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I'm learning D from 8 years in C++. My question is with regards to D garbage collection - when do I use delete, and when don't I need to?
You don't. Delete is not to be used with D version 2 and intended to be removed from the language. What the hold up is, I am not sure. Instead you use a function, destroy(object), which calls the destructor where you can free resources that are not GC memory. The destructor will be caused again during GC collection of the objects own memory. This is explained in "The D Programming Language".
The idea is to reclaim resources earlier than what the GC would provide and prevents memory corruption from dangling pointers. To be less safe the core.memory module provides GC.free(object) which can be used to free the memory, after calling destroy(object).
As I'm not a C++ programmer, I don't really know the RAII pattern, but this and reference counting is the expected strategy if you wish to avoid the GC.
Take a look at Garbage Collection in the D documentation. As noted, it is (almost) never necessary to explicitly manage memory. Of course after spending a handful of bullet points trying to convince you of the GC's power, they include a few scenarios where garbage collection falls short. To address these short-falls (they call them constraints), Digital Mars offers tips for Memory Management.
If possible, let the D garbage collector do its thing. Ignore explicit memory management. In a few very specific scenarios, there's a potential for an unacceptable GC pause or memory that can't be reclaimed. If your app includes one of these scenarios (test and profile to prove it), isolate the cause of the problem and explicitly manage memory where necessary. D lets you start as an optimist. If things don't work out perfectly, it's reassuring that you can fall back on explicit memory management.
I was reading the C# entry on Wikipedia, and came across:
Managed memory cannot be explicitly freed; instead, it is automatically garbage collected.
Why is it that in languages with automatic memory management, manual management isn't even allowed? I can see that in most cases it wouldn't be necessary, but wouldn't it come in handy where you are tight on memory and don't want to rely on the GC being smart?
Languages with automatic memory management are designed to provide substantial memory safety guarantees that can't be offered in the presence of any manual memory management.
Among the problems prevented are
Double free()s
Calling free() on a pointer to memory that you do not own, leading to illegal access in other places
Calling free() on a pointer that was not the return value of an allocation function, such as taking the address of some object on the stack or in the middle of an array or other allocation.
Dereferencing a pointer to memory that has already been free()d
Additionally, automatic management can result in better performance when the GC moves live objects to a consolidated area. This improves locality of reference and hence cache performance.
Garbage collection enforces the type safety of a memory allocator by guaranteeing that memory allocations never alias. That is, if a piece of memory is currently being viewed as a type T, the memory allocator can guarantee (with garbage collection) that while that reference is alive, it will always refer to a T. More specifically, it means that the memory allocator will never return that memory as a different type.
Now, if a memory allocator allows for manual free() and uses garbage collection, it must ensure that the memory you free()'d is not referenced by anyone else; in other words, that the reference you pass in to free() is the only reference to that memory. Most of the time this is prohibitively expensive to do given an arbitrary call to free(), so most memory allocators that use garbage collection do not allow for it.
That isn't to say it is not possible; if you could express a single-referrent type, you could manage it manually. But at that point it would be easier to either stop using a GC language or simply not worry about it.
Calling GC.Collect is almost always the better than having an explicit free method. Calling free would make sense only for pointers/object refs that are referenced from nowhere. That is something that is error prone, since there is a chance that your call free for the wrong kind of pointer.
When the runtime environment does reference counting monitoring for you, it knows which pointers can be freed safely, and which not, so letting the GC decide which memory can be freed avoids a hole class of ugly bugs. One could think of a runtime implementation with both GC and free where explicitly calling free for a single memory block might be much faster than running a complete GC.Collect (but don't expect freeing every possible memory block "by hand" to be faster than the GC). But I think the designers of C#, CLI (and other languages with garbage collectors like Java) have decided to favor robustness and safety over speed here.
