Knowing your garbage collector

Francisco Fernandez Castano

Rushmore.fm

francisco.fernandez.castano@gmail.com @fcofdezc

September 27, 2014

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Overview

1 IntroductionMotivationConcepts

2 AlgorithmsCPython RCPyPy

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Motivation

Managing memory manually is hard.

Who owns the memory?

Should I free these resources?

What happens with double frees?

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Dangling pointers

int *func(void)

{

int num = 1234;

/* ... */

return #

}

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John Maccarthy

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Basic concepts

Heap

A data structure in which objects may be allocated or deallocated in anyorder.

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Basic concepts

Heap

A data structure in which objects may be allocated or deallocated in anyorder.

Mutator

The part of a running program which executes application code.

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Basic concepts

Heap

A data structure in which objects may be allocated or deallocated in anyorder.

Mutator

The part of a running program which executes application code.

Collector

The part of a running program responsible of garbage collection.

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Garbage collection

Definition

Garbage collection is automatic memory management. While themutator runs , it routinely allocates memory from the heap. If morememory than available is needed, the collector reclaims unused memoryand returns it to the heap.

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CPython GC

CPython implementation has garbage collection.

CPython GC algorithm is Reference counting with cycle detector

It also has a generational GC.

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Young objects

[elem * 2 for elem in elements]

balance = (a / b / c) * 4

’asdadsasd -xxx’.replace(’x’, ’y’). replace(’a’, ’b’)

foo.bar()

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PyObject

typedef struct _object {

_PyObject_HEAD_EXTRA

Py_ssize_t ob_refcnt;

struct _typeobject *ob_type;

} PyObject;

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PyTypeObject

typedef struct _typeobject {

PyObject_VAR_HEAD

const char *tp_name;

Py_ssize_t tp_basicsize , tp_itemsize;

destructor tp_dealloc;

printfunc tp_print;

getattrfunc tp_getattr;

setattrfunc tp_setattr;

void *tp_reserved;

.

.

} PyTypeObject;

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Reference Counting Algorithm

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Reference Counting Algorithm

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Reference Counting Algorithm

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Reference Counting Algorithm

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Reference Counting Algorithm

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Cycles

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Cycles

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PyObject

typedef struct _object {

_PyObject_HEAD_EXTRA

Py_ssize_t ob_refcnt;

struct _typeobject *ob_type;

} PyObject;

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PyTypeObject

typedef struct _typeobject {

PyObject_VAR_HEAD

const char *tp_name;

Py_ssize_t tp_basicsize , tp_itemsize;

destructor tp_dealloc;

printfunc tp_print;

getattrfunc tp_getattr;

setattrfunc tp_setattr;

void *tp_reserved;

.

.

} PyTypeObject;

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Demo

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Reference counting

Pros: Is incremental, as it works, it frees memory.

Cons: Detecting Cycles could be hard.

Cons: Size overhead on objects.

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PyPy

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Mark and Sweep Algorithm

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Mark and Sweep Algorithm

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Mark and Sweep Algorithm

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Mark and Sweep Algorithm

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Mark and sweep

Pros: Can collect cycles.

Cons: Basic implementation stops the world

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Questions?

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The End

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