From the book, G1 collector uses the primitive snapshot approach to solve the tri-color marker under-marking problem.
The G1 collector uses the TAMS pointer to solve the problem of storing newly created objects during concurrent collection, implicitly marking newly created objects as alive and not included in the collection.
What happens if an object that has been marked as black adds a reference to a white object that was created historically in the free state? (The black object has already been scanned by GC as a living object, the white object is historically created and is no longer in the GCRoots reference chain when it is scanned)
Related
I am debugging the my Cocoa application and noticed using the Instruments.app allocation profile that the model object graph is not deallocating as I expect. Basically when I remove a root model object from the my NSDocument I was expecting the whole object graph for that object to be deallocated. That doesn't happen, which means that there is a strong reference to my root model object somewhere else in the application.
Is it possible to get a list of object which have a reference to a specific object in Cocoa either programmatically or using Instruments.app? If I could know where the strong reference is held this would help with debugging this problem.
I found this similar question, How to get the reference count of an NSObject?, but this simply says which how many references there are, not which objects hold the references.
Not in this way. The only information that is stored inside of a running process is the number of retains on an object (number of times "retain" has been called net the number of times "release" has been called). This is not the same as the number of references (number of "strong" pointers to the memory). Most memory locations do not have a side table of things that point to them in Cocoa.
In Instruments, you can turn on "record reference counts" and see everywhere the retain count is increased or decreased. See Instruments Allocations track alloc and dealloc of objects of user defined classes for a good explanation of how to do that. This won't tell you where you've made your mistake, but it will tell you where the retains are occurring.
I'm learning Objectiv C, and I hear the term "live in the heap" constantly, from what I understand its some kind of unknown area that a pointer lives in, but trying to really put head around the exact term...like "we should make our property strong so it won't live in the heap. He said that since the property is private. I know it'ss a big difference It's pretty clear that we want to make sure that we want to count the reference to this object so the autorelease wont clean it (we want to "retain" it from what i know so far), but I want to make sure I understand the term since it's being use pretty often.
Appreciate it
There are three major memory areas used by C (and by extension, Objective C) programs for storing the data:
The static area
The automatic area (also known as "the stack"), and
The dynamic area (also known as "the heap").
When you allocate objects by sending their class a new or alloc message, the resultant object is allocated in the dynamic storage area, so the object is said to live in the heap. All Objective-C objects are like that (although the pointers that reference these objects may be in any of the three memory data areas). In contrast, primitive local variables and arrays "live" on the stack, while global primitive variables and arrays live in the static data storage.
Only the heap objects are reference counted, although you can allocate memory from the heap using malloc/calloc/realloc, in which case the allocation would not be reference-counted: your code would be responsible for deciding when to free the allocated dynamic memory.
Both are exactly the same thing, except that "abandoned memory" refers to a whole object graph leaked rather than just a single object. Right?
First, you need to understand the notion of a "memory object graph" or "application object graph" (or, simply, "object graph" as it applies to allocated buffers). In this case, "object" refers to any allocation in your application, be it an object or a simple malloc()ed buffer. The "graph" part if it is that any object can contain a reference to -- a pointer -- to other objects.
The "live object graph" of an application are all of the allocations that can be reached, directly or indirectly, from the various "roots" in the application. A "root" is something that, on its own, represents a live reference to an object, regardless of whether or not anything else explicitly references the root.
For example, global variables are roots; by referring to an object, a global variable, by definition, makes that object part of the app's live object graph. And, by implication, any objects that the object referred to by the global variable are also considered to be live; not leaked.
The same goes for the stack; any object referred to by any thread's live stack is, itself, considered live.
With this in mind, a leak and abandoned memory actually do have two distinct meanings.
Leak
A leak is a piece of memory for which there are no references to the allocation from any live object in the application's live object graph.
I.e. the memory is unreachable and, thus, there is no way that it can ever be referred to again (barring bugs). It is dead memory.
Note that if object A points to object B and object B points to A, but nothing in the live object graph points to either A or B, it is still a leak. If the B->A and A->B references are both retained references, you got yourself a retain cycle & a leak.
Abandoned Memory
Allocations that are in the app's live object graph but are no longer reachable due to application logic issues are considered abandoned, but not leaked.
For example, say you have a cache whose entries are instances of NSData that were downloaded from some URL where the URL contains a session ID in the URL (a common pattern) and that session ID + URL are used as the key to look up stuff in the cache. Now, say the user logs out, causing the session ID to be destroyed. If the cache isn't also pruned of all entries specific to that session ID, then all of those NSData objects will be abandoned, but not leaked as they can still be reached via the cache.
In reality, there is little use in making this strong of a distinction between the two save for that fixing either requires very different strategies.
Fixing a leak is to figure out where the extra retain came from (or where a missing call to free() might need to be inserted, in the case of a malloc() based leak). Since a detected leak cannot be reached from the live object graph, fixing a leak is really this straightforward.
Fixing abandoned memory can be considerably trickier for a couple of reasons.
First, the memory is still reachable from the live object graph. Thus, by definition, there is an algorithmic problem in your application that is keeping the memory alive. Finding and fixing that can often be much more difficult and potentially disruptive then fixing a mere leak.
Secondly, there might be non-zeroing non-retained weak references to the abandoned allocation. That is, if you figure out where to prune the strong references and make the allocation actually go away, that doesn't mean that your work is done; if there are any remaining non-zeroing weak references, they will now be dangling pointers and..... BOOM.
As Amit indicated, Heapshot Analysis is quite adept at finding both leaks, abandoned memory and -- quite important -- overall "Undesirable Memory Growth".
