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Internals of CPython memory

Vlad Sydorenko, Senior Software Engineer, SoftServe

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Agenda

  • Prerequisites
    • Learn C in 2 minutes
    • Heap vs Stack
  • Object structure
    • Why number 5 consumes 28 bytes of memory
    • Immutable object structure in Python
    • Mutable object structure
  • Low-level memory management (memory allocation)

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Learn C in 2 minutes

Pointers

Variable, that stores some address in memory.

Example:

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Structures

Structure is data type available in C that allows to combine data items of different kinds.

Example:

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Macroses

A Macro is typically an abbreviated name given to a piece of code or a value.

Example:

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Heap vs Stack

Heap

  • Used for dynamic memory allocation.
  • All next slides will relate to it.

Stack

  • Used for static memory allocation

​

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Why number 5 consumes 28 bytes

Warning: all tests were ran on x64 system

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Why nothing(!) consumes 16 bytes

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Structure of object in Python

Source: https://www.youtube.com/watch?v=d7qEzpnkWaY

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Structure of object in Python

Include/object.h

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Structure of immutable object in Python

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Structure of int object in Python

Source: https://www.youtube.com/watch?v=d7qEzpnkWaY

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Structure of int object in Python

Include/object.h:112

Include/longintrepr.h:85

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Interesting fact about int

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Interesting fact about int

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Structure of mutable object in Python

Source: https://www.youtube.com/watch?v=d7qEzpnkWaY

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Structure of mutable object header in Python

Include/objimpl.h:252

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How to see size of PyGC_HEAD

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We need to go deeper

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Source: https://rushter.com/blog/python-memory-managment/

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Python memory allocators families

Source: https://docs.python.org/3/c-api/memory.html#c.PyMemAllocatorDomain

PYMEM_DOMAIN_RAW PEP445

Functions:

Thread-safe

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PYMEM_DOMAIN_MEM

Functions:

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PYMEM_DOMAIN_OBJ

Functions:

  • PyObject_Malloc()
  • PyObject_Realloc()
  • PyObject_Calloc()
  • PyObject_Free()

​

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PYMEM_DOMAIN_MEM… or not?

Objects/obmalloc.c

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Objects/obmalloc.c

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Objects/obmalloc.c

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Source: https://rushter.com/blog/python-memory-managment/

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Source: https://rushter.com/blog/python-memory-managment/

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Source: http://blogs.microsoft.co.il/terfin3/2016/01/14/python-small-memory-allocator/

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Pool header structure

Objects/pymalloc.h

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Useful links

  1. http://net-informations.com/faq/net/stack-heap.htm - about difference between stack and heap
  2. https://www.youtube.com/watch?v=XGF3Qu4dUqk - “Stepping through CPython”, Larry Hastings
  3. https://www.youtube.com/watch?v=d7qEzpnkWaY - “The memory chronicles”, Kavya Joshi
  4. https://www.youtube.com/watch?v=F6u5rhUQ6dU - “Memory management in Python”, Nina Zakharenko
  5. https://docs.python.org/3/c-api/memory.html - official documentation
  6. http://www.linuxjournal.com/article/6390 - memory allocation on Linux level
  7. https://rushter.com/blog/python-memory-managment/ - one more article about memory management in Python

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Thanks!

Email: vladyslav.sydorenko@gmail.com

LinkedIn: linkedin.com/in/vlad-sydorenko/

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