In systems that allow objects to be manually freed, the allocation routines have to search through a list of freed memory areas to find some free memory. In a garbage-collection-based system, any immediately-available free memory is going to be at the end of the heap. It's generally faster and easier for the system to ignore unused areas of memory in the middle of the heap than it would be to try to allocate them.
Interestingly enough, you do have access to the garbage collector through System.GC -- Though from everything I've read, it's highly recommended that you allow the GC manage itself.
I was advised once to use the following 2 lines by a 3rd party vendor to deal with a garbage collection issue with a DLL or COM object or some-such:
// Force garbage collection (cleanup event objects from previous run.)
GC.Collect(); // Force an immediate garbage collection of all generations
GC.GetTotalMemory(true);
That said, I wouldn't bother with System.GC unless I knew exactly what was going on under the hood. In this case, the 3rd party vendor's advice "fixed" the problem that I was dealing with regarding their code. But I can't help but wonder if this was actually a workaround for their broken code...
If you are in situation that you "don't want to rely on the GC being smart" then most probably you picked framework for your task incorrectly. In .net you can manipulate GC a bit (http://msdn.microsoft.com/library/system.gc.aspx), in Java not sure.
I think you can't call free because you start doing one task of GC. GC's efficiency can be somehow guaranteed overall when it does things the way it finds it best and it does them when it decides. If developers will interfere with GC it might decrease it's overall efficiency.
I can't say that it is the answer, but one that comes to mind is that if you can free, you can accidentally double free a pointer/reference or even worse, use one after free. Which defeats the main point of using languages like c#/java/etc.
Of course one possible solution to that, would be to have your free take it's argument by reference and set it to null after freeing. But then, what if they pass an r-value like this: free(whatever()). I suppose you could have an overload for r-value versions, but don't even know if c# supports such a thing :-P.
In the end, even that would be insufficient because as has been pointed out, you can have multiple references to the same object. Setting one to null would do nothing to prevent the others from accessing the now deallocated object.
Many of the other answers provide good explanations of how the GC work and how you should think when programming against a runtime system which provides a GC.
I would like to add a trick that I try to keep in mind when programming in GC'd languages. The rule is this "It is important to drop pointers as soon as possible." By dropping pointers I mean that I no longer point to objects that I no longer will use. For instance, this can be done in some languages by setting a variable to Null. This can be seen as a hint to the garbage collector that it is fine to collect this object, provided there are no other pointers to it.
Why would you want to use free()? Suppose you have a large chunk of memory you want to deallocate.
One way to do it is to call the garbage collector, or let it run when the system wants. In that case, if the large chunk of memory can't be accessed, it will be deallocated. (Modern garbage collectors are pretty smart.) That means that, if it wasn't deallocated, it could still be accessed.
Therefore, if you can get rid of it with free() but not the garbage collector, something still can access that chunk (and not through a weak pointer if the language has the concept), which means that you're left with the language's equivalent of a dangling pointer.
The language can defend itself against double-frees or trying to free unallocated memory, but the only way it can avoid dangling pointers is by abolishing free(), or modifying its meaning so it no longer has a use.
Why is it that in languages with automatic memory management, manual management isn't even allowed? I can see that in most cases it wouldn't be necessary, but wouldn't it come in handy where you are tight on memory and don't want to rely on the GC being smart?
In the vast majority of garbage collected languages and VMs it does not make sense to offer a free function although you can almost always use the FFI to allocate and free unmanaged memory outside the managed VM if you want to.
There are two main reasons why free is absent from garbage collected languages:
Memory safety.
No pointers.
Regarding memory safety, one of the main motivations behind automatic memory management is eliminating the class of bugs caused by incorrect manual memory management. For example, with manual memory management calling free with the same pointer twice or with an incorrect pointer can corrupt the memory manager's own data structures and cause non-deterministic crashes later in the program (when the memory manager next reaches its corrupted data). This cannot happen with automatic memory management but exposing free would open up this can of worms again.
