What's the difference between memory allocation and garbage collection, please? - memory-management

I understand that 'Garbage Collection' is a form of memory management and that it's a way to automatically reclaim unused memory.
But what is 'memory allocation' and the conceptual difference from 'Garbage Collection'?

They are Polar opposites. So yeah, pretty big difference.
Allocating memory is the process of claiming a memory space to store things.
Garbage Collection (or freeing of memory) is the process of releasing that memory back to the pool of available memory.
Many newer languages perform both of these steps in the background for you when variables are declared/initialized, and fall out of scope.

Memory allocation is the act of asking for some memory to the system to use it for something.
Garbage collection is a process to check if some memory that was previously allocated is no longer really in use (i.e. is no longer accessible from the program) to free it automatically.
A subtle point is that the objective of garbage collection is not actually "freeing objects that are no longer used", but to emulate a machine with infinite memory, allowing you to continue to allocate memory and not caring about deallocating it; for this reason, it's not a substitute for the management of other kind resources (e.g. file handles, database connections, ...).

A simple pseudo-code example:
void myFoo()
{
LinkedList<int> myList = new LinkedList<int>();
return;
}
This will request enough new space on the heap to store the LinkedList object.
However, when the function body is over, myList dissapears and you do not have anymore anyway of knowing where this LinkedList is stored (the memory address). Hence, there is absolutely no way to tell to the system to free that memory, and make it available to you again later.
The Java Garbage Collector will do that for you automatically, in the cost of some performance, and with also introducing a little non-determinism (you cannot really tell when the GC will be called).
In C++ there is no native garbage collector (yet?). But the correct way of managing memory is by the use of smart_pointers (eg. std::auto_ptr (deprecated in C++11), std::shared_ptr) etc etc.

You want a book. You go to the library and request the book you want. The library checks to see if they have the book (in which case they do) and you gladly take it and know you must return it later.
You go home, sit down, read the book and finish it. You return the book back to the library the next day because you are finished with it.
That is a simple analogy for memory allocation and garbage collection. Computers have limited memory, just like libraries have limited copies of books. When you want to allocate memory you need to make a request and if the computer has sufficient memory (the library has enough copies for you) then what you receive is a chunk of memory. Computers need memory for storing data.
Since computers have limited memory, you need to return the memory otherwise you will run out (just like if no one returned the books to the library then the library would have nothing, the computer will explode and burn furiously before your very eyes if it runs out of memory... not really). Garbage collection is the term for checking whether memory that has been previously allocated is no longer in use so it can be returned and reused for other purposes.

Memory allocation asks the computer for some memory, in order to store data. For example, in C++:
int* myInts = new int[howManyIntsIWant];
tells the computer to allocate me enough memory to store some number of integers.
Another way of doing the same thing would be:
int myInts[6];
The difference here is that in the second example, we know when the code is written and compiled exactly how much space we need - it's 6 * the size of one int. This lets us do static memory allocation (which uses memory on what's called the "stack").
In the first example we don't know how much space is needed when the code is compiled, we only know it when the program is running and we have the value of howManyIntsIWant. This is dynamic memory allocation, which gets memory on the "heap".
Now, with static allocation we don't need to tell the computer when we're finished with the memory. This relates to how the stack works; the short version is that once we've left the function where we created that static array, the memory is swallowed up straight away.
With dynamic allocation, this doesn't happen so the memory has to be cleaned up some other way. In some languages, you have to write the code to deallocate this memory, in other it's done automatically. This is garbage collection - some automatic process built into the language that will sweep through all of the dynamically allocated memory on the heap, work out which bits aren't being used and deallocate them (i.e. free them up for other processes and programs).
So: memory allocation = asking for memory for your program. Garbage collection = where the programming language itself works out what memory isn't being used any more and deallocates it for you.

Related

How to release memory allocated by gcnew?

