I am a bit confused about the fact that in C# only the reference types get garbage collected.
That means GC picks only the reference types for memory de-allocation.
So what happens with the value types as they also occupy memory on stack ?
For a start, whether they're on the stack or part of the heap depends on what context they're part of - if they're within a reference type, they'll be on the heap anyway. (You should consider how much you really care about the stack/heap divide anyway - as Eric Lippert has written, it's largely an implementation detail.)
However, basically value type memory is reclaimed when the context is reclaimed - so when the stack is popped by you returning from a method, that "reclaims" the whole stack frame. Likewise if the value type value is actually part of an object, then the memory is reclaimed when that object is garbage collected.
The short answer is that you don't have to worry about it :) (This assumes you don't have anything other than the memory to worry about, of course - if you've got structs with references to native handles that need releasing, that's a somewhat different scenario.)
I am a bit confused about the fact that in C# only the reference types get garbage collected.
This is not a fact. Or, rather, the truth or falsity of this statement depends on what you mean by "get garbage collected". The garbage collector certainly looks at value types when collecting; those value types might be alive and holding on to a reference type:
struct S { public string str; }
...
S s = default(S); // local variable of value type
s.str = M();
when the garbage collector runs it certainly looks at s, because it needs to determine that s.str is still alive.
My suggestion: clarify precisely what you mean by the verb "gets garbage collected".
GC picks only the reference types for memory de-allocation.
Again, this is not a fact. Suppose you have an instance of
class C { int x; }
the memory for the integer will be on the garbage-collected heap, and therefore reclaimed by the garbage collector when the instance of C becomes unrooted.
Why do you believe the falsehood that only the memory of reference types is deallocated by the garbage collector? The correct statement is that memory that was allocated by the garbage collector is deallocated by the garbage collector, which I think makes perfect sense. The GC allocated it so it is responsible for cleaning it up.
So what happens with the value types as they also occupy memory on stack ?
Nothing at all happens to them. Nothing needs to happen to them. The stack is a million bytes. The size of the stack is determined when the thread starts up; it starts at a million bytes and it stays a million bytes throughout the entire execution of the thread. Memory on the stack is neither created nor destroyed; only its contents are changed.
There are too many verbs used in this question, like destroyed, reclaimed, deallocated, removed. That doesn't correspond well with what actually happens. A local variable simply ceases to be, Norwegian parrot style.
A method has a single point of entry, first thing that happens is that the CPU stack pointer is adjusted. Creating a "stack frame", storage space for the local variables. The CLR guarantees that this space is initialized to 0, not otherwise a feature you use strongly in C# because of the definite assignment rule.
A method has a single point of exit, even if you method code is peppered with multiple return statements. At that point, the stack pointer is simply restored to its original value. In effect it "forgets" that the local variables where ever there. Their values are not 'scrubbed' in any way, the bytes are still there. But they won't last long, the next call in your program are going to overwrite them again. The CLR zero-initialization rule ensures that you can never observe those old values, that would be insecure.
Very, very fast, takes no more than a single processor cycle. A visible side-effect of this behavior in the C# language is that value types cannot have a finalizer. Ensuring no extra work has to be done.
A value type on the stack is removed from the stack when it goes out of scope.
Value types are destroyed as soon as they go out of scope.
value types would get deallocated when the stack frame is removed after it has been executed i would assume
Would also like to add that stack is at a thread level, and heap is at application domain level.
So, when a thread ends it would reclam the stack memory used by that specific thread.
Every value type instance in .NET will be part of something else, which could be a larger enclosing value type instance, a heap object, or a stack frame. Whenever any of those things come into being, any structures within them will also come into being; those structures will then continue to exist as long as the thing containing them does. When the thing that contains the structure ceases to exist, the structure will as well. There is no way to destroy a structure without destroying the container, and there is no way to destroy something that contains one or more structures without destroying the structures contained therein.
Related
In the following code, what happens to the memory myArray initially pointed to once it's reassigned in the 2nd line? Is that memory lost, or does the C# garbage collector take care of it?
static void Main(string[] args)
{
double[] myArray = new Double[10];
myArray = new Double[3];
}
As far as I've been reading, there's no way to explicitly free up the memory, so I'm hoping C# automatically does it.
Of course, C# automatically releases memory that was associated with myArray prior to the assignment. This does not happen right away, but as soon as garbage collector realizes there are no remaining references to that Double[10] array object*, the memory allocated to the object gets reclaimed.
If you change your program slightly to create a second reference to the same array, like this
double[] myArray = new Double[10];
double[] myArray2 = myArray; // Second reference
myArray = new Double[3];
garbage collector would not release the object, as long as a reference to it remains accessible to your program.
