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When is it acceptable to call GC.Collect?
From what I know the CLR does all this garbage collection for you, but is there a reason to manually call GC.Collect()?
Is it for cases when you close a file, dispose of an image resource or an unmanaged resource, that you should call GC.Collect() to quickly clean up the unused memory immediately?
Is it for cases when you close a file, dispose of an image resource or an unmanaged resource, that you should call GC.Collect() to quickly clean up the unused memory immediately?
No, not really. This is typically handled via IDisposable.
There are very few reasons to call GC.Dispose() directly, and doing so often causes a lot of harm, since it interferes with the internal heuristics of the GC in .NET. There are, however, rare occurrences when it's useful.
For example, if you have a operation that is a rare operation which uses a large object graph, and you know that your going to be "idle" afterwards, you might want to call GC.Collect immediately afterwards to release the memory used by the objects. That being said, this is often still best left up to the system to handle.
For the most part, I've found the most useful scenario for GC.Collect() is for debugging. It can help you guarantee that you don't have a leak, since it allows you to force a full Gen2 collection.
http://blogs.msdn.com/b/ricom/archive/2003/12/02/40780.aspx
http://blogs.msdn.com/b/ricom/archive/2004/11/29/271829.aspx
Right from the equine's oral orifice.
I usually use GC.Collect() to bring the heap to a mostly-known state. For example, when benchmarking, you need to make sure that you start each run from a known state, and GC.Collect() helps with this.
It should not be used to dispose of unmanaged resources -- for that you should use using or manually call Dispose.
I can just think of two cases where GC.Collect() might be useful:
In unit tests. Call GC.Collect() before and after some test to find potential memory leaks. In this case consider using GC.WaitForPendingFinalizers(), because finalizers are executed in a separate thread. This means that classes with finalizers do not immediately release all ressources after calling GC.Collect().
In long living processes like Windows Services where there is a long idle time. Call GC.Collect() before it goes idle. Reason: If a process is idle, the garbage collector will not kick in, thus unused memory will be not released during idle time.
GC.Collect() should not be called for the purpose of "quickly clean up the unused memory immediately". Having a forced release of memory does not pay off the overall performance caused by an active garbage collection.
The only time I've ever used it in code that wasn't specifically to compare memory usage of two or more different approaches to something had the following scenario:
In a web application (and hence, long-running), there were a few very large collections that would generally be re-built at most a few times a day, often much less frequently.
Several objects within that collection would be equivalent, and hence a lot of memory could be saved by replacing references to one such object to a reference to the other (after building the collection was read-only in use, and hence the aliasing involved was safe). So first the collection would be built, but then it would be reduced in size, killing many objects.
This meant that there was a sudden spike in the number of objects destroyed per second, that would then not happen again for several hours at least. As such the GC would not correctly judge the amount of collection needed, and that memory was about to be needed to build the second large collection. Hence doing a manual collection every thousand of clean-up operations did have a positive effect on performance (enough to sometimes go from the application crashing on the third collection, to it being dependable).
A lot of measurement was done to make sure it was indeed beneficial in this case.
The two factors that made this beneficial were:
A lot of object deaths happened.
The event causing this was rare in the lifetime of the application.
Without both of those being true, the manual call would be needless. Without a lot of object deaths, there's no point, and if it wasn't rare in the application's lifetime, the GC would have self-tuned to cope.
Related
I'm in a situation where I'm having to create thousands of objects at once, and the cost of instantiating the objects and garbage collecting them is impacting the performance of the application, and the impact of the garbage collector running hurts the performance more since this is on older hardware, so I'm mostly trying to prevent the creation of garbage. I believe a memory pool would solve my issue but I'm not sure how the memory pool would know when a resource in the pool was freed up for re-use. The tricky part is that receiver of objects from the pool end up passing that object around throughout the program and it would be very difficult to know when it could be manually freed up. I'd like it to be like a WeakReference, where I could know when nobody was using it anymore. But my understanding is that if I use a WeakReference in the memory pool then it would eventually get garbage collected from the pool itself and I need these objects to remain pretty much forever so they'll continue to get recycled. Sometimes the program can go for awhile without needing the objects so I imagine the garbage collector would collect them before the next time when they were needed and that would then trigger another performance hit as another thousand of these objects were made.
Is there a way I can make sure these objects are never collected, but know when there are no references to them aside from the memory pool itself? Do I need to implement reference counting for these objects somehow?