Regarding pointers, the free function releases a block of allocated memory at a location specified by a pointer back to the memory manager. Garbage collected languages and VMs replace pointers with a more abstract concept called references. Most production GCs are moving which means the high-level code holds a reference to a value or object but the underlying location in memory can change as the VM is capable of moving allocated blocks of memory around without the high-level language knowing. This is used to compact the heap, preventing fragmentation and improving locality.
So there are good reasons not to have free when you have a GC.
Manual management is allowed. For example, in Ruby calling GC.start will free everything that can be freed, though you can't free things individually.
I'd like to know the best alternative for a garbage collector, with its pros and cons. My priority is speed, memory is less important. If there is garbage collector which doesn't make any pause, let me know.
I'm working on a safe language (i.e. a language with no dangling pointers, checking bounds, etc), and garbage collection or its alternative has to be used.
I suspect you will be best sticking with garbage collection (as per the JVM) unless you have a very good reason otherwise. Modern GCs are extremely fast, general purpose and safe. Unless you can design your language to take advantage of a very specific special case (as in one of the above allocators) then you are unlikely to beat the JVM.
The only really compelling reason I see nowadays as an argument against modern GC is latency issues caused by GC pauses. These are small, rare and not really an issue for most purposes (e.g. I've successfully written 3D engines in Java), but they still can cause problems in very tight realtime situations.
Having said that, there may still be some special cases where a different memory allocation scheme may make sense so I've listed a few interesting options below:
An example of a very fast, specialised memory management approach is the "per frame" allocator used in many games. This works by incrementing a single pointer to allocate memory, and at the end of a time period (typically a visual "frame") all objects are discarded at once by simply setting the pointer back to the base address and overwriting them in the next allocation. This can be "safe", however the constraints of object lifetime would be very strict. Might be a winner if you can guarantee that all memory allocation is bounded in size and only valid for the scope of handling e.g. a single server request.
Another very fast approach is to have dedicated object pools for different classes of object. Released objects can just be recycled in the pool, using something like a linked list of free object slots. Operating systems often used this kind of approach for common data structures. Again however you need to watch object lifetime and explicitly handle disposals by returning objects to the pool.
Reference counting looks superficially good but usually doesn't make sense because you frequently have to dereference and update the count on two objects whenever you change a pointer value. This cost is usually worse than the advantage of having simple and fast memory management, and it also doesn't work in the presence of cyclic references.
Stack allocation is extremely fast and can run safely. Depending on your language, it is possible to make do without a heap and run entirely on a stack based system. However I suspect this will somewhat constrain your language design so that might be a non-starter. Still might be worth considering for certain DSLs.
Classic malloc/free is pretty fast and can be made safe if you have sufficient constraints on object creation and lifetime which you may be able to enforce in your language. An example would be if e.g. you placed significant constraints on the use of pointers.
Anyway - hope this is useful food for thought!
If speed matters but memory does not, then the fastest and simplest allocation strategy is to never free. Allocation is simply a matter of bumping a pointer up. You cannot get faster than that.
Of course, never releasing anything has a huge potential for overflowing available memory. It is very rare that memory is truly "unimportant". Usually there is a large but finite amount of available memory. One strategy is called "region based allocation". Namely you allocate memory in a few big blocks called "regions", with the pointer-bumping strategy. Release occurs only by whole regions. This strategy can be applied with some success if the problem at hand can be structured into successive "tasks", each having its own region.
For more generic solutions, if you want real-time allocation (i.e. guaranteed limits on the response time from allocation requests) then garbage collection is the way to go. A real-time GC may look like this: objects are allocated with a pointer-bumping strategy. Also, on every allocation, the allocator performs a little bit of garbage collection, in which "live" objects are copied somewhere else. In a way the GC runs "at the same time" than the application. This implies a bit of extra work for accessing objects, because you cannot move an object and update all pointers to point to the new object location while keeping the "real-time" promise. Solutions may imply barriers, e.g. an extra indirection. Generational GC allow for barrier-free access to most objects while keeping pause times under strict bounds.
