1. Allowed memory
size of X bytes exhausted
Piotr Pasich
piotr.pasich @xsolve.pl
@piotrpasich

2. „A good understanding
of how variables are stored
and manipulated
is essential to becoming
a Hacker.”
php.net

3. „A good understanding
of how variables are stored
and manipulated
is essential to becoming
a Hacker.”
php.net

4. Have you ever thought
why don't you do this?

5. public function foo()
{
$a = new stdClass;
$b = array();
//(...)
unset($a);
unset($b);
return;
}

6. It might save memory

7. But it doesn't

8. It...

9. S...KS

10. STACKS

12. First in, last out

13. It's mostly used to
store variables in functions

14. So, variables live only in functions

15. Not PHP functions,
but C functions

16. One PHP function != One C function

17. They are
allocated and freed
automatically

18. At function return the stack is popped

19. public function foo($file1, $file2){
$obj = new Obj();
$data = array();
$data[] = $obj->importAFile($file1);
$data[] = $obj->importAFile($file2);
return $data;
}

20. Does the memory allocated
for $obj get freed after the return?

21. Yes, it is

22. And when it isn't?

23. public function foo($file1, $file2){
$obj = new Obj();
$data = array();
$data[] = $obj->importAFile($file1);
$data[] = $obj->importAFile($file2);
return $data;
}

24. public function
importAFile($file)
{
//(...)
return $this;
}

25. It frees all variables to which
there are no references left

26. Size limited by OS

27. HEAPS

29. Fragmented

30. No limit on memory size*

31. You have to manage it

32. Variables can be accessed globally

33. Extensions
Parsed code
Variables

34. EXTENSIONS

35. Consume memory

36. memory_get_usage()
profiles the heap

37. echo memory_get_usage(true);

38. PHP 5.3
786 432

39. PHP 5.4
262 144

40. Upgrade!

41. Also extensions

42. PARSED CODE

43. more code == more memory

44. more code == more time

45. but...

46. OPcache

47. sudo pecl install zendopcache-7.0.2

48. Substitute for APC

49. Stores precompiled
script bytecode
in shared memory

51. Difference
ab -n 10000 -c 20

52. Without cache
1743.994 seconds

53. OPcache
658.408 seconds

54. VARIABLES

55. $startMemory = memory_get_usage();
$array = range(1, 100000);
echo memory_get_usage() - $startMemory, ' bytes';

56. C
100 000 * 8 bytes = 800 000 bytes

57. PHP
14 649 016 bytes

58. !?

59. All variables
are represented
by one structure

60. The zval

61. typedef struct _zval_struct {
zvalue_value value;
/* variable value */
zend_uint refcount__gc; /* reference counter */
zend_uchar type;
/* value type */
zend_uchar is_ref__gc; /* reference flag */
} zval;

62. typedef union _zvalue_value {
long lval;
/* long value */
double dval;
/* double value */
struct {
char *val;
int len;
/* this will always be set for strings */
} str;
/* string (always has length) */
HashTable *ht;
/* an array */
zend_object_value obj; /* stores an object store handle,
and handlers */
} zvalue_value;

63. Each variable
needs much more
memory than in C

64. | 64 bit | 32 bit
--------------------------------------------------zval
| 24 bytes | 16 bytes
+ cyclic GC info
|
8 bytes | 4 bytes
+ allocation header
| 16 bytes | 8 bytes
===================================================
zval (value) total
| 48 bytes | 28 bytes
===================================================
bucket
| 72 bytes | 36 bytes
+ allocation header
| 16 bytes | 8 bytes
+ pointer
|
8 bytes | 4 bytes
===================================================
bucket (array element) total | 96 bytes | 48 bytes
===================================================
total total
| 144 bytes | 76 bytes

65. 144 bytes * 100 000 = 1 440 0000

66. What happened to
the rest 249 016 bytes?

67. Unfilled buckets in C

68. 2^17 = 131 072
131 072 – 100 000 = 31 072 pointers

69. 31 072 pointers * 8 bytes
=
248 576 missing bytes

70. PHP ain’t C

71. We can use SplFixedArray

72. That’s 56 bytes per element

73. SplFixedArray is
writting 33 % faster
reading 10% faster

74. Uff, now you know a lot

75. „Am I a hacker now?”

76. Not yet, wait a second

77. Be like a ninja!

78. Looking for leaks

79. PHP Fatal Error: Allowed memory
size of 8388608 bytes exhausted

80. ini_set("memory_limit","12M");

81. ini_set("memory_limit",”512M");

82. ini_set("memory_limit",”64G");

83. Fatal error: Allowed memory size of
33554432 bytes exhausted (tried
to allocate 2798048 bytes) in
/***/app/http.php on line 6331

84. That’s the one way

85. Let’s move to another

86. Xdebug will help you

87. Memory needs time

88. You can debug it
step by step

90. or by
xdebug profiler

91. xdebug.profiler_enable=1
xdebug.profiler_output_dir=/var/logs/profiler

92. xdebug profiler

93. Holy moly!

94. xdebug.trace_output_name

95. xdebug.collect_params

96. Default
0.0004 114272 -> str_split() ../trace.php:8
0.0153 117424 -> ret_ord() ../trace.php:10
1
0.0004
0.0007
114272
117424
-> str_split(string(6)) ../trace.php:8
-> ret_ord(string(1)) ../trace.php:10
3
0.0004
0.0007
114272
117424
-> str_split('Xdebug') ../trace.php:8
-> ret_ord('X') ../trace.php:10
4
0.0004
0.0007
114272
117424
-> str_split('Xdebug') ../trace.php:8
-> ret_ord($c = 'X') ../trace.php:10

97. What's inside?

98. What's inside?

99. You can use Webgrind

100. You can use Webgrind

101. Analyzer in PHPStorm

104. After all

105. Everything consumes memory

106. Thank you!