Memory Handling and Garbage Collection in Python

Memory Handling and Garbage Collection in Python

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  Memory Handling & Garbage Collectionin Python Prepared by: Ms. Pooja Mehta ITSNS Branch, GTU-CDAC-BISAG ME Program, Gandhinagar 1
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  Memory Management 3 March2016By: Pooja Mehta 2  The process of binding values to memory locations  Whether done automatically (as in Python), or partially by the programmer (as in C/C++), dynamic memory management is an important part of programming language design.
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  Three Categories ofMemory (for Data Storage) 3 March 2016By: Pooja Mehta 3  Static: storage requirements are known prior to run time; lifetime is the entire program execution  Run-time stack: memory associated with active functions Structured as stack frames  Heap: dynamically allocated storage; the least organized and most dynamic storage area
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  Cont.. 3 March 2016By:Pooja Mehta 4
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  Cont.. 3 March 2016By:Pooja Mehta 5
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  Memory Leaks andGarbage Collection 3 March 2016By: Pooja Mehta 6  Three types of storage  Static  Stack  Heap  Problems with heap storage:  Memory leaks (garbage): failure to free storage when pointers (references) are reassigned  Dangling pointers: when storage is freed, but references to the storage still exist.
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  Garbage 3 March 2016By:Pooja Mehta 7  Any block of heap memory that cannot be accessed by the program; i.e., there is no stack pointer to the block; but which the runtime system thinks is in use.  Garbage is created in several ways:  A function ends without returning the space allocated to a local array or other dynamic variable.  A node is deleted from a linked data structure, but isn’t freed
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  Garbage Collection 3 March2016By: Pooja Mehta 8  All inaccessible blocks of storage are identified and returned to the free list.  The heap may also be compacted at this time: allocated space is compressed into one end of the heap, leaving all free space in a large block at the other end.
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  Implementing Automated Garbage Collection 3March 2016By: Pooja Mehta 9  If programmers were perfect, garbage collection wouldn’t be needed.  There are three major approaches to automating the process:  Reference counting  Mark-sweep  Copy collection
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  Reference Counting 10  Priorto Python version 2.0, the Python interpreter only used reference counting for memory management.  Reference counting works by counting the number of times an object is referenced by other objects in the system.  When references to an object are removed, the reference count for an object is decremented.  When the reference count becomes zero the object is de allocated. 3 March 2016By: Pooja Mehta
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  Mark and Sweep 3March 2016By: Pooja Mehta 11  Reference counting tries to find unreachable objects by finding objects with no incoming references.  Imprecise because we forget which references those are.
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  Cont.. 3 March 2016By:Pooja Mehta 12  The root set is the set of memory locations in the program that are known to be reachable.  Any objects reachable from the root set are reachable.  Any objects not reachable from the root set are not reachable.
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  Cont.. 3 March 2016By:Pooja Mehta 13 Marking phase: Find reachable objects.  Add the root set to a worklist.  While the worklist isn't empty:  Remove an object from the worklist.  If it is not marked, mark it and add to the worklist all objects reachable from that object. Sweeping phase: Reclaim free memory.  For each allocated object:  If that object isn't marked, reclaim its memory.  If the object is marked, unmark it (so on the next mark-and sweep iteration we have to mark it again).
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  Copy Collection 3 March2016By: Pooja Mehta 14  Divide heap into two “semispaces”.  Allocate from one space (fromspace) till full.  Copy live data into other space (tospace).  Switch roles of the spaces.  Requires fixing pointers to moved data (forwarding).  Eliminates fragmentation.  DFS improves locality, while BFS does not require any extra storage.
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  References 15  https://docs.python.org/2/c-api/memory.html  http://foobarnbaz.com/2012/07/08/understanding- python-variables/ https://www.digi.com/wiki/developer/index.php/Pyt hon_Garbage_Collection 3 March 2016By: Pooja Mehta
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  16 Thank you...!