I've been googling for a couple of hours now and have seen no implementation of a Memory Pool that doesn't require the user to let the memory pool know when they're done with it. I find it hard to believe there is no way to do this in C#.
Is there a way I can make sure these objects are never collected, but know when there are no references to them aside from the memory pool itself?
Usually an object pool only holds references to available objects (you can check ObjectPool implementation in Roslyn). With that in mind, you can use the finalizer to resurrect the object and return it to the pool when it is unreachable.
However, I don't think it will improve performance. The whole pool will very soon reach generation 2, so the unreachable objects will need a full garbage collection to be returned to the pool. Depending on the memory usage pattern in your program, it might not happen very often. You can GC.Collect() and GC.WaitForPendingFinalizers() of course, but it will also hurt performance. You can try it out and check if it helps.
Another problem is the design - your objects are coupled to the pool.
I'd rather try to return the objects to the pool explicitly. Remember that not all objects have to be returned. If there are no more available objects, the pool can create new ones. The ones that were not returned will just be garbage collected. Check if there are some code paths where you are sure the objects are not needed anymore. If you can't find any, try to refactor your code.
During an interview,
I was asked to come up with a way to ensure a block of code in c# can run in consistent time to meet a hypothetical time requirement.
The interviewer mentioned that one way is to invoke the garbage collector to collect before the block of code is executed such that it significantly reduces the probability of the GC running again during that block of code. It was applied to accurate time-based measurements of a medical device, where garbage collection can affect those measurements.
It makes sense to me, but I could not find any information backing that. The general consensus I reviewed is to never call GC.Collect(), and the exceptions did not include this scenario.
Can running GC.Collect() truly reduce the probability of it running anytime soon? Is this the right approach to do this using the .net framework? Does GC.Collect() collect for other CLR programs too or is it only applicable to the current process?
Jared's answer is of course excellent. To add a few other points:
Can running GC.Collect() truly reduce the probability of it running anytime soon?
Yes.
Is this the right approach to do this using the .net framework?
I would not want the correctness of software that has human-life-safety-critical functions to depend on probabalistic guesses about the undocumented behaviour of the collector.
That said: you should do a WaitForPendingFinalizers after the collection. Remember, finalizers run on their own thread, and that might be taking up processor time.
Does GC.Collect() collect for other CLR programs too or is it only applicable to the current process?
Collecting cleans up managed memory in the current process, so if you have multiple appdomains in one process, collecting in one appdomain collects in the others. It does not go cross process.
It is definitely possible that running a GC.Collect will reduce the probability that it happens during the code that immediately follows. There are many definitive scenarios you could draw up under which it would have this exact behavior. For example if the very next allocation would cause a collect and the GC.Collect freed at least the amount of memory allocated during the scenario.
However it's something I would never rely on being true. There is absolutely no guarantee that this will be the case. In fact it's possible that the collection wouldn't occur during the scenario and the time to run the forced GC.Collect could exceed the scenario time itself.
The only way to guarantee that GC.Collect won't run during a given scenario is to
Don't allocate any memory
Don't let anyone else call GC.Collect
Extremely hard to guarantee.
what you could possibly do, is specify to disable concurrent/background garbage collection through app.config:
Concurrent Garbage Collection on MSDN
I have an application that creates trees of nodes, then tosses them and makes new trees.
The application allocates about 20 MB upon startup. But I tried loading a large file of nodes many times, and the allocated memory went above 700 MB. I thought I would see memory being freed by the garbage collector on occasion.
The machine I'm on has 12 GB of RAM, so maybe it's just that such a "small" amount of memory being allocated doesn't matter to the GC.
I've found a lot of good info on how the GC works, and that it's best not to tell it what to do. But I wanted to verify that it's actually doing anything, and that I'm not somehow doing something wrong in the code that prevents my objects from being cleaned up.
The GC generally runs when either of the scenarios below occur:
You call GC.Collect (which you shouldn't)
Gen0's budget is exhausted
There are some other scenarios as well, but I'll skip those for now.
You didn't tell us how you measured the memory usage, but if you're looking at the memory usage of process itself (e.g. through task manager), then you may not see the numbers you expect. Remember that the .NET runtime essentially has its own memory manager that handles memory usage on behalf of you managed application. The runtime tries to be smart about it so it doesn't allocate and free memory to the OS all the time (those are expensive operations). This question may be relevant as well.