This article is a must-read for whoever wants to study memory allocation, in particular garbage collection.
With C++ it's possible to make a heap allocation ONCE for your objects, then reuse that memory for subsequent objects, I've seen it work and it was blindingly fast.
It's only applicable to a certian set of problems, and it's difficult to do it right, but it is possible.
One of the joys of C++ is you have complete control over memory management, you can decide to use classic new/delete, or implement your own reference counting or Garbage Collection.
However - here be dragons - you really, really need to know what you're doing.
If memory doesn't matter, then what #Thomas says applies. Considering the gargantuan memory spaces of modern hardware, this may very well be a viable option -- it really depends on the process.
Manual memory management doesn't necessarily solve your problems directly, but it does give you complete control over WHEN memory events happen. Generic malloc, for example, is not an O(1) operation. It does all sorts of potentially horrible things in there, both within the heap managed by malloc itself as well as the operating system. For example, ya never know when "malloc(10)" may cause the VM to page something out, now your 10 bytes of RAM have an unknown disk I/O component -- oops! Even worse, that page out could be YOUR memory, which you'll need to immediately page back in! Now c = *p is a disk hit. YAY!
But if you are aware of these, then you can safely set up your code so that all of the time critical parts effectively do NO memory management, instead they work off of pre-allocated structures for the task.
With a GC system, you may have a similar option -- it depends on the collector. I don't think the Sun JVM, for example, has the ability to be "turned off" for short periods of time. But if you work with pre-allocated structures, and call all of your own code (or know exactly what's going on in the library routine you call), you probably have a good chance of not hitting the memory manager.
Because, the crux of the matter is that memory management is a lot of work. If you want to get rid of memory management, the write old school FORTRAN with ARRAYs and COMMON blocks (one of the reasons FORTRAN can be so fast). Of course, you can write "FORTRAN" in most any language.
With modern languages, modern GCs, etc., memory management has been pushed aside and become a "10%" problem. We are now pretty sloppy with creating garbage, copying memory, etc. etc., because the GCs et al make it easy for us to be sloppy. And for 90% of the programs, this is not an issue, so we don't worry about. Nowadays, it's a tuning issue, late in the process.
So, your best bet is set it all up at once, use it, then toss it all away. The "use it" part is where you will get consistent, reliable results (assuming enough memory on the system of course).
As an "alternative" to garbage collection, C++ specifically has smart pointers. boost::shared_ptr<> (or std::tr1::shared_ptr<>) works exactly like Python's reference counted garbage collection. In my eyes, shared_ptr IS garbage collection. (although you may need to do a few weak_ptr<> stuff to make sure that circular references don't happen)
I would argue that auto_ptr<> (or in C++0x, the unique_ptr<>...) is a viable alternative, with its own set of benefits and tradeoffs. Auto_ptr has a clunky syntax and can't be used in STL containers... but it gets the job done. During compile-time, you "move" the ownership of the pointer from variable to variable. If a variable owns the pointer when it goes out of scope, it will call its destructor and free the memory. Only one auto_ptr<> (or unique_ptr<>) is allowed to own the real pointer. (at least, if you use it correctly).
As another alternative, you can store everything on the stack and just pass references around to all the functions you need.
These alternatives don't really solve the general memory management problem that garbage collection solves. Nonetheless, they are efficient and well tested. An auto_ptr doesn't use any more space than the pointer did originally... and there is no overhead on dereferencing an auto_ptr. "Movement" (or assignment in Auto_ptr) has a tiny amount of overhead to keep track of the owner. I haven't done any benchmarks, but I'm pretty sure they're faster than garbage collection / shared_ptr.