After some tests with help of Task Manager, I understood one thing about gcnew — memory allocated for local variables remaines allocated even if control leaves function, and is re-allocated only when control re-enters this function — so I'm in perplexity, how to deallocate memory myself. Here is some example of the problem:
void Foo(void)
{
System::Text::StringBuilder ^ t = gcnew System::Text::StringBuilder("");
int i = 0;
while(++i < 20000000) t->Append(i);
return;
}
As I mentioned, memory for variable t remains after leaving Foo(), delete not work as it works for new, and calling Foo() once, only gives me pointless allocated memory.
This is gcnew, which means garbage collected allocation. It will be disposed and deallocated by GC thread
Your function uses memory for code and data. The code is a fixed amount and will be used the entire time the library or program is loaded. The data is only used when the function is executing.
Data used by a program is either static or dynamic. Static data is laid out by the compiler and is basically equivalent to code (except that it might be marked as non-executable and/or read-only to prevent accidents). Dynamic data is temporary and allocated from a stack or heap (or CPU registers).
In a classic program, the stack and heap share the same memory address range with the stack at one end, growing toward the heap and the heap at the other end, trying not to grow into the stack. However, with modern address ranges on the order of 1TB, a heap generally has a lot of room.
Keep in mind that when a program requests an address range, it's just signaling to the operating system that it's okay to use that address for data reading, data writing and/or code execution. Until it actually puts something there, there is no load on the system. Also keep in mind with a virtual memory system, process memory is effectively allocated on the swap file/device (hard drive) with optimizations especially using RAM for caching, copy on write and many other techniques. (Data written to a memory address might never make it to the swap file, but that's up to the operating system.)
The data your function needs is for the two variables: t and i. t is a reference to a garbage collected object. i is an integer. Both are quite small and short-lived. You could think of them as being on the stack. When the function returns, the stack frame is popped and their memory is reused by the next stack operation. If you are looking at memory allocation, there won't be a change because the amount of memory allocated to the stack would not be changed.
Now in the execution of your function, a new object is created and, the way it's filled with data, it takes up quite a bit of memory. You could consider that object to be created in the heap. You don't need to delete it since it is a garbage collection object. When the garbage collector runs by walking all objects reachable from a set of root objects, it will find that the object is not reachable and add its space to a free list. When space for a new object is needed that doesn't fit into any blocks on the free list, more of the heap's address range will be used.
The CLR heap is compactable, which means it can move objects around in order to coalesce free blocks. Using this ability, it can move objects out of areas of allocated memory and give it back to the operating system, thereby freeing up space in the swap file.
So, there are three things that have to happen for you to see a reduction in the amount of memory allocated to the process:
The garbage collection has run to find unreachable objects.
The heap has been compacted.
The heap allocation has been reduced.
None of these things are really necessary until the swap file can't grow anymore. Obviously, the system has been designed for performance and to be a good citizen so it wouldn't take it that far. You can influence when garbage collection runs but this is only very rarely helpful and is generally not done.

Does free() free the memory immediately

In my program, I am using malloc to allocate large amounts of memory (several hundred mbs, in chunks of say 25mb to 75mb at a time), I am subsequently freeing some of the chunks, then again reallocating some more. My question is when I use free() to free memory, does it immediately free the concerned block of memory, or it merely marks it for freeing. If it is merely marking for freeing later, is there some standard C library function to force it to be freed immediately.
I am actually required to develop my program to be portable between linux and vxworks. In Vxworks, in one library I am using(vsipl) , I find 'free' is not freeing up, immediately on the call.
The answer depends upon malloc and free implementation. However, I can safely say that in nearly any implementation the memory does not get deallocated. Instead it goes back into a pool where it can be reused by the process.
If you are using large blocks of memory, it is usually better to use operating system memory functions and do your own memory management. However, it is not a good idea to map pages in and out of memory.

What does freeing memory mean? Does it mean setting all bits to zeros?

