I am reading a book on C# memory management which says:
What’s important to understand is that a finalizer increases the life
of an object. Because the finalization code also has to run, the .NET
Framework keeps a reference to the object in a special finalization
queue. An additional thread runs all the finalizers at a time deemed
appropriate based on the execution context. This delays garbage
collection for types that have a finalizer.
As per my knowledge finalizer would run on garbage collection only, not before it. So, how can it delay its garbage collection?
After going through MSDN links posted by comments and deleted answers, below are the details of the whole process:
When a finalizable object is discovered (by GC) to be dead, its finalizer is put in a queue so that its cleanup actions are executed, but the object itself is promoted to the next generation. Therefore, you have to wait until the next garbage collection that occurs on that generation (which is not necessarily the next garbage collection) to determine whether the object has been reclaimed.
Following is the code to demonstrate the same:
using System;
class Program
{
static void CreateDestructorReferences()
{
for (int i = 0; i < 1000; i++)
_ = new Destructor();
}
static void CreateWithoutDestructorReferences()
{
for (int i = 0; i < 1000; i++)
_ = new WithoutDestructor();
}
static void Main(string[] args)
{
CreateDestructorReferences();
DemoGCProcess("****Objects With Destructors*****");
CreateWithoutDestructorReferences();
DemoGCProcess("****Objects Without Destructors*****");
Console.ReadLine();
}
private static void DemoGCProcess(string text)
{
Console.WriteLine(text);
var memory = GC.GetTotalMemory(false);
GC.Collect(0);
GC.WaitForPendingFinalizers();
var memory1 = GC.GetTotalMemory(false);
Console.WriteLine("Memory freed on first Garbage Collection on Generation 0:" + (memory - memory1));
GC.Collect(1);
var memory2 = GC.GetTotalMemory(false);
Console.WriteLine("Memory freed on second Garbage Collection on Generation 0 and Generation 1:" + (memory1 - memory2));
}
}
class Destructor
{
~Destructor()
{
//Console.WriteLine("Destructor is called");
}
}
class WithoutDestructor
{
}
And here is the output of the above program:
I've read this article and this too.
I was trying to implement destructor in simple code.
class Program
{
static void Main(string[] args)
{
CreateSubscriber();
Console.Read();
}
static void CreateSubscriber()
{
Subscriber s = new Subscriber();
s.list.Clear();
}
}
public class Subscriber
{
public List<string> list = new List<string>();
public Subscriber()
{
for(long i = 0; i < 1000000; i++)
{
list.Add(i.ToString());
}
}
~Subscriber()
{
//this line is only performed on explicit GC.Collect()
Console.WriteLine("Destructor Called - Sub");
}
}
As when code reached line of Console.Read(), instance of Subscriber was no longer in scope (I was expecting it to be eligible for Garbage collection). I left above code running for almost 2 hours waiting for destructor of Subscriber. but that never called, neither memory taken by the code released.
I understood, in c# we cannot call destructors programmatic and it is automatically called on Garbage collection , So I tried to call GC.Collect() explicitly.
By doing that, I could see that destructor was called. So In my above code, Garbage collection was not being done! But Why?
Is it because, program is single threaded and that thread is waiting for user input on Console.Read() ?
Or it does have something with List of string ? if so what is it
Update (for future readers)
as Fabjan suggested in his answer
Most likely somewhere when a new object is created and memory is allocated for it, GC does perform a check of all references and collects the first object.
and suggested to try
CreateSubscriber();
Console.Read();
CreateSubscriber();
Console.Readkey();
I updated the code like below,
class Program
{
static void Main(string[] args)
{
CreateSubscriber(true);
Console.ReadLine();
CreateSubscriber(false);
Console.ReadLine();
}
static void CreateSubscriber(bool toDo)
{
Subscriber s = new Subscriber(toDo);
s.list.Clear();
}
}
public class Subscriber
{
public List<string> list = new List<string>();
public Subscriber(bool toDo)
{
Console.WriteLine("In Consutructor");
if (toDo)
{
for (long i = 0; i < 5000000; i++)
list.Add(i.ToString());
}
else
{
for (long i = 0; i < 2000000; i++)
list.Add(i.ToString());
}
Console.WriteLine("Out Consutructor");
}
~Subscriber()
{
Console.WriteLine("Destructor Called - Sub");
}
}
output:
and as he expected, on second instance creation of Subscriber, i could see GC being collected (finalizer being called).