* Sometimes your program finishes execution before garbage collector gets to complete its analysis. The memory still gets released, though.
When your variable goes out of scope and the memory is no longer required then it becomes eligible for garbage collection.
MS explains all, as seen here: https://msdn.microsoft.com/en-us/library/aa691138(v=vs.71).aspx
C# employs automatic memory management, which frees developers from manually allocating and freeing the memory occupied by objects.
Automatic memory management policies are implemented by a garbage
collector. The memory management life cycle of an object is as
follows:
When the object is created, memory is allocated for it, the constructor is run, and the object is considered live.
If the object, or any part of it, cannot be accessed by any possible continuation of execution, other than the running of
destructors, the object is considered no longer in use, and it becomes
eligible for destruction. The C# compiler and the garbage collector
may choose to analyze code to determine which references to an object
may be used in the future. For instance, if a local variable that is
in scope is the only existing reference to an object, but that local
variable is never referred to in any possible continuation of
execution from the current execution point in the procedure, the
garbage collector may (but is not required to) treat the object as no
longer in use.
Once the object is eligible for destruction, at some unspecified later time the destructor (Section 10.12) (if any) for the
object is run. Unless overridden by explicit calls, the destructor for
the object is run once only.
Once the destructor for an object is run, if that object, or any part of it, cannot be accessed by any possible continuation of
execution, including the running of destructors, the object is
considered inaccessible and the object becomes eligible for
collection.
Finally, at some time after the object becomes eligible for collection, the garbage collector frees the memory associated with
that object.
If you want to go into more detail then you can look here: https://learn.microsoft.com/en-us/dotnet/standard/garbage-collection/index
or I'm sure you can also find some blogs on the topic.
I was asked this question in an interview: How does memory leakage problem occur in C# as of all know Garbage Collector responsible for all the memory management related work? So how is it possible?
From MSDN:-
A memory leak occurs when memory is allocated in a program and is
never returned to the operating system, even though the program does
not use the memory any longer. The following are the four basic types of memory leaks:
In a manually managed memory environment: Memory is dynamically allocated and referenced by a pointer. The pointer is erased before the memory is freed. After the pointer is erased, the memory can no longer be accessed and therefore cannot be freed.
In a dynamically managed memory environment: Memory is disposed of but never collected, because a reference to the object is still active. Because a reference to the object is still active, the garbage collector never collects that memory. This can occur with a reference that is set by the system or the program.
In a dynamically managed memory environment: The garbage collector can collect and free the memory but never returns it to the operating system. This occurs when the garbage collector cannot move the objects that are still in use to one portion of the memory and free the rest.
In any memory environment: Poor memory management can result when many large objects are declared and never permitted to leave scope. As a result, memory is used and never freed.
Dim DS As DataSet
Dim cn As New SqlClient.SqlConnection("data source=localhost;initial catalog=Northwind;integrated security=SSPI")
cn.Open()
Dim da As New SqlClient.SqlDataAdapter("Select * from Employees", cn)
Dim i As Integer
DS = New DataSet()
For i = 0 To 1000
da.Fill(DS, "Table" + i.ToString)
Next
Although this code is obviously inefficient and not practical, it is meant to demonstrate that if objects are added to a collection (such as adding the tables to the DataSet collection), the objects are kept active as long as the collection remains alive. If a collection is declared at the global level of the program, and objects are declared throughout the program and added to that collection, this means that even though the objects are no longer in scope, the objects remain alive because they are still being referenced.
You may also check this reference:-
Identify And Prevent Memory Leaks In Managed Code
The above link gives a very good conclusion
Although .NET reduces the need for you to be concerned with memory,
you still must pay attention to your application's use of memory to
ensure that it is well-behaved and efficient. Just because an
application is managed doesn't mean you can throw good software
engineering practices out the window and count on the GC to perform
magic.
Just holding on to a reference to an object when you actually don't have any use for it anymore is a good way to leak. Particularly so when you store it in a static variable, that creates a reference that lives for the life of the AppDomain unless you explicitly set it back to null. If such a reference is stored in a collection then you can get a true leak that can crash your program with OOM. Not usual.
Such leaks can be hard to find. Particularly events can be tricky that way, they will get the event source object to add a reference to the event handler object when you subscribe an event handler. Hard to see in C# code because you never explicitly pass this when you subscribe the event. If the event source object lives for a long time then you can get in trouble with all of the subscriber objects staying referenced.
The tricky ones do require a memory profiler to diagnose.
Example would be a Class Child containing ClickEventHandler method subscribed to an Event ClickEvent of another class Parent.