If you're concerned about memory leaks have a look at some of the answers here.
When does the .Net 3.5 garbage collector run?
I thought I would see memory being freed by the garbage collector on occasion.
Since the GC is non-deterministic, you won't be able to necessarily determine when it is going to issue a collection. Short answer: It will run when needed. Trying to analyze your code and predict or assume it should be running at a certain time usually ends up down a rabbit hole.
Answer to: do I leak objects or GC have not need to run yet?
Use memory profiler to see what objects are allocated. As basic step - force garbage collection (GC.Collect) and check out if allocated memory (GC.GetTotalMemory) seems to be reasonable.
If you want to make sure that you're not leaving any unwanted object behind you can use dotTrace memory profiler. It allows you to take two snapshots of objects in memory (taken some time apart) and compare them. You will can clearly see if any old nodes are still hanging around and what is keeping a reference to them and stops them from being collected.
You may have the managed equivalent of a memory leak. Are you maintaining stale references to these objects (i.e., do you have a List<T> or some other object which tracks these nodes)?
Are the subscribing to an event of an object that is not going out of scope? A reference to the subscribee of an event is maintained, so if you don't detach it will keep your objects alive.
You may also be forgetting to Dispose of objects that implement IDisposable. Can't say without seeing your code.
The exact behavior of the GC is implementation defined. You should design your application such that it does not matter. If you need deterministic memory management then you are using the wrong language. Use a tool (RedGate's ANTS profiler will do) to see if you are leaking references somewhere.
This comic is the best way to explain this topic :)
You can monitor the garbage collector activity in .NET 4.0 and beyond with GC.RegisterForFullGCNotification as described in this link: http://www.abhisheksur.com/2010/08/garbage-collection-notifications-in-net.html
I need to dispose of an object so it can release everything it owns, but it doesn't implement the IDisposable so I can't use it in a using block. How can I make the garbage collector collect it?
You can force a collection with GC.Collect(). Be very careful using this, since a full collection can take some time. The best-practice is to just let the GC determine when the best time to collect is.
Does the object contain unmanaged resources but does not implement IDisposable? If so, it's a bug.
If it doesn't, it shouldn't matter if it gets released right away, the garbage collector should do the right thing.
If it "owns" anything other than memory, you need to fix the object to use IDisposable. If it's not an object you control this is something worth picking a different vendor over, because it speaks to the core of how well your vendor really understands .Net.
If it does just own memory, even a lot of it, all you have to do is make sure the object goes out of scope. Don't call GC.Collect() — it's one of those things that if you have to ask, you shouldn't do it.
You can't perform garbage collection on a single object. You could request a garbage collection by calling GC.Collect() but this will effect all objects subject to cleanup. It is also highly discouraged as it can have a negative effect on the performance of later collections.
Also, calling Dispose on an object does not clean up it's memory. It only allows the object to remove references to unmanaged resources. For example, calling Dispose on a StreamWriter closes the stream and releases the Windows file handle. The memory for the object on the managed heap does not get reclaimed until a subsequent garbage collection.
Chris Sells also discussed this on .NET Rocks. I think it was during his first appearance but the subject might have been revisited in later interviews.
http://www.dotnetrocks.com/default.aspx?showNum=10
This article by Francesco Balena is also a good reference:
When and How to Use Dispose and Finalize in C#
http://www.devx.com/dotnet/Article/33167/0/page/1
Garbage collection in .NET is non deterministic, meaning you can't really control when it happens. You can suggest, but that doesn't mean it will listen.
Tells us a little bit more about the object and why you want to do this. We can make some suggestions based off of that. Code always helps. And depending on the object, there might be a Close method or something similar. Maybe the useage is to call that. If there is no Close or Dispose type of method, you probably don't want to rely on that object, as you will probably get memory leaks if in fact it does contain resourses which will need to be released.
If the object goes out of scope and it have no external references it will be collected rather fast (likely on the next collection).
BEWARE: of f ra gm enta tion in many cases, GC.Collect() or some IDisposal is not very helpful, especially for large objects (LOH is for objects ~80kb+, performs no compaction and is subject to high levels of fragmentation for many common use cases) which will then lead to out of memory (OOM) issues even with potentially hundreds of MB free. As time marches on, things get bigger, though perhaps not this size (80 something kb) for LOH relegated objects, high degrees of parallelism exasperates this issue due simply due to more objects in less time (and likely varying in size) being instantiated/released.