Note that: in else condition of constructor of Subscriber, i m adding less items in list then if condition - to notice if RAM usage of application is being decreased accordingly, Yes it is being decreased too.
there in else condition, i could have left list of string empty (so memory usage will be significantly decreased). But doing so, GC is not being collected. Most likely because of the reason which M.Aroosi has mentioned in question's comment.
in addition to what's said above, the GC will only collect once a generation gets full(or due to an explicit call), and just 1 object created wouldn't trigger it. Yes the object is elligible for finalization, but there's no reason for the GC to collect it.
As when code reached line of Console.Read(), instance of Subscriber
was no longer in scope (I was expecting it to be eligible for Garbage
collection).
When GC detects that the reference for the instance of Subscriber is lost (out of scope) it will mark this object to be collected on one of the next rounds.
But only GC knows when exactly will this next round be.
Is it because, program is single threaded and that thread is waiting
for user input on Console.Read() ?
No, if we run this code in a separate thread the result will be the same.
However if we change this:
CreateSubscriber();
Console.Read();
To:
CreateSubscriber();
Console.Read();
CreateSubscriber();
Console.Readkey();
We could see that GC will collect the garbage and run the finalizer after Console.Read(). Why?
Most likely somewhere when a new object is created and memory is allocated for it, GC does perform a check of all references and collects the first object.
Let's summarize it a bit:
When we only create an object and there is no reference in code that points
to this obj or its class until program ends - GC allows program to end and
collect the garbage before exiting.
When we create an object and there is some sort of reference to obj
or its class - GC does performs a check and collects the garbage.
There's some complex logic behind how and when does the GC run collect and how and when the lifetime of the obj ends.
A quote from Eric Lippert's answer:
The lifetime may be extended by a variety of mechanisms, including
capturing outer variables by a lambda, iterator blocks, asynchronous
methods, and so on
It's very rare that we need to execute some code on object's destruction. In that specific scenario instead of guessing when the obj will be destroyed we could run GC.Collect explicitly.
More often though we might need to free some managed resources and for that we could use IDisposable interface and using statement that will automatically call Dispose before the control flow leaves the block of code (it'll create a try {} finally {} clause and in finally it will call Dispose for us).
using(myDisposable)
{
...
} // dispose is called here
I am implementing an infinite task that runs every so often via a .net console application. However I am concerned that the below will result in a memory leak. Since Main is static (this is where my knowledge on garbage collection gets foggy) does it not mean that the object that I create within the try and catch won't be picked up by the garbage collector until Main finishes (which is never)?
Could someone please explain how the garbage collector would behave in this case?
public static void Main(string[] args)
{
Log.Logger = new LoggerConfiguration()
.MinimumLevel.Debug()
.WriteTo.RollingFile("Logs/log-{Date}.txt")
.CreateLogger();
while (true)
{
try
{
Thread.Sleep(1000);
new UpdatePlayer().Run();
}
catch (Exception ex)
{
Log.Error(ex.ToString());
}
}
}
You'll have no memory leak: Main doesn't have any reference to UpdatePlayer() instance:
...
try
{
Thread.Sleep(1000);
// Instance created, executed
new UpdatePlayer().Run();
}
// And can be safely collected here when the instance is out of scope (`try {...}`)
// the anonymous instance can't be accessed and that's enough
A sample with memory leak:
// please notice, that anchor is outside the `while(true)` scope
List<Object> anchor = new List<Object>();
...
while (true)
{
try
{
Thread.Sleep(1000);
// Instance created
var item = new UpdatePlayer();
// anchored
anchor.Add(item);
// and executed
item.Run();
}
// item can't be collected here: anchor has a reference to it
// And the item potentially can be accessed by, say, anchor[0]
Edit: and if you move the collection into the while(true) scope, the code will be without memory leak:
try
{
List<object> pseudoAnchor = new List<object>();
// Instance created
var item = new UpdatePlayer();
// tried to be anchored (illusion)
pseudoAnchor.Add(item);
// and executed
item.Run();
}
...
// the only reference to item is pseudoAnchor, but
// pseudoAnchor has not been referenced, and so GC
// can safely collect entire pseudoAnchor with all its items
// (just one `item` in fact)
Since Main is static (this is where my knowledge on garbage collection gets foggy) does it not mean that the object that I create within the try and catch won't be picked up by the garbage collector until Main finishes (which is never)?
No, that is false.
The Garbage Collector is able to free the memory of any object it can prove can no longer be accessed from managed code. In this case, it's clear that after the Run method ends (unless you store a reference to that object somewhere else) that object will no longer be accessible, and so the GC is allowed to free it (it may take some time to do so, but it's allowed to).