GC of Child class would be blocked until Parent class goes out of scope..Even if Child goes out of scope it won't be collected by GC until Parent goes out of scope
All such subscribers subscribing to a Broadcaster(Event) would not be collected by GC until the broadcaster goes out of scope.
So, its a one way relation
Broadcaster(ClickEvent) -> Subscribers(ClickEventHandler)
GC of all the ClickEventHandlers would be blocked until ClickEvent goes out of scope!
As example:
You have application with main form and static variable Popups[] collectionOfPopups.
Store all application popups objects into static array and never delete them.
So with each new popup will take memory and GC will never release it.
Try to read how GC works
http://msdn.microsoft.com/en-us/library/ee787088.aspx
This will explains everything.
There can be multiple reasons, but here is one:
Consider two classes:
class A
{
private B b;
}
class B
{
private A a;
}
If you create an object for each of those classes, and cross-link them, and after that both those objects go out of the scope, you will still have link to each of them within another one. It is very difficult for GC to catch these sorts of crosslinks, and it may continue to believe that both objects are still in use.
Resharper recommends that these vars:
List<string> senderDeviceIDList;
string senderDeviceID;
. . .
foreach (var item in PlatypiIds)
{
senderDeviceIDList = await GetSenderDeviceIDForSenderID(item);
senderDeviceID = senderDeviceIDList[0];
...can be declared in inner scope, like so:
foreach (var item in PlatypiIds)
{
List<string> senderDeviceIDList = await GetSenderDeviceIDForSenderID(item);
string senderDeviceID = senderDeviceIDList[0];
...but is that really "more better"? Doesn't that cause the vars to be declared N times (once for each foreach loop)?
There is no any benefit in terms of performance or memory allocation here, as variables inside or oustside of the if scope are declared in the IL, by the way.
The only benefit is localization of the variable scope. Move it to the scope where it's used, which brings goodness like:
easy refactoring (may be the most important)
readability. If you see variable inside the scope, you know that its used only inside that scope and if you see some variable that apparently is not inside, you know that changing it can affect other parts of the code. So changing it introduce some potential danger.
In short, it's about readability and usability of the code you're writing and does not introduce any performance or memory consumption benefits.
Doesn't that cause the vars to be declared N times (one for each foreach loop)?
Logically, from a conceptual point of view yes, and that's the point! Logically they exist once per loop and make no sense outside of the scope of the loop.
As an implementation detail, no, it will not result in multiple local variables being created. The method will only have a single variable and it will be re-used in the general case (and when it's allowed to). There are exceptional cases, such as when you close over the variable with an anonymous method, in which it can't re-use a variable.
Note that because C# forces you to initialize all local variables before using them the runtime isn't even responsible for clearing it after each loop, the compiler won't let you re-use the garbage that was in the before (unless you explicitly initialize it at the start of the loop to the default value).
You are assigning instances of those objects once per iteration anyways, the only thing that's different in the initial approach is that you are declaring the references once, rather than every iteration as in the 2nd example.
If you needed to use those objects in their final state at the end of the foreach loop (hairy), then you may want to go with the 1st approach.
Sometimes benefit exists.
If resulting array is huge, then moving it to the inner scope — i.e. reducing scope and lifetime — may prevent it to be shifted to later garbage collection generations, and to be garbage collected with a tangible delay.
Decided to explain my old answer [after getting surprised by an upvote] and partially give up from my words (27.10.2021)
Detail #0.
Two your options differ in visibility scope and, hence, lifetime. A variable declared inside "for" loop lives starting from the declaration until } of this loop, i.e. until the end of each iteration. This means that the variable doesn't even exist after the end of each iteration until it reaches the declaration line of the next iteration!
And a variable declared outside "for" loop lives for the entire duration of the method body (I assume here that in your first example it is declared at the top level of some method's body).
Ok, what it affects?
Variables may represent a pointer to an object. And objects live only when at least someone points at them. Because if no one points at them, how can anyone refer to them? And vice versa - if someone points to an object, it may refer to it at some point in the future, so it must live. That's exactly the logic of the garbage collector (GC).
So, when you leave the visibility scope of a variable, which points to some object, GC can stop taking this pointer into consideration. And if that object had only one pointer to itself (your variable), then this object already doesn't exist as no one points to it. However, it continues to live in terms of occupied memory - it's a garbage in your heap that waits for GC to be collected (at the next garbage collection run or later - it's up to GC when to free the memory).
Getting back to your options - there're points during the method execution when these options differ in terms of whether GC is allowed or not collecting an object to which your variable is/were pointing. As long as garbage collection may happen at any moment, moving a variable declaration to the inner scope just increases probability that that object could be collected.