I directly started with managed languages and have barely any experience with C++, hence this question might be too basic.
In a managed language like .net, GC frees the memory. From what I read, in C++ this is done by calling delete. But what does it do to free memory? Does it it set all the bits at a memory location to zero? Or does it in some other way tell the operating system that the memory is available for reuse?
Update:
I have been thru this before and I know what GC does. But thats not my question. I am not trying to ask how GC works. What I am trying to understand is, how do you tell some memory is free?
delete does three different things:
Runs the destructor of the object (or of all objects in the array in the case of delete[]).
Marks the chunk of memory previously used by the object as free.
If possible, informs the operating system that a chunk of memory is free for other programs to use.
Your question is about #2 and #3 together, but they are very different things. To understand how #2 works, remember that the (typically) single "heap" provided by the operating system is segmented into smaller chunks of different sizes. When you allocate a chunk of memory with new, you get a pointer to a previously free part of the heap, and the runtime performs the necessary bookkeeping that marks that region as unavailable for further allocations. delete does the reverse: it performs the bookkeeping that marks the region as available again, optionally coalescing it with adjacent free regions to reduce fragmentation. Subsequent calls to new will consider that region when looking for free memory to return.
In other words, it is wrong to ask what happens with the memory when it is deallocated. The real magic happens in the bookkeeping region! To learn about implementation of generic allocators, google for implementation of malloc.
As for #3, it is an optional step and in many cases impossible to perform. It is only possible to "give back" freed memory that happens to reside at the very end of the allocated heap. A single allocation situated after a large region will remove the possibility of giving back.
In C++, if you allocate memory using "new", that portion of memory will be allocated by the OS to that particular process, until you release that memory or until that process exits.
If some portion of memory allocated for a process means that OS does not allow other process to use that portion of memory until that process release that memory. In C++, you have to use "delete" to release memory.
By releasing memory portion, process just inform the OS that it does not use this portion any more so that OS can allocate that memory portion whenever other processes request memory. In that case content of the memory portion will not be changed.
Garbage collection is just automatic memory management (so you never need to delete anything, the system will take care of it for you). I'm not 100% on whether it sets memory locations to 0, but I would assume it doesnt, since when you delete in c++, thats not what happens, it just allows the space to be used for storage. Writing zeros over everything is much more inefficient and not necessary. Here's some links that might be able to help explain this more thoroughly:
How does garbage collection and scoping work in C#?
http://en.wikipedia.org/wiki/Garbage_collection_(computer_science)
Inside each application, dynamic memory are managed by "heaps". When your code asks for a block of memory, it asks the heap manager to allocate a block of memory, when your application frees that block of memory, it returns it back to the heap manager. In a traditional application, you must explicitly return each memory you allocated. Otherwise you will eventually run out of memory.
In languages like C# or Java, the runtime offers a garbage collector. A garbage collector automatically identify "unreachable" memory block and free them. An unreable memory block is a block that is no longer referenced by any variables. For example, if you have a global variable p1 that points to a block of memory, because p1 is global, so it is visible to anywhere in your code, then it is always reachable. Thus it will never be released by garbage collector. On the other hand, if you have local variable p2 in one of your function Foo, vairable p2 is no longer reachable after Foo has returned. The garbage collector is able to identify such variables and free any memory block pointed by them.
As application/garbage collector interact with the heap, the heap may decide to ask for more memory from the OS or return it to the OS. The OS manages all these memory request from different process and it then decide how to allocate the actual physical memory to different process.
No, it does not set the bits to zero. In a very simplified explanation,
First the garbage collector must determine, not what objects are no longer accessible ("not reachable"), but which ones are still accessible or reachable. It does this by simply listing all object roots. A root is a memory location containing a pointer to a reference object (an object on the heap). Then, recursively, it flags as "reachable" every object referenced by a root, or referenced by a field or property of a object already flagged as reachable.
There are four types of roots.
static variables containing reference objects
reference objects on the stack for any currently active thread.
reference types in method parameters
reference objects pointed to by CPU registers.
After determining what reference objects are still accessible (reachable) by any code in the App Domain, it takes all those objects that are still reachable, and if there are any gaps in physical memory between them, it "defragments" them by moving some of them so they are all contiguous, then it sets the pointer which represents the "end" od "used" memory to the end of this new compressed defragmented list. All new memory allocations, for newly instantiated objects, are then allocated from the memory immediately after this pointer location.
If there are no gaps in the memory used by the reachable objects, it just resets the pointer to the end of the last reachable object in the list.
No, deleting a pointer does not set the bytes to zero.
It's not in the standard of course, but it would be a performance overhead and serious implementations don't bother doing it, and it does not even make sense, when the memory is used for complex objects (floats, objects, strings, etc)
You can always try it out.
Declare a pointer to an int, write an integer, delete the pointer.
Then read the content of the deleted pointer again.
Does it have the same content?
int *ptr = new int;
*ptr = 13;
cout << "Before delete: " << *ptr << endl;
delete ptr;
cout << "After delete: " << *ptr << endl;
Yes probably it will, BUT ptr is just a dangling pointer you have there, the memory has been returned to the system and it will be available when you allocate memory again, it's likely that when you allocate another int* it will be pointing where ptr was pointing.