Not sure if there's a standard terminology, but there's also the possibility of having memory around which does have a reference, but will never be used. (The Heap Shot feature of the Leaks instrument can help track this down.) I call this "bloat" to distinguish it from a true leak. Both are wasted memory.
Abandoned memory are the memory leaks. Heapshot Analysis will help you to Find Undesirable Memory Growth. This is good article about that. http://www.friday.com/bbum/2010/10/17/when-is-a-leak-not-a-leak-using-heapshot-analysis-to-find-undesirable-memory-growth/
I turned on garbage collection (objective-c 2.0) and the image for the status item disappeared when I restarted my application. I am manually memory managing the image and status item.
This application works normal when garbage collection is off (i.e. the status item doesn't disappear.) Is there a way to make the garbage collection not collect specific variables or should I turn it off completely? When turning on garbage collection, my app's memory drops from 100mb to 2mb.
How are you "manually managing the memory of the image and status item"? If you are using release and retain, they are actually ignored under GC.
Under GC, objects stick around if (a) you have a strong reference to them in GC scanned memory or you (b) CFRetain them.
Just keep a reference to it as an instance variable of some object e.g. the controller of the view it appears in.
I want to cache the instances of a certain class. The class keeps a dictionary of all its instances and when somebody requests a new instance, the class tries to satisfy the request from the cache first. There is a small problem with memory management though: The dictionary cache retains the inserted objects, so that they never get deallocated. I do want them to get deallocated, so that I had to overload the release method and when the retain count drops to one, I can remove the instance from cache and let it get deallocated.
This works, but I am not comfortable mucking around the release method and find the solution overly complicated. I thought I could use some hashing class that does not retain the objects it stores. Is there such? The idea is that when the last user of a certain instance releases it, the instance would automatically disappear from the cache.
NSHashTable seems to be what I am looking for, but the documentation talks about “supporting weak relationships in a garbage-collected environment.” Does it also work without garbage collection?
Clarification: I cannot afford to keep the instances in memory unless somebody really needs them, that is why I want to purge the instance from the cache when the last “real” user releases it.
Better solution: This was on the iPhone, I wanted to cache some textures and on the other hand I wanted to free them from memory as soon as the last real holder released them. The easier way to code this is through another class (let’s call it TextureManager). This class manages the texture instances and caches them, so that subsequent calls for texture with the same name are served from the cache. There is no need to purge the cache immediately as the last user releases the texture. We can simply keep the texture cached in memory and when the device gets short on memory, we receive the low memory warning and can purge the cache. This is a better solution, because the caching stuff does not pollute the Texture class, we do not have to mess with release and there is even a higher chance for cache hits. The TextureManager can be abstracted into a ResourceManager, so that it can cache other data, not only textures.
Yes, you can use an NSHashTable to build what is essentially a non-retaining dictionary. Alternatively, you can call CFDictionaryCreate with NULL for release and retain callbacks. You can then simply typecast the result to a NSDictionary thanks to tollfree bridging, and use it just like a normal NSDictionary except for not fiddling with retain counts.
If you do this the dictionary will not automatically zero the reference, you will need to make sure to remove it when you dealloc an instance.
What you want is a zeroing weak reference (it's not a "Graal of cache managing algorithms", it's a well known pattern). The problem is that Objective C provides you with zeroing weak references only when running with garbage collection, not in manual memory managed programs. And the iPhone does not provide garbage collection (yet).
All the answers so far seem to point you to half-solutions.
Using a non-reataining reference is not sufficient because you will need to zero it out (or remove the entry from the dictionary) when the referenced object is deallocated. However this must be done BEFORE the -dealloc method of that object is called otherwise the very existence of the cache expose you to the risk that the object is resurrected. The way to do this is to dynamically subclass the object when you create the weak reference and, in the dynamically created subclass, override -release to use a lock and -dealloc to zero out the weak reference(s).
This works in general but it fails miserably for toll-free bridged Core Foundation objects. Unfortunately the only solution, if you need to to extend the technique to toll-free bridged objects, requires some hacking and undocumented stuff (see here for code and explanations) and is therefore not usable for iOS or programs that you want to sell on the Mac App Store.
If you need to sell on the Apple stores and must therefore avoid undocumented stuff, your best alternative is to implement locked access to a retaining cache and then scavenge it for references with a current -retainCount value of 1 when you want to release memory. As long as all accesses to the cache are done with the lock held, if you observe a count of 1 while holding the lock you know that there's no-one that can resurrect the object if you remove it from the cache (and therefore release it) before relinquishing the lock.
For iOS you can use UIApplicationDidReceiveMemoryWarningNotification to trigger the scavenging. On the mac you need to implement your own logic: maybe just a periodical check or even simply a periodical scavenging (both solutions would also work on iOS).
I've just implemented this kind of thing by using an NSMutableDictionary and registering for UIApplicationDidReceiveMemoryWarningNotification. On a memory warning I remove anything from the dictionary with a retainCount of 1...
Use [NSValue valueWithNonretainedObject:] to wrap the instance in an NSValue and put that in the dictionary. In the instance dealloc method, remove the corresponding entry from the dictionary. No messing with retain.
My understanding is that you want to implement the Graal of cache managing algorithms: drop items that will no longer be used.
You may want to consider other criteria, such as dropping the least recently requested items.
I think the way I would approach this is to maintain a separate count or a flag somewhere to indicate if the object in the cache is being used or not. You could then check this when you're done with an object, or just run a check every n seconds to see if it needs to be released or not.
I would avoid any solution involving releasing the object before removing it from the dictionary (using NSValue's valueWithNonretainedObject: would be another way to accomplish this). It would just cause you problems in the long run.