Aaaaaand.. 99.9% of the time you should not worry about it, because this object will be collected at some point in the future anyway! But..
Detail #1
No "but". Really :)
Dunno, I probably was mistaken back in 2016 by thinking that objects in LOH (Large Object Heap) also have generations, so it does matter how quickly you can allow GC to collect them - because the higher the generation the more rare its collection happens. Therefore generation 2 objects are very-very long lived objects. This's not true now (see docs) - LOH objects are generation 2 objects for their entire lifetime. So, no rush with variables to LOH objects - you are already late :)
Even if these objects are large, but not large enough for LOH, you probably still should not worry until you find evidences that you have to investigate this matter (memory footprint and performance).
The moral
So, my answer should be Sometimes benefit exists [but it's negligible].
Specifying the right scope is a help to yourself and your teammates in the first place, not a fancy performance trick.
private button btnNew=new button();
btnNew.addclickhandler(this);
private DataGrid grid;
private void onClick(event click) {grid=new DataGrid();}
Hello ,I write a code like this sample ,I want to know that every time a user click on btnNew,what is going on in heap and stack memory?for example does a new block in heap memory assign to this grid?Or an older block remove and this new block replace it ?Or an older block remains in heap memory and also new block assign to it.
Is this block of code allocate a huge memory on several click?
**The DataGrid could be replace with any component I want to know about this type of new statement usage and memory allocation **
sorry, for my bad english!
.
what is going on with respect to heap
and stack memory?
since the button is reference type and declared in global will be allocated in heap, not in stack.
Is a new block in heap memory assigned to this button?
yes if memory is available, else unreached references will be removed and this one is allocated
Does this block of code allocate a
large amount of memory on a single
click?
No, but it will, if you add thousand buttons
Check out this cool article Memory in .NET - what goes where by Jon Skeet to understand the memory internals better..
Cheers
This is a huge topic. This is akin to asking "you type www.amazon.com into a browser. What happens next?" To answer that question fully you have to explain the architecture of the entire internet. To answer your question fully you have to understand the entire memory model of a modern operating system.
You should start by reading about the fundamentals of memory and garbage collection, here:
http://msdn.microsoft.com/en-us/library/ee787088.aspx
and then ask more specific questions about things you don't understand.
Using the new statement allocates memory on the heap. In general new memory is allocated. If the btnNew pointer was the only pointer associated with a button object, it should become a target to the garbage collector. So the memory will be freed again. For multiple clicks the same will happen, but you should be aware that the garbage collector does not work in real time. So in a high-frequency loop allocating large objects - "new" can become a problem in c#.
if button is a class (ref type), it is allocated on the heap. (value-types are allocated on the stack in the current CLR implementation, unless they are contained by another reference type or captured in a closure - in which case they are on the heap.).
The garbage collector has pre-allocated segments of memory of different sizes corresponding to generations 0, 1 and 2. WHen you new up an object, it is allocated in generation 0. And this allocation is really fast, since it is just moving a pointer by a delta = size of the object. The CLR clears the values in the object to default values as a prereq step before executing the ctor.
Periodically all threads are paused and the garbage collector runs. It creates a graph of reachable objects by traversing "roots". All unreachable objects are discarded. The generation segments are moved around / compacted to avoid fragmentation. Gen 0 is collected more frequently than 1 and so on... (since Gen-0 objects are likely to be short-lived objects). After the collection, the app threads resume.
For more on this, refer to documents explaining the garbage collector and generations. Here's one.
I have a problem where a couple 3 dimensional arrays allocate a huge amount of memory and the program sometimes needs to replace them with bigger/smaller ones and throws an OutOfMemoryException.
Example: there are 5 allocated 96MB arrays (200x200x200, 12 bytes of data in each entry) and the program needs to replace them with 210x210x210 (111MB). It does it in a manner similar to this:
array1 = new Vector3[210,210,210];
Where array1-array5 are the same fields used previously. This should set the old arrays as candidates for garbage collection but seemingly the GC does not act quickly enough and leaves the old arrays allocated before allocating the new ones - which causes the OOM - whereas if they where freed before the new allocations the space should be enough.
What I'm looking for is a way to do something like this:
GC.Collect(array1) // this would set the reference to null and free the memory
array1 = new Vector3[210,210,210];
I'm not sure if a full garbage collecion would be a good idea since that code may (in some situations) need to be executed fairly often.
Is there a proper way of doing this?
This is not an exact answer to the original question, "how to force GC', yet, I think it will help you to reexamine your issue.