Why is free() not allowed in garbage-collected languages?

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.

Does calling free or delete ever release memory back to the "system"

Here's my question: Does calling free or delete ever release memory back to the "system". By system I mean, does it ever reduce the data segment of the process?
Let's consider the memory allocator on Linux, i.e ptmalloc.
From what I know (please correct me if I am wrong), ptmalloc maintains a free list of memory blocks and when a request for memory allocation comes, it tries to allocate a memory block from this free list (I know, the allocator is much more complex than that but I am just putting it in simple words). If, however, it fails, it gets the memory from the system using say sbrk or brk system calls. When a memory is free'd, that block is placed in the free list.
Now consider this scenario, on peak load, a lot of objects have been allocated on heap. Now when the load decreases, the objects are free'd. So my question is: Once the object is free'd will the allocator do some calculations to find whether it should just keep this object in the free list or depending upon the current size of the free list it may decide to give that memory back to the system i.e decrease the data segment of the process using sbrk or brk?
Documentation of glibc tells me that if the allocation request is much larger than page size, it will be allocated using mmap and will be directly released back to the system once free'd. Cool. But let's say I never ask for allocation of size greater than say 50 bytes and I ask a lot of such 50 byte objects on peak load on the system. Then what?
From what I know (correct me please), a memory allocated with malloc will never be released back to the system ever until the process ends i.e. the allocator will simply keep it in the free list if I free it. But the question that is troubling me is then, if I use a tool to see the memory usage of my process (I am using pmap on Linux, what do you guys use?), it should always show the memory used at peak load (as the memory is never given back to the system, except when allocated using mmap)? That is memory used by the process should never ever decrease(except the stack memory)? Is it?
I know I am missing something, so please shed some light on all this.
Experts, please clear my concepts regarding this. I will be grateful. I hope I was able to explain my question.
There isn't much overhead for malloc, so you are unlikely to achieve any run-time savings. There is, however, a good reason to implement an allocator on top of malloc, and that is to be able to trace memory leaks. For example, you can free all memory allocated by the program when it exits, and then check to see if your memory allocator calls balance (i.e. same number of calls to allocate/deallocate).
For your specific implementation, there is no reason to free() since the malloc won't release to system memory and so it will only release memory back to your own allocator.
Another reason for using a custom allocator is that you may be allocating many objects of the same size (i.e you have some data structure that you are allocating a lot). You may want to maintain a separate free list for this type of object, and free/allocate only from this special list. The advantage of this is that it will avoid memory fragmentation.
No.
It's actually a bad strategy for a number of reasons, so it doesn't happen --except-- as you note, there can be an exception for large allocations that can be directly made in pages.
It increases internal fragmentation and therefore can actually waste memory. (You can only return aligned pages to the OS, so pulling aligned pages out of a block will usually create two guaranteed-to-be-small blocks --smaller than a page, anyway-- to either side of the block. If this happens a lot you end up with the same total amount of usefully-allocated memory plus lots of useless small blocks.)
A kernel call is required, and kernel calls are slow, so it would slow down the program. It's much faster to just throw the block back into the heap.
Almost every program will either converge on a steady-state memory footprint or it will have an increasing footprint until exit. (Or, until near-exit.) Therefore, all the extra processing needed by a page-return mechanism would be completely wasted.
It is entirely implementation dependent. On Windows VC++ programs can return memory back to the system if the corresponding memory pages contain only free'd blocks.
