1. memory_get_usage function
int memory_get_usage ([ bool $real_usage = false ] )
Memory used by PHP script (excluding memory_get_usage() The memory occupied by the function itself)
The parameter $real_usage of memory_get_usage() defaults to FALSE; when set to TRUE, the memory value obtained is the memory occupied by PHP memory management.
//php分配到的内存 var_dump(memory_get_usage(true)); //php使用内内存 var_dump(memory_get_usage());
Result
int 262144
int 239368
variables, and when the space is not enough, apply for a new space.
memory_get_usage(true) is the pre-allocated memory.var_dump(memory_get_usage()); $a = "Hello World"; var_dump(memory_get_usage()); $b = $a; var_dump(memory_get_usage()); unset($b); var_dump(memory_get_usage()); unset($a); var_dump(memory_get_usage());Resultint 239776
int 239912
int 240000
int 239912
int 239776
##239912-239776=136
240000-239912=88
The memory used by $a and $b assignments is different. The reason is that $b assignment does not allocate memory. These 88 bytes are occupied by the output function.
In addition, destroying $b does not affect the memory used by $a. This is the reason for memory management reference counting.
You can clearly understand the relationship between variable assignment and memory by changing the example
var_dump(memory_get_usage()); $a = "Hello World"; var_dump(memory_get_usage()); $b = "Hello World"; var_dump(memory_get_usage()); unset($b); var_dump(memory_get_usage()); unset($a); var_dump(memory_get_usage());
Result
int 239816
int 239952int 240088
int 239952
int 239816
239952-239816=136
240088-239952=136
In addition, the memory occupied by reference assignment is similar to that of direct assignment, which also involves the reference counting rules of memory management
3. PHP garbage collection mechanism, this part is not understood deeply enough, simply record a function
xdebug_debug_zval needs to install the xdebug extension
$a = "Hello World"; xdebug_debug_zval('a');
Output
a:
(refcount=1, is_ref=0),
string 'Hello World' (length=11)
refcount points to The number of variables of this value
is_ref represents whether there is an address reference
Type
Value
When assigning a variable: is_ref is false and refcount is 1
Manual address
Basic knowledge of reference counting
http://php.net/manual/zh/features.gc.refcounting-basics.php
Each
php variable exists in a variable container called "zval". A zval variable container contains, in addition to the type and value of the variable, two bytes of additional information. The first one is "is_ref", which is a bool value used to identify whether this variable belongs to the reference set. Through this byte, the PHP engine can distinguish ordinary variables from reference variables. Since PHP allows users to use custom references by using &, there is also an internal reference counting mechanism in the zval variable container to optimize memory usage. The second extra byte is "refcount", which is used to indicate the number of variables (also called symbols) pointing to this zval variable container. