Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West .Net Tecnology unit II

Garbage collection Algo. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Garbage collection Algo. The basic goal of garbage collection is to look for objects that are not being used by an application. To be more technical, objects that are not referenced by code within the application can be cleaned up. During the cleaned up process : * An object’s finalize method is called. Finalize method is written by the developer and provides object specific cleanup code.

Garbage collection Algo. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Garbage collection Algo. Finalized method conti… The memory associated with the object is reclaimed. The trick to garbage collection is to determine which objects are not referenced. To determine this, the garbage collection algo traverses active objects within an application. To determine what objects are active or referenced following rules are used :

Garbage collection Algo. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Garbage collection Algo. All object contained on a thread’s stack are referenced, as are the objects they contain. All global objects and the object they contain are referenced. Static object also considered to be global and are hence part of this root. All objects referred to by the cpu’s registers are referenced, as are the objects they contain.

Garbage collection Algo. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Garbage collection Algo. When the garbage collection process an object, it makes sure that the object has not already been added to the referenced list. An object that was already traversed is skipped. It is more efficient to skip an object that is already referenced. This also prevents loops. If such object were not skipped, a set of objects implementing a circular queue would prove “infinitely” dangerous.

Garbage collection Algo. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Garbage collection Algo. Once every referenced object has been added to the graph, the garbage collection algo. Walks up the managed heap (from bottom to top). Regions in the heap that contained non referenced objects are discarded and referenced objects are moved down in the heap. An invalid pointer now refers to each object that is moved down the heap. The garbage collection algorithm now traverse the active objects and update the pointers contained in each reference contained in each reference to an object that has been moved.

Garbage collection Algo. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Garbage collection Algo. The final step is to reset the next object pointer. This is the point at which the next object is allocated .

Managed Heap Organization.. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Managed Heap Organization.. Automatic memory management is one of the services that the common language runtime provides during managed execution. Clr’s GC manages the allocation and release of memory for an application. For developers, this means that they don’t have to write code to perform memory management tasks when you develop managed application. Automatic Memory management can eliminate common problems, such as forgetting to free an object and causing a memory leak or attempting to access memory for an object that has already been freed.

Managed Heap Organization.. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Managed Heap Organization.. The main task in memory managed heap organization is done by GC are : Allocating Memory : when new process is initialized, the runtime reserves a contiguous region of address space for the process. This reserved address space is known as managed heap.

Managed Heap Organization.. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Managed Heap Organization.. The managed heap maintains a pointer to the address where next object in the heap will be allocated. Initially this pointer is set to the managed heap’s base address. All ref. typed objects are allocated in heap. When next object is created by application, the GC allocates memory for it in the address space immediately following the 1st object. As long as space is available GC continues to allocate space for new objects in this manner.

Managed Heap Organization.. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Managed Heap Organization.. Allocating memory from the managed heap is faster than unmanaged memory allocation. 2) Releasing Memory : The GC’s optimizing engine determines the best time to perform a collection based on the allocations being made. When the garbage collector performs a collection, it releases the memory for the objects that are no longer being used by the application. Every application has a set of roots. Each root either refers to an object on the managed heap or is set to null.

Managed Heap Organization.. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Managed Heap Organization.. An application’s roots include global and static object pointers, local variables and referenced object parameters on a thread’s stack and cpu’s registers. The GC has access to the list of active roots that the JIT (Just-in-Time) and the runtime maintain. Using this list, it examines an application’s roots and in the process it creates a graph that contains all the objects that are reachable from the roots.

Managed Heap Organization.. Created By: Asst. Prof. Ashish Shah, J.M.Patel College, Goregoan West Managed Heap Organization.. Objects that are not in the graph are unreachable from the application’s root. The garbage collector considers unreachable objects garbage and will release the memory allocated for them.