If you truly want no pauses at all, disallow all memory allocation except for stack allocation, region-based buffers, and static allocation. Despite what you may have been told, malloc() can actually cause severe pauses if the free list becomes fragmented, and if you often find yourself building massive object graphs, naive manual free can and will lose to stop-and-copy; the only way to really avoid this is to amortize over preallocated pages, such as the stack or a bump-allocated pool that's freed all at once. I don't know how useful this is, but I know that the proprietary graphical programming language LabVIEW by default allocates a static region of memory for each subroutine-equivalent, requiring programmers to manually enable stack allocation; this is the kind of thing that's useful in a hard-real-time environment where you need absolute guarantees on memory usage.
If what you want is to make it easy to reason about pauses and give your developers control over allocation and placement, then there is already a language called Rust that has the same stated goals as your language; while not a completely safe language, it does have a safe subset, allowing you to create safe abstractions for raw bit-twiddling. It uses pointer type annotations to eliminate use-after-free bugs. It also doesn't have null pointers in safe code, because null pointers cost a billion dollars at least.
If bounded pauses are enough, though, there are a wide variety of algorithms that will work. If you really have a small working set compared to available memory, then I would recommend the MOS collector (aka the Train Algorithm), which collects incrementally and provably always makes progress toward freeing unreferenced objects.
It's a common fallacy that managed languages are not suitable for high performance low latency scenarios. Yes, with limited resources (such as an embedded platform) and sloppy programming you can shoot yourself in the foot just as spectacularly as with C++ (and that can be VERY VERY spectacular).
This problem has come whilst developing games in Java/C# and the solution was to utilise a memory pool and not let object die, hence not needing garbage collector to run when you don't expect it. This is really the same approach as with low latency unmanaged systems - TO TRY REALLY REALLY HARD NOT TO ALLOCATE MEMORY.
So, considering the fact that implementing such system in Java/C# is very similar to C++, the advantage of doing it the girly man way(managed), you have the "niceness" of other language features that free up your mental clock cycles to concentrate on important things.
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Garbage collection has been around since the early days of LISP, and now - several decades on - most modern programming languages utilize it.
Assuming that you're using one of these languages, what reasons would you have to not use garbage collection, and instead manually manage the memory allocations in some way?
Have you ever had to do this?
Please give solid examples if possible.
I can think of a few:
Deterministic deallocation/cleanup
Real time systems
Not giving up half the memory or processor time - depending on the algorithm
Faster memory alloc/dealloc and application-specific allocation, deallocation and management of memory. Basically writing your own memory stuff - typically for performance sensitive apps. This can be done where the behavior of the application is fairly well understood. For general purpose GC (like for Java and C#) this is not possible.
EDIT
That said, GC has certainly been good for much of the community. It allows us to focus more on the problem domain rather than nifty programming tricks or patterns. I'm still an "unmanaged" C++ developer though. Good practices and tools help in that case.
Memory allocations? No, I think the GC is better at it than I am.
But scarce resource allocations, like file handles, database connections, etc.? I write the code to close those when I'm done. GC won't do that for you.
I do a lot of embedded development, where the question is more likely to be whether to use malloc or static allocation and garbage collection is not an option.
I also write a lot of PC-based support tools and will happily use GC where it is available & fast enough and it means that I don't have to use pedant::std::string.
I write a lot of compression & encryption code and GC performance is usually not good enough unless I really bend the implementation. GC also requires you to be very careful with address aliasing tricks. I normally write performance sensitive code in C and call it from Python / C# front ends.
So my answer is that there are reasons to avoid GC, but the reason is almost always performance and it's then best to code the stuff that needs it in another language rather than trying to trick the GC.
If I develop something in MSVC++, I never use garbage collection. Partly because it is non-standard, but also because I've grown up without GC in C++ and automatically design in safe memory reclamation. Having said this, I think that C++ is an abomination which fails to offer the translation transparency and predictability of C or the scoped memory safety (amongst other things) of later OO languages.
Real time applications are probably difficult to write with a garbage collector. Maybe with an incremental GC that works in another thread, but this is an additional overhead.
One case I can think of is when you are dealing with large data sets amounting to hundreads of megabytes or more. Depending on the situation you might want to free this memory as soon as you are done with it, so that other applications can use it.