After seeing your comment,
Putting the GC.Collect(); does seem to help, altought it still does not solve the problem completely - for some reason the program still crashes when about 1.3GB are allocated (I'm using System.GC.GetTotalMemory( false ); to find the real amount allocated).
I will suspect you may have memory fragmentation. If the object is large (85000 bytes under .net 2.0 CLR if I remember correctly, I do not know whether it has been changed or not), the object will be allocated in a special heap, Large Object Heap (LOH). GC does reclaim the memory being used by unreachable objects in LOH, yet, it does not perform compaction, in LOH as it does to other heaps (gen0, gen1, and gen2), due to performance.
If you do frequently allocate and deallocate large objects, it will make LOH fragmented and even though you have more free memory in total than what you need, you may not have a contiguous memory space anymore, hence, will get OutOfMemory exception.
I can think two workarounds at this moment.
Move to 64-bit machine/OS and take advantage of it :) (Easiest, but possibly hardest as well depending on your resource constraints)
If you cannot do #1, then try to allocate a huge chuck of memory first and use them (it may require to write some helper class to manipulate a smaller array, which in fact resides in a larger array) to avoid fragmentation. This may help a little bit, yet, it may not completely solve the issue and you may have to deal with the complexity.
Seems you've run into LOH (Large object heap) fragmentation issue.
Large Object Heap
CLR Inside Out Large Object Heap Uncovered
You can check to see if you're having loh fragmentation issues using SOS
Check this question for an example of how to use SOS to inspect the loh.
Forcing a Garbage Collection is not always a good idea (it can actually promote the lifetimes of objects in some situations). If you have to, you would use:
array1 = null;
GC.Collect();
array1 = new Vector3[210,210,210];
Isn't this just large object heap fragmentation? Objects > 85,000 bytes are allocated on the large object heap. The GC frees up space in this heap but never compacts the remaining objects. This can result in insufficent contiguous memory to successfully allocate a large object.
Alan.
If I had to speculate you problem is not really that you are going from Vector3[200,200,200] to a Vector3[210,210,210] but that most likely you have similar previous steps before this one:
i.e.
// first you have
Vector3[10,10,10];
// then
Vector3[20,20,20];
// then maybe
Vector3[30,30,30];
// .. and so on ..
// ...
// then
Vector3[200,200,200];
// and eventually you try
Vector3[210,210,210] // and you get an OutOfMemoryException..
If that is true, I would suggest a better allocation strategy. Try over allocating - maybe doubling the size every time as opposed to always allocating just the space that you need. Especially if these arrays are ever used by objects that need to pin the buffers (i.e. if that have ties to native code)
So, instead of the above, have something like this:
// first start with an arbitrary size
Vector3[64,64,64];
// then double that
Vector3[128,128,128];
// and then.. so in thee steps you go to where otherwise
// it would have taken you 20..
Vector3[256,256,256];
They might not be getting collected because they're being referenced somewhere you're not expecting.
As a test, try changing your references to WeakReferences instead and see if that resolves your OOM problem. If it doesn't then you're referencing them somewhere else.
I understand what you're trying to do and pushing for immediate garbage collection is probably not the right approach (since the GC is subtle in its ways and quick to anger).
That said, if you want that functionality, why not create it?
public static void Collect(ref object o)
{
o = null;
GC.Collect();
}
An OutOfMemory exception internally triggers a GC cycle automatically once and attempts the allocation again before actually throwing the exception to your code. The only way you could be having OutOfMemory exceptions is if you're holding references to too much memory. Clear the references as soon as you can by assigning them null.
Part of the problem may be that you're allocating a multidimensional array, which is represented as a single contiguous block of memory on the large object heap (more details here). This can block other allocations as there isn't a free contiguous block to use, even if there is still some free space somewhere, hence the OOM.
Try allocating it as a jagged array - Vector3[210][210][210] - which spreads the arrays around memory rather than as a single block, and see if that improves matters
John, Creating objects > 85000 bytes will make the object end up in the large object heap. The large object heap is never compacted, instead the free space is reused again.
This means that if you are allocating larger arrays every time, you can end up in situations where LOH is fragmented, hence the OOM.
you can verify this is the case by breaking with the debugger at the point of OOM and getting a dump, submitting this dump to MS through a connect bug (http://connect.microsoft.com) would be a great start.
What I can assure you is that the GC will do the right thing trying to satisfy you allocation request, this includes kicking off a GC to clean the old garbage to satisfy the new allocation requests.
I don't know what is the policy of sharing out memory dumps on Stackoverflow, but I would be happy to take a look to understand your problem more.