I think that you have all the information you need to answer your own question. pmap shows the memory that is currenly being used by the process. So, if you call pmap before the process achieves peak memory, then no it will not show peak memory. if you call pmap just before the process exits, then it will show peak memory for a process that does not use mmap. If the process uses mmap, then if you call pmap at the point where maximum memory is being used, it will show peak memory usage, but this point may not be at the end of the process (it could occur anywhere).
This applies only to your current system (i.e. based on the documentation you have provided for free and mmap and malloc) but as the previous poster has stated, behavior of these is implmentation dependent.
This varies a bit from implementation to implementation.
Think of your memory as a massive long block, when you allocate to it you take a bit out of your memory (labeled '1' below):
111
If I allocate more more memory with malloc it gets some from the system:
1112222
If I now free '1':
___2222
It won't be returned to the system, because two is in front of it (and memory is given as a continous block). However if the end of the memory is freed, then that memory is returned to the system. If I freed '2' instead of '1'. I would get:
111
the bit where '2' was would be returned to the system.
The main benefit of freeing memory is that that bit can then be reallocated, as opposed to getting more memory from the system. e.g:
33_2222
I believe that the memory allocator in glibc can return memory back to the system, but whether it will or not depends on your memory allocation patterns.
Let's say you do something like this:
void *pointers[10000];
for(i = 0; i < 10000; i++)
pointers[i] = malloc(1024);
for(i = 0; i < 9999; i++)
free(pointers[i]);
The only part of the heap that can be safely returned to the system is the "wilderness chunk", which is at the end of the heap. This can be returned to the system using another sbrk system call, and the glibc memory allocator will do that when the size of this last chunk exceeds some threshold.
The above program would make 10000 small allocations, but only free the first 9999 of them. The last one should (assuming nothing else has called malloc, which is unlikely) be sitting right at the end of the heap. This would prevent the allocator from returning any memory to the system at all.
If you were to free the remaining allocation, glibc's malloc implementation should be able to return most of the pages allocated back to the system.
If you're allocating and freeing small chunks of memory, a few of which are long-lived, you could end up in a situation where you have a large chunk of memory allocated from the system, but you're only using a tiny fraction of it.
Here are some "advantages" to never releasing memory back to the system:
Having already used a lot of memory makes it very likely you will do so again, and
when you release memory the OS has to do quite a bit of paperwork
when you need it again, your memory allocator has to re-initialise all its data structures in the region it just received
Freed memory that isn't needed gets paged out to disk where it doesn't actually make that much difference
Often, even if you free 90% of your memory, fragmentation means that very few pages can actually be released, so the effort required to look for empty pages isn't terribly well spent
Many memory managers can perform TRIM operations where they return entirely unused blocks of memory to the OS. However, as several posts here have mentioned, it's entirely implementation dependent.
But lets say I never ask for allocation of size greater than say 50 bytes and I ask a lot of such 50 byte objects on peak load on the system. Then what ?
This depends on your allocation pattern. Do you free ALL of the small allocations? If so and if the memory manager has handling for a small block allocations, then this may be possible. However, if you allocate many small items and then only free all but a few scattered items, you may fragment memory and make it impossible to TRIM blocks since each block will have only a few straggling allocations. In this case, you may want to use a different allocation scheme for the temporary allocations and the persistant ones so you can return the temporary allocations back to the OS.

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