Array’s are the usual suspects for this problem (it’s also often hard to identify due to non-specific exceptions and assertions from the runtime, something like “high % of large object heap fragmentation” would be swell), the prognosis for code suffering from this problem is to implement an aggressive re-use strategy.
A class in Systems.Collections.Concurrent.ObjectPool from the parallel extensions beta1 samples helps (unfortunately there is not a simple ubiquitous pattern which I have seen, like maybe some attached property/extension methods?), it is simple enough to drop in or re-implement for most projects, you assign a generator Func<> and use Get/Put helper methods to re-use your previous object’s and forgo usual garbage collection. It is usually sufficient to focus on array’s and not the individual array elements.
It would be nice if .NET 4 updated all of the .ToArray() methods everywhere to include .ToArray(T target).
Getting the hang of using SOS/windbg (.loadby sos mscoreei for CLRv4) to analyze this class of issue can help. Thinking about it, the current garbage collection system is more like garbage re-cycling (using the same physical memory again), ObjectPool is analogous to garbage re-using. If anybody remembers the 3 R’s, reducing your memory use is a good idea too, for performance sakes ;)
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Closed 10 years ago.
What are some tips I can use to avoid memory leaks in my applications? Are there any gotchas or pitfalls that I can look out for?
Call Dispose on IDisposable objects or use the using clause. That should take care of most of the leaks I can think of.
Watch that you remove any event handlers that you use. In .NET, they are the most common cause of leaked memory.
As mentioned by ocdecio be sure to call Dispose on Idisposable objects, and remember to remove event handlers when you're done with an object. When building classes that works with unmanaged resources, be sure to implement Idisposable, so the user will know that there are critical resources that'll need to be disposed of.
Also, even though garbage collections do quite a bit a work for you, you should get rid of references to objects that you're done with. Else they'll still have a root, and they won't be GC'ed.
Don't underestimate the helpfulness of tools in these situations. The .NET memory profilers are fairly mature and robust nowadays, and if you have a complicated application where an object that you think should be freed is still held as a reference by something else the ability to pinpoint that reference is invaluable.
I've just finished hunting down a memory leak where a WPF tab page hosted a Windows Form control (since these tabs held a lot of data and you could open and close them at will simply waiting until the GC cleared the memory on close was not an option). I used the YourKit profiler to take a snapshot of the memory before the tab was opened, opened a tab, closed it again and took another snapshot. Inside the profiler you could visually compare the two snapshots and see what objects had survived the process and follow their references back to the GC root. I have no experience with other profilers, but I know there are a few out there if YourKit doesn't fulfil your needs.
Edited to add:
Okay, this isn't avoiding memory leaks, it's fixing them. But I'll leave it here since I think it is useful information and I don't think that enough .NET developers know about these tools.
I know some people are going to advise garbage collection as the solution. But there are lots of cases where garbage collection doesn't give you the results that you expect. It is easy to end up holding on to a stray reference which prevents whole chains of memory from being freed. Read about how this torpedoed a DARPA Grand Challenge entry. You can argue these aren't memory leaks but if the program expects that memory to be freed it is still a problem. Just like in C programming, after a couple of months you get the hang of how to make sure you don't leave unwanted references behind.
Memory leaks are bugs, so in general - the question could be answered the same as "how to code without bugs"? In the long run - it is not possible to have no bugs, but you can limit the chance for having those in the released code.
Start with caring about developed code quality and following the guidelines mentioned by others.
Simplicity is golden - the more simple the code - the less a chance for bugs or leaks
Be careful when using unmanaged resources - Windows Handles, DB connections, GDI objects, etc.
Use using for IDisposables
Implement IDisposable for classes that carry unmanaged resources
Make sure to remove any references to unused objects - including the tricky event handlers
On top of these - implement tests to see if the memory leaks - unit, concurrency, stress and load tests could help here most. See if memory leaks by checking metrics (perf counters). You could also log object creations and destructions and analyze the logs at the end of a test.
I've ran into issues where an object (Ping) implemented Dispose() twice, by implementing the IDisposable interface and inheriting it at the same time. The inherited method did nothing, and as a result you had to cast the object to IDisposable when calling Dispose() or it would leak memory. Here's a post I wrote a few years ago with more detail.
Wrap anything which is disposable in a using construct.