Also, when dealing with some unmanaged code there might be a situation where you might want to prevent the GC from collecting some data because it's still being used by the unmanaged part. Though I still have to think of a good reason why simply keeping a reference to it might not be good enough. :P
One situation I've dealt with is image processing. While working on an algorithm for cropping images, I've found that managed libraries just aren't fast enough to cut it on large images or on multiple images at a time.
The only way to do processing on an image at a reasonable speed was to use non-managed code in my situation. This was while working on a small personal side-project in C# .NET where I didn't want to learn a third-party library because of the size of the project and because I wanted to learn it to better myself. There may have been an existing third-party library (perhaps Paint.NET) that could do it, but it still would require unmanaged code.
Two words: Space Hardening
I know its an extreme case, but still applicable. One of the coding standards that applied to the core of the Mars rovers actually forbid dynamic memory allocation. While this is indeed extreme, it illustrates a "deploy and forget about it with no worries" ideal.
In short, have some sense as to what your code is actually doing to someone's computer. If you do, and you are conservative .. then let the memory fairy take care of the rest. While you develop on a quad core, your user might be on something much older, with much less memory to spare.
Use garbage collection as a safety net, be aware of what you allocate.
There are two major types of real time systems, hard and soft. The main distinction is that hard real time systems require that an algorithm always finish in a particular time budget where as a soft system would like it to normally happen. Soft systems can potentially use well designed garbage collectors although a normal one would not be acceptable. However if a hard real time system algorithm did not complete in time then lives could be in danger. You will find such sorts of systems in nuclear reactors, aeroplanes and space shuttles and even then only in the specialist software that the operating systems and drivers are made of. Suffice to say this is not your common programming job.
People who write these systems don't tend to use general purpose programming languages. Ada was designed for the purpose of writing these sorts of real time systems. Despite being a special language for such systems in some systems the language is cut down further to a subset known as Spark. Spark is a special safety critical subset of the Ada language and one of the features it does not allow is the creation of a new object. The new keyword for objects is totally banned for its potential to run out of memory and its variable execution time. Indeed all memory access in Spark is done with absolute memory locations or stack variables and no new allocations on the heap is made. A garbage collector is not only totally useless but harmful to the guaranteed execution time.
These sorts of systems are not exactly common, but where they exist some very special programming techniques are required and guaranteed execution times are critical.
Just about all of these answers come down to performance and control. One angle I haven't seen in earlier posts is that skipping GC gives your application more predictable cache behavior in two ways.
In certain cache sensitive applications, having the language automatically trash your cache every once in a while (although this depends on the implementation) can be a problem.
Although GC is orthogonal to allocation, most implementations give you less control over the specifics. A lot of high performance code has data structures tuned for caches, and implementing stuff like cache-oblivious algorithms requires more fine grained control over memory layout. Although conceptually there's no reason GC would be incompatible with manually specifying memory layout, I can't think of a popular implementation that lets you do so.
Assuming that you're using one of these languages, what reasons would you have to not use garbage collection, and instead manually manage the memory allocations in some way?
Potentially, several possible reasons:
Program latency due to the garbage collector is unacceptably high.
Delay before recycling is unacceptably long, e.g. allocating a big array on .NET puts it in the Large Object Heap (LOH) which is infrequently collected so it will hang around for a while after it has become unreachable.
Other overheads related to garbage collection are unacceptably high, e.g. the write barrier.
The characteristics of the garbage collector are unnacceptable, e.g. redoubling arrays on .NET fragments the Large Object Heap (LOH) causing out of memory when 32-bit address space is exhausted even though there is theoretically plenty of free space. In OCaml (and probably most GC'd languages), functions with deep thread stacks run asymptotically slower. Also in OCaml, threads are prevented from running in parallel by a global lock on the GC so (in theory) parallelism can be achieved by dropping to C and using manual memory management.
Have you ever had to do this?