Consider the code below:
using System;
namespace memoryEater
{
internal class Program
{
private static void Main(string[] args)
{
Console.WriteLine("alloc 1");
var big1 = new BigObject();
Console.WriteLine("alloc 2");
var big2 = new BigObject();
Console.WriteLine("null 1");
big1 = null;
//GC.Collect();
Console.WriteLine("alloc3");
big1 = new BigObject();
Console.WriteLine("done");
Console.Read();
}
}
public class BigObject
{
private const uint OneMeg = 1024 * 1024;
private static int _idCnt;
private readonly int _myId;
private byte[][] _bigArray;
public BigObject()
{
_myId = _idCnt++;
Console.WriteLine("BigObject {0} creating... ", _myId);
_bigArray = new byte[700][];
for (int i = 0; i < 700; i++)
{
_bigArray[i] = new byte[OneMeg];
}
for (int j = 0; j < 700; j++)
{
for (int i = 0; i < OneMeg; i++)
{
_bigArray[j][i] = (byte)i;
}
}
Console.WriteLine("done");
}
~BigObject()
{
Console.WriteLine("BigObject {0} finalised", _myId);
}
}
}
I have a class, BigObject, which creates a 700MiB array in its constructor, and has a finalise method which does nothing other than print to console. In Main, I create two of these objects, free one, and then create a third.
If this is compiled for 32 bit (so as to limit memory to 2 gigs), an out of memory exception is thrown when creating the third BigObject. This is because, when memory is requested for the third time, the request cannot be satisfied and so the garbage collector runs. However, the first BigObject, which is ready to be collected, has a finaliser method so instead of being collected is placed on the finalisation queue and is finalised. The garbage collecter then halts and the exception is thrown. However, if the call to GC.Collect is uncommented, or the finalise method is removed, the code will run fine.
My question is, why does the garbage collector not do everything it can to satisfy the request for memory? If it ran twice (once to finalise and again to free) the above code would work fine. Shouldn't the garbage collector continue to finalise and collect until no more memory can be free'd before throwing the exception, and is there any way to configure it to behave this way (either in code or through Visual Studio)?
Its undeterministic when GC will work and try to reclaim memory.
If you add this line after big1 = null . However you should be carefult about forcing GC to collect. Its not recommended unless you know what you are doing.
GC.Collect();
GC.WaitForPendingFinalizers();
Best Practice for Forcing Garbage Collection in C#
When should I use GC.SuppressFinalize()?
Garbage collection in .NET (generations)
I guess its because the time the finalizer executes during garbage collection is undefined. Resources are not guaranteed to be released at any specific time (unless calling a Close method or a Dispose method.), also the order that finalizers are run is random so you could have a finalizer on another object waiting, while your object waits for that.
Summary: C#/.NET is supposed to be garbage collected. C# has a destructor, used to clean resources. What happen when an object A is garbage collected the same line I try to clone one of its variable members? Apparently, on multiprocessors, sometimes, the garbage collector wins...
The problem
Today, on a training session on C#, the teacher showed us some code which contained a bug only when run on multiprocessors.
I'll summarize to say that sometimes, the compiler or the JIT screws up by calling the finalizer of a C# class object before returning from its called method.
The full code, given in Visual C++ 2005 documentation, will be posted as an "answer" to avoid making a very very large questions, but the essential are below:
The following class has a "Hash" property which will return a cloned copy of an internal array. At is construction, the first item of the array has a value of 2. In the destructor, its value is set to zero.
The point is: If you try to get the "Hash" property of "Example", you'll get a clean copy of the array, whose first item is still 2, as the object is being used (and as such, not being garbage collected/finalized):
public class Example
{
private int nValue;
public int N { get { return nValue; } }
// The Hash property is slower because it clones an array. When
// KeepAlive is not used, the finalizer sometimes runs before
// the Hash property value is read.
private byte[] hashValue;
public byte[] Hash { get { return (byte[])hashValue.Clone(); } }
public Example()
{
nValue = 2;
hashValue = new byte[20];
hashValue[0] = 2;
}
~Example()
{
nValue = 0;
if (hashValue != null)
{
Array.Clear(hashValue, 0, hashValue.Length);
}
}
}
But nothing is so simple...
