch of stuff, including some mongo operations with callbacks.
When the operations that are performed within the object constructor are complete, the object is no longer needed or wanted.
After some experiments involving complexity (i.e. storage of the object in an array element or as a property of a 'master' object and then deleting it, I tried the simple approach.
var x = new DisposableObject(message);
delete x;
This seems to work fine. There may be many instances of DisposableObject in existence at any time. DisposableObject is created and does everything expected of it in tests.
I am assuming that when I delete 'x' that this does not affect the object itself, just the reference to it that is contained in 'x'. The objects callbacks occur and all is good.
So four questions:
Will this simple mechanism allow the garbage collector (node/V8) to get rid of the object?
Is there some awful trap door awaiting my code because I am doing this?
Is there any issue with multiple instances of DisposableObject floating around waiting for callbacks when there is no active reference left to them in my program?
Is there a better way to create the objects, then make sure they can be garbage collected after all their operations are completed?
Javascript uses a nongenerational mark-and-sweep garbage collector, in javascript objects are automatically garbage collected when they are no longer needed , so you need not worry about garbage collection.
but you have to keep these points in mind (from this answer):
what you are trying with delete does not really delete an object, delete is only effective on an object's properties. It has no effect on variable or function names., Use delete statements whenever you create an object using a new statement, pair it with a delete statement. This ensures that all of the memory associated with the object, including its property name, is available for garbage collection. The delete statement is discussed more in “Freeing Objects.”
Use the var keyword. Any variable created without the var keyword is created at the global scope and is never eligible for garbage collection, presenting the opportunity for a memory leak.
In case of global variables, Global variables are never disposed of by the GC in the sense that a global variable will always exist. Setting it to null will allow the memory that it references to be collected. the memory used by the object will be eligible for collection. but The variable still exists though, and it simply references null(some more here)
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;
I have a fairly complex Javascript app, which has a main loop that is called 60 times per second. There seems to be a lot of garbage collection going on (based on the 'sawtooth' output from the Memory timeline in the Chrome dev tools) - and this often impacts the performance of the application.
So, I'm trying to research best practices for reducing the amount of work that the garbage collector has to do. (Most of the information I've been able to find on the web regards avoiding memory leaks, which is a slightly different question - my memory is getting freed up, it's just that there's too much garbage collection going on.) I'm assuming that this mostly comes down to reusing objects as much as possible, but of course the devil is in the details.
The app is structured in 'classes' along the lines of John Resig's Simple JavaScript Inheritance.
I think one issue is that some functions can be called thousands of times per second (as they are used hundreds of times during each iteration of the main loop), and perhaps the local working variables in these functions (strings, arrays, etc.) might be the issue.
I'm aware of object pooling for larger/heavier objects (and we use this to a degree), but I'm looking for techniques that can be applied across the board, especially relating to functions that are called very many times in tight loops.
What techniques can I use to reduce the amount of work that the garbage collector must do?
And, perhaps also - what techniques can be employed to identify which objects are being garbage collected the most? (It's a farly large codebase, so comparing snapshots of the heap has not been very fruitful)
A lot of the things you need to do to minimize GC churn go against what is considered idiomatic JS in most other scenarios, so please keep in mind the context when judging the advice I give.
Allocation happens in modern interpreters in several places:
When you create an object via new or via literal syntax [...], or {}.
When you concatenate strings.
When you enter a scope that contains function declarations.
When you perform an action that triggers an exception.
When you evaluate a function expression: (function (...) { ... }).
When you perform an operation that coerces to Object like Object(myNumber) or Number.prototype.toString.call(42)
When you call a builtin that does any of these under the hood, like Array.prototype.slice.
When you use arguments to reflect over the parameter list.
When you split a string or match with a regular expression.
Avoid doing those, and pool and reuse objects where possible.
Specifically, look out for opportunities to:
Pull inner functions that have no or few dependencies on closed-over state out into a higher, longer-lived scope. (Some code minifiers like Closure compiler can inline inner functions and might improve your GC performance.)
Avoid using strings to represent structured data or for dynamic addressing. Especially avoid repeatedly parsing using split or regular expression matches since each requires multiple object allocations. This frequently happens with keys into lookup tables and dynamic DOM node IDs. For example, lookupTable['foo-' + x] and document.getElementById('foo-' + x) both involve an allocation since there is a string concatenation. Often you can attach keys to long-lived objects instead of re-concatenating. Depending on the browsers you need to support, you might be able to use Map to use objects as keys directly.