No, I have never had to do that. I have done it for fun. For example, I wrote a garbage collector in F# (a .NET language) and, in order to make my timings representative, I adopted an allocationless style in order to avoid GC latency. In production code, I have had to optimize my programs using knowledge of how the garbage collector works but I have never even had to circumvent it from within .NET, much less drop .NET entirely because it imposes a GC.
The nearest I have come to dropping garbage collection was dropping the OCaml language itself because its GC impedes parallelism. However, I ended up migrating to F# which is a .NET language and, consequently, inherits the CLR's excellent multicore-capable GC.
I don't quite understand the question. Since you ask about a language that uses GC, I assume you are asking for examples like
Deliberately hang on to a reference even when I know it's dead, maybe to reuse the object to satisfy a future allocation request.
Keep track of some objects and close them explicitly, because they hold resources that can't easily be managed with the garbage collector (open file descriptors, windows on the screen, that sort of thing).
I've never found a reason to do #1, but #2 is one that comes along occasionally. Many garbage collectors offer mechanisms for finalization, which is an action that you bind to an object and the system runs that action before the object is reclaimed. But oftentimes the system provides no guarantees about whether or if finalizers actually run, so finalization can be of limited utility.
The main thing I do in a garbage-collected language is to keep a tight watch on the number of allocations per unit of other work I do. Allocation is usually the performance bottleneck, especially in Java or .NET systems. It is less of an issue in languages like ML, Haskell, or LISP, which are typically designed with the idea that the program is going to allocate like crazy.
EDIT: longer response to comment.
Not everyone understands that when it comes to performance, the allocator and the GC must be considered as a team. In a state-of-the-art system, allocation is done from contiguous free space (the 'nursery') and is as quick as test and increment. But unless the object allocated is incredibly short-lived, the object incurs a debt down the line: it has to be copied out of the nursery, and if it lives a while, it may be copied through several generatations. The best systems use contiguous free space for allocation and at some point switch from copying to mark/sweep or mark/scan/compact for older objects. So if you're very picky, you can get away with ignoring allocations if
You know you are dealing with a state-of-the art system that allocates from continuous free space (a nursery).
The objects you allocate are very short-lived (less than one allocation cycle in the nursery).
Otherwise, allocated objects may be cheap initially, but they represent work that has to be done later. Even if the cost of the allocation itself is a test and increment, reducing allocations is still the best way to improve performance. I have tuned dozens of ML programs using state-of-the-art allocators and collectors and this is still true; even with the very best technology, memory management is a common performance bottleneck.
And you'd be surprised how many allocators don't deal well even with very short-lived objects. I just got a big speedup from Lua 5.1.4 (probably the fastest of the scripting language, with a generational GC) by replacing a sequence of 30 substitutions, each of which allocated a fresh copy of a large expression, with a simultaneous substitution of 30 names, which allocated one copy of the large expression instead of 30. Performance problem disappeared.
In video games, you don't want to run the garbage collector in between a game frame.
For example, the Big Bad is in front
of you and you are down to 10 life.
You decided to run towards the Quad
Damage powerup. As soon as you pick up
the powerup, you prepare yourself to
turn towards your enemy to fire with
your strongest weapon.
When the powerup disappeared, it would
be a bad idea to run the garbage
collector just because the game world
has to delete the data for the
powerup.
Video games usually manages their objects by figuring out what is needed in a certain map (this is why it takes a while to load maps with a lot of objects). Some game engines would call the garbage collector after certain events (after saving, when the engine detects there's no threat in the vicinity, etc).
Other than video games, I don't find any good reasons to turn off garbage collecting.
Edit: After reading the other comments, I realized that embedded systems and Space Hardening (Bill's and tinkertim's comments, respectively) are also good reasons to turn off the garbage collector
The more critical the execution, the more you want to postpone garbage collection, but the longer you postpone garbage collection, the more of a problem it will eventually be.
Use the context to determine the need:
1.
Garbage collection is supposed to protect against memory leaks
Do you need more state than you can manage in your head?