Avoid COM objects.
Check to see that all event hanlders are being removed properly.
Check to see that all data bindings are being removed properly.
Keep it simple
If your application logic is getting needlessly complex, you might start ending up with memory leaks. If you keep your classes small and follow general coding practices you probably won't run into any memory leaks with managed code. It is possible, but not as likely as it use to be.
If you suspect a memory leak, use a profiler to see if any objects are being kept around longer than needed.
The last time I ran into a serious memory leak was .NET 1.1, it turned out there was a bug in the framework.
Having a basic understanding of how the garbage collector works will help you avoid abusing memory. For example, if you are keeping a reference to an object you no longer need, the gc won't be able to collect it.
Along the same lines, if you're storing data the the user enters, or data that is added over time, you should consider some kind of limitations so that your memory usage doesn't grow indefinitely.
Most memory leaks that I have encountered in .NET has been related to using COM objects and not releasing them properly. As soon as I see a reference to a COM object, I think "memory leak".
The most common case of memory not being destroyed by the GC is when you have event handlers that did not get unhooked properly. You can unhook in Dispose() if you want.
I describe the problem more in detail, and I have a way to write tests to determine if you leak the object here.
As others have said call Dispose() (or use a using statement), but additionally consider whether classes use resources and if so implement IDisposeable. This is most often the problem in my code is that I have a class with member that doesn't get cleaned up till a GC.
It's managed code, that c#, so you have to try hard to leak memory :P
Try google:
http://www.google.com/search?hl=en&client=firefox-a&rls=org.mozilla%3Aen-US%3Aofficial&hs=Mbp&q=c%23+memory+leaks&btnG=Search
Types which implement a finalizer may leak, if any one finalizer blocks for some reason. I have seen finalizers block due to locking and thread apartment issues.
As instances of types with finalizers are not collected until their respective finalizers have run a single blocking finalizer will block any other finalizable objects pending collection.
First, let me bring share my strict understanding of a memory leak. My definition of a memory leak is when you have memory you have allocated and no longer a reference to it, so it is not possible to free that memory. Having said that, it is impossible to have a memory leak in .net objects (instances of CTS types that live in the managed heap, I mean). Unreferenced memory is precisely what the GC looks for to free.
Having said that, one can have a more lax understanding of what a memory leak is. If you consider a memory leak to be a uncontrolled growth of the memory being used, well, that is very easy. Just make a big misuse of static variables, mostly ones that reference huge lists. If you keep those objects referenced, the GC will never clean them, promoting them to higher generations and making them even harder to collect. Even though this is not a memory leak in the strict sense, in the end of the day it can lead to similar symptoms. A good way to try to detect this kind of "leak" is to use the CLR Profiler.
Another source of "leaks" is through improper use of event handlers as previously stated. Every time that object A registers one of its instance methods with an event at object B, object B ends keeping an indirect reference to object A, which means that while B is alive, A will be kept alive. Please note however, that there is no circularity here. As soon as neither B or A have any root reference, no matter how many cross references they have, they will eventually be collected.
Finally, one can actually induce a memory leak in .net, but never (at least theoretically) when talking about managed memory, as the GC does an excellent job in clearing that. If any of your objects maintains a reference to unmanaged memory, for instance through interop, then that memory needs to be explicitly cleaned. Failing to do so can lead to a memory leak, indeed. Even though I've never experienced such a scenario, at least in theory this can happen. As previously stated, objects that hold such references ought to implement IDiposable in order to clear that memory and their usage should guarantee Dispose is invoked for that purpose, mainly through the usage of the using clause. Notice that Dispose will not free the object's memory, but will only ask the object to release any unmanaged memory it is referring to.
One special kind of unmanaged memory is the one needed by COM objects used in interop scenarios. These objects are accessed through Runtime Callable Wrappers, RCWs for friends, but have no Dispose. "Using" will not work. The way to release the underlying COM objects is through the static method:
System.Runtime.InteropServices.Marshal.ReleaseComObject(object);
Since "using" is only syntax sugar to call IDisposable.Dispose() in a finally block, it is not usable with RCWs, hence don't forget to place the call in ReleaseComObject(object) in a finally block yourself so you ensure it is really called.
Use "using" keyword to automatically call Dispose() method of IDisposable object.
For any COM interop you have to manually release all resources.
You may find my new article useful: How to detect and avoid memory and resources leaks in .NET applications