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Author
: Thomas Kurian Ambattu |
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A
word for beginners: - This article is for beginners who
want to know the mechanism of Garbage Collection in. NET.
I suggest that people who read this have a basic understanding
of C & C++ .I’m trying to explain the mechanism
of GC without the influence of C & C++ but as I explain
some points I can’t avoid at least some of the basics
of C & C++ like pointers, memory allocation in heap
etc. Any way I’ll try my best to explain this mechanism
in the simplest way so that it will be easy for a beginner
to understand.
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One
of the important advantage perhaps I would say the most
attractive feature of .NET is the concept of Garbage Collection.
Now the question is what is this Garbage Collection? Garbage
Collection is an automatic memory management mechanism
in .NET. This mechanism helps to free the memory automatically
when they are no longer used by the application. The advantage
is that developers need not worry about the memory management
when writing the code i.e. the GC will do it for you as
and when needed. I think C & C++ programmers will
understand the above said point better than anybody else.
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A
little about C, C++: - In C & C++ if we allocate the
memory using new operator we have to free it by ourselves.
There is no mechanism that will automatically free the
memory that is allocated. So people often make two common
mistakes, they either forgets to free the memory allocated
or try to use the memory they already freed. The consequences
of these two bugs will be very severe and the results
will be unpredictable. This is called a resource leaks
or memory leaks in application which will be a headache
to programmers.
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Again
back to our topic, The Common Language Runtime of .NET
requires all the resources to be allocated from the managed
heap. When a program is loaded into the memory the runtime
reserves a region of address space for that application.
This reserved memory space is called Managed Heap. This
space initially will have no storage allocated. This space
is only used when an object of this particular program
is loaded into the memory. So now the question is, how
is the object inserted in the heap? The answer is not
that easy, In order to indicate where the object is to
be inserted the heap always maintains a pointer. The heap
maintains this pointer in the position just below the
last object inserted. Initially this pointer is set to
the base address of the reserved space region.
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Managed
Heap
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Now
let us see how the object is created and allocated in
the heap. An application creates an object with the new
operator. The operator checks for whether the space required
by the objects meets the reserved space. If it meets the
space then the object is inserted at the address location
where the pointer is points. Now the pointer changes the
location where it points. The pointer is shifted to the
location just below the newly inserted object. The figure
below shows the details of this
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The
Garbage Collection Process
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Consider
a situation where an application creates an object and
there is not enough space in the heap to allocate for
this situation. The heap has a mechanism by which it checks
whether there is enough space for the object to be allocated.
The heap checks this by adding the size of the object
to our NextObjPointer. If the NextObjPointer is beyond
the reserved address space region, then the heap is full
and so it should make a collection to free some memory.
So the GC now travels through the heap and find outs the
objects that are no longer used by the application and
those dead objects are cleared to free the memory and
the free space is created in the heap. Although I said
the process in a simple way the real situation is not
that easy it’s a little complicated. Actually a
collection occurs when the Generation 0 is full. The Generation
is a mechanism by the Garbage Collector to improve the
performance, another thing is the garbage collection is
performed not in a simple manner as what I said above,
it is actually done on the basis of an algorithm.
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Hope
you all got an idea about the Garbage Collection in .NET.
I’ll discuss about generations & Garbage Collection
Algorithm in my next articles Garbage
Collection Internals I II & III. Do
mail me your feedback on this article to feedback@netkidoos.com.
All suggestions & views will be appreciated. |
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