2.
Returning memory by destroying objects with no references can be unpredictable
Do you need more pointers than you can manage in your head?
3.
Resource starvation can be caused by garbage collection
Do you have more CPU and memory than you can manage in your head?
4.
Garbage collection cannot address files and sockets
Do you have I/O as your primary concern?
In systems that use garbage collection, weak pointers are sometimes used to implement a simple caching mechanism because objects with no strong references are deallocated only when memory pressure triggers garbage collection. However, with ARC, values are deallocated as soon as their last strong reference is removed, making weak references unsuitable for such a purpose.
References
GC FAQ
Smart Pointer Guidelines
Transitioning to ARC Release Notes
Accurate Garbage Collection with LLVM
Memory management in various languages
jwz on Garbage Collection
Apple Could Power the Web
How Do The Script Garbage Collectors Work?
Minimize Garbage Generation: GC is your Friend, not your Servant
Garbage Collection in IE6
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I'm designing a high level language, and I want it to have the speed of C++ (it will use LLVM), but be safe and high level like C#. Garbage collection is slow, and new/delete is unsafe. I decided to try to use "region based memory management" (there are a few papers about it on the web, mostly for functional languages). The only "useful" language using it is Cyclone, but that also has GC. Basically, objects are allocated on a lexical stack, and are freed when the block closes. Objects can only refer to other objects in the same region or higher, to prevent dangling references. To make this more flexible, I added parallel regions that can be moved up and down the stack, and retained through loops. The type system would be able to verify assignments in most cases, but low overhead runtime checks would be necessary in some places.
Ex:
region(A) {
Foo#A x=new Foo(); //x is deleted when this region closes.
region(B,C) while(x.Y) {
Bar#B n=new Bar();
n.D=x; //OK, n is in lower region than x.
//x.D=n; would cause error: x is in higher region than n.
n.DoSomething();
Bar#C m=new Bar();
//m.D=n; would cause error: m and n are parallel.
if(m.Y)
retain(C); //On the next iteration, m is retained.
}
}
Does this seem practical? Would I need to add non-lexically scoped, reference counted regions? Would I need to add weak variables that can refer to any object, but with a check on region deletion? Can you think of any algorithms that would be hard to use with this system or that would leak?
I would discourage you from trying regions. The problem is that in order to make regions guaranteed to be safe, you need a very sophisticated type system---I'm sure you've looked at the papers by Tofte and Talpin and you have an idea of the complexities involved. Even if you do get regions working successfully, the chances are very hight that your program will require a whose lifetime is the lifetime of the program---and that region at least has to be garbage collected. (This is why Cyclone has regions and GC.)
Since you're just getting started, I'd encourage you to go with garbage collection. Modern garbage collectors can be made pretty fast without a lot of effort. The main issue is to allocate from contiguous free space so that allocation is fast. It helps to be targeting AMD64 or other machine with spare registers so you can use a hardware register as the allocation pointer.
There are lots of good ideas to adapt; one of the easiest to implement is a page-based collector like Joel Bartlett's mostly-copying collector, where the idea is you allocate only from completely empty pages.
If you want to study existing garbage collectors, Lua has a fairly sophisticated incremental garbage collector (so there are no visible pause times) and the implementation is only 700 lines. It is fast enough to be used in a lot of games, where performance matters.
If I were implementing a language with region based memory management, I would probably read A language-independent framework for region inference. That said, it's been a while since I looked into this stuff, and I'm sure the state of the art has moved on, if I ever even knew what the state of the art was.
Well you should go study Apples memory management. It has release pools and zones, which sure sound a lot like what you're doing here.
I won't comment on the "GC is slow" remark,
You can start by Tofte and Talpin's papers about region-based memory management.
How would it return a dynamically created object? Who would "own" it and be responsible for freeing the memory?
Refcounting or GC are so common because they are almost always the best choices. Generational garbage collectors can be very efficient.