Avoid catching exceptions on normal code-paths. Instead of try { op(x) } catch (e) { ... }, do if (!opCouldFailOn(x)) { op(x); } else { ... }.
When you can't avoid creating strings, e.g. to pass a message to a server, use a builtin like JSON.stringify which uses an internal native buffer to accumulate content instead of allocating multiple objects.
Avoid using callbacks for high-frequency events, and where you can, pass as a callback a long-lived function (see 1) that recreates state from the message content.
Avoid using arguments since functions that use that have to create an array-like object when called.
I suggested using JSON.stringify to create outgoing network messages. Parsing input messages using JSON.parse obviously involves allocation, and lots of it for large messages. If you can represent your incoming messages as arrays of primitives, then you can save a lot of allocations. The only other builtin around which you can build a parser that does not allocate is String.prototype.charCodeAt. A parser for a complex format that only uses that is going to be hellish to read though.
The Chrome developer tools have a very nice feature for tracing memory allocation. It's called the Memory Timeline. This article describes some details. I suppose this is what you're talking about re the "sawtooth"? This is normal behavior for most GC'ed runtimes. Allocation proceeds until a usage threshold is reached triggering a collection. Normally there are different kinds of collections at different thresholds.
Garbage collections are included in the event list associated with the trace along with their duration. On my rather old notebook, ephemeral collections are occurring at about 4Mb and take 30ms. This is 2 of your 60Hz loop iterations. If this is an animation, 30ms collections are probably causing stutter. You should start here to see what's going on in your environment: where the collection threshold is and how long your collections are taking. This gives you a reference point to assess optimizations. But you probably won't do better than to decrease the frequency of the stutter by slowing the allocation rate, lengthening the interval between collections.
The next step is to use the Profiles | Record Heap Allocations feature to generate a catalog of allocations by record type. This will quickly show which object types are consuming the most memory during the trace period, which is equivalent to allocation rate. Focus on these in descending order of rate.
The techniques are not rocket science. Avoid boxed objects when you can do with an unboxed one. Use global variables to hold and reuse single boxed objects rather than allocating fresh ones in each iteration. Pool common object types in free lists rather than abandoning them. Cache string concatenation results that are likely reusable in future iterations. Avoid allocation just to return function results by setting variables in an enclosing scope instead. You will have to consider each object type in its own context to find the best strategy. If you need help with specifics, post an edit describing details of the challenge you're looking at.
I advise against perverting your normal coding style throughout an application in a shotgun attempt to produce less garbage. This is for the same reason you should not optimize for speed prematurely. Most of your effort plus much of the added complexity and obscurity of code will be meaningless.
As a general principle you'd want to cache as much as possible and do as little creating and destroying for each run of your loop.
The first thing that pops in my head is to reduce the use of anonymous functions (if you have any) inside your main loop. Also it'd be easy to fall into the trap of creating and destroying objects that are passed into other functions. I'm by no means a javascript expert, but I would imagine that this:
var options = {var1: value1, var2: value2, ChangingVariable: value3};
function loopfunc()
{
//do something
}
while(true)
{
$.each(listofthings, loopfunc);
options.ChangingVariable = newvalue;
someOtherFunction(options);
}
would run much faster than this:
while(true)
{
$.each(listofthings, function(){
//do something on the list
});
someOtherFunction({
var1: value1,
var2: value2,
ChangingVariable: newvalue
});
}
Is there ever any downtime for your program? Maybe you need it to run smoothly for a second or two (e.g. for an animation) and then it has more time to process? If this is the case I could see taking objects that would normally be garbage collected throughout the animation and keeping a reference to them in some global object. Then when the animation ends you can clear all the references and let the garbage collector do it's work.
Sorry if this is all a bit trivial compared to what you've already tried and thought of.
I'd make one or few objects in the global scope (where I'm sure garbage collector is not allowed to touch them), then I'd try to refactor my solution to use those objects to get the job done, instead of using local variables.
Of course it couldn't be done everywhere in the code, but generally that's my way to avoid garbage collector.
P.S. It might make that specific part of code a little bit less maintainable.