The code using this class is wokring inside a thread, and of course, for the test, the app is heavily multithreaded:
public static void Main(string[] args)
{
Thread t = new Thread(new ThreadStart(ThreadProc));
t.Start();
t.Join();
}
private static void ThreadProc()
{
// running is a boolean which is always true until
// the user press ENTER
while (running) DoWork();
}
The DoWork static method is the code where the problem happens:
private static void DoWork()
{
Example ex = new Example();
byte[] res = ex.Hash; // [1]
// If the finalizer runs before the call to the Hash
// property completes, the hashValue array might be
// cleared before the property value is read. The
// following test detects that.
if (res[0] != 2)
{
// Oops... The finalizer of ex was launched before
// the Hash method/property completed
}
}
Once every 1,000,000 excutions of DoWork, apparently, the Garbage Collector does its magic, and tries to reclaim "ex", as it is not anymore referenced in the remaning code of the function, and this time, it is faster than the "Hash" get method. So what we have in the end is a clone of a zero-ed byte array, instead of having the right one (with the 1st item at 2).
My guess is that there is inlining of the code, which essentially replaces the line marked [1] in the DoWork function by something like:
// Supposed inlined processing
byte[] res2 = ex.Hash2;
// note that after this line, "ex" could be garbage collected,
// but not res2
byte[] res = (byte[])res2.Clone();
If we supposed Hash2 is a simple accessor coded like:
// Hash2 code:
public byte[] Hash2 { get { return (byte[])hashValue; } }
So, the question is: Is this supposed to work that way in C#/.NET, or could this be considered as a bug of either the compiler of the JIT?
edit
See Chris Brumme's and Chris Lyons' blogs for an explanation.
http://blogs.msdn.com/cbrumme/archive/2003/04/19/51365.aspx
http://blogs.msdn.com/clyon/archive/2004/09/21/232445.aspx
Everyone's answer was interesting, but I couldn't choose one better than the other. So I gave you all a +1...
Sorry
:-)
Edit 2
I was unable to reproduce the problem on Linux/Ubuntu/Mono, despite using the same code on the same conditions (multiple same executable running simultaneously, release mode, etc.)
It's simply a bug in your code: finalizers should not be accessing managed objects.
The only reason to implement a finalizer is to release unmanaged resources. And in this case, you should carefully implement the standard IDisposable pattern.
With this pattern, you implement a protected method "protected Dispose(bool disposing)". When this method is called from the finalizer, it cleans up unmanaged resources, but does not attempt to clean up managed resources.
In your example, you don't have any unmanaged resources, so should not be implementing a finalizer.
What you're seeing is perfectly natural.
You don't keep a reference to the object that owns the byte array, so that object (not the byte array) is actually free for the garbage collector to collect.
The garbage collector really can be that aggressive.
So if you call a method on your object, which returns a reference to an internal data structure, and the finalizer for your object mess up that data structure, you need to keep a live reference to the object as well.
The garbage collector sees that the ex variable isn't used in that method any more, so it can, and as you notice, will garbage collect it under the right circumstances (ie. timing and need).
The correct way to do this is to call GC.KeepAlive on ex, so add this line of code to the bottom of your method, and all should be well:
GC.KeepAlive(ex);
I learned about this aggressive behavior by reading the book Applied .NET Framework Programming by Jeffrey Richter.
this looks like a race condition between your work thread and the GC thread(s); to avoid it, i think there are two options:
(1) change your if statement to use ex.Hash[0] instead of res, so that ex cannot be GC'd prematurely, or
(2) lock ex for the duration of the call to Hash
that's a pretty spiffy example - was the teacher's point that there may be a bug in the JIT compiler that only manifests on multicore systems, or that this kind of coding can have subtle race conditions with garbage collection?
I think what you are seeing is reasonable behavior due to the fact that things are running on multiple threads. This is the reason for the GC.KeepAlive() method, which should be used in this case to tell the GC that the object is still being used and that it isn't a candidate for cleanup.
Looking at the DoWork function in your "full code" response, the problem is that immediately after this line of code:
byte[] res = ex.Hash;
the function no longer makes any references to the ex object, so it becomes eligible for garbage collection at that point. Adding the call to GC.KeepAlive would prevent this from happening.
Yes, this is an issue that has come up before.
Its even more fun in that you need to run release for this to happen and you end up stratching your head going 'huh, how can that be null?'.
Interesting comment from Chris Brumme's blog
http://blogs.msdn.com/cbrumme/archive/2003/04/19/51365.aspx
class C {<br>
IntPtr _handle;
Static void OperateOnHandle(IntPtr h) { ... }
void m() {
OperateOnHandle(_handle);
...
}
...
}
class Other {
void work() {
if (something) {
C aC = new C();
aC.m();
... // most guess here
} else {
...
}
}
}
So we can’t say how long ‘aC’ might live in the above code. The JIT might report the reference until Other.work() completes. It might inline Other.work() into some other method, and report aC even longer. Even if you add “aC = null;” after your usage of it, the JIT is free to consider this assignment to be dead code and eliminate it. Regardless of when the JIT stops reporting the reference, the GC might not get around to collecting it for some time.
It’s more interesting to worry about the earliest point that aC could be collected. If you are like most people, you’ll guess that the soonest aC becomes eligible for collection is at the closing brace of Other.work()’s “if” clause, where I’ve added the comment. In fact, braces don’t exist in the IL. They are a syntactic contract between you and your language compiler. Other.work() is free to stop reporting aC as soon as it has initiated the call to aC.m().
That's perfectly nornal for the finalizer to be called in your do work method as after the
ex.Hash call, the CLR knows that the ex instance won't be needed anymore...
Now, if you want to keep the instance alive do this:
private static void DoWork()
{
Example ex = new Example();
byte[] res = ex.Hash; // [1]
// If the finalizer runs before the call to the Hash
// property completes, the hashValue array might be
// cleared before the property value is read. The
// following test detects that.
if (res[0] != 2) // NOTE
{
// Oops... The finalizer of ex was launched before
// the Hash method/property completed
}
GC.KeepAlive(ex); // keep our instance alive in case we need it.. uh.. we don't
}
GC.KeepAlive does... nothing :) it's an empty not inlinable /jittable method whose only purpose is to trick the GC into thinking the object will be used after this.
WARNING: Your example is perfectly valid if the DoWork method were a managed C++ method... You DO have to manually keep the managed instances alive manually if you don't want the destructor to be called from within another thread. IE. you pass a reference to a managed object who is going to delete a blob of unmanaged memory when finalized, and the method is using this same blob. If you don't hold the instance alive, you're going to have a race condition between the GC and your method's thread.
And this will end up in tears. And managed heap corruption...
The Full Code
You'll find below the full code, copy/pasted from a Visual C++ 2008 .cs file. As I'm now on Linux, and without any Mono compiler or knowledge about its use, there's no way I can do tests now. Still, a couple of hours ago, I saw this code work and its bug:
using System;
using System.Threading;
public class Example
{
private int nValue;
public int N { get { return nValue; } }
// The Hash property is slower because it clones an array. When
// KeepAlive is not used, the finalizer sometimes runs before
// the Hash property value is read.
private byte[] hashValue;
public byte[] Hash { get { return (byte[])hashValue.Clone(); } }
public byte[] Hash2 { get { return (byte[])hashValue; } }
public int returnNothing() { return 25; }
public Example()
{
nValue = 2;
hashValue = new byte[20];
hashValue[0] = 2;
}
~Example()
{
nValue = 0;
if (hashValue != null)
{
Array.Clear(hashValue, 0, hashValue.Length);
}
}
}
public class Test
{
private static int totalCount = 0;
private static int finalizerFirstCount = 0;
// This variable controls the thread that runs the demo.
private static bool running = true;
// In order to demonstrate the finalizer running first, the
// DoWork method must create an Example object and invoke its
// Hash property. If there are no other calls to members of
// the Example object in DoWork, garbage collection reclaims
// the Example object aggressively. Sometimes this means that
// the finalizer runs before the call to the Hash property
// completes.
private static void DoWork()
{
totalCount++;
// Create an Example object and save the value of the
// Hash property. There are no more calls to members of
// the object in the DoWork method, so it is available
// for aggressive garbage collection.
Example ex = new Example();
// Normal processing
byte[] res = ex.Hash;
// Supposed inlined processing
//byte[] res2 = ex.Hash2;
//byte[] res = (byte[])res2.Clone();
// successful try to keep reference alive
//ex.returnNothing();
// Failed try to keep reference alive
//ex = null;
// If the finalizer runs before the call to the Hash
// property completes, the hashValue array might be
// cleared before the property value is read. The
// following test detects that.
if (res[0] != 2)
{
finalizerFirstCount++;
Console.WriteLine("The finalizer ran first at {0} iterations.", totalCount);
}
//GC.KeepAlive(ex);
}
public static void Main(string[] args)
{
Console.WriteLine("Test:");
// Create a thread to run the test.
Thread t = new Thread(new ThreadStart(ThreadProc));
t.Start();
// The thread runs until Enter is pressed.
Console.WriteLine("Press Enter to stop the program.");
Console.ReadLine();
running = false;
// Wait for the thread to end.
t.Join();
Console.WriteLine("{0} iterations total; the finalizer ran first {1} times.", totalCount, finalizerFirstCount);
}
private static void ThreadProc()
{
while (running) DoWork();
}
}
For those interested, I can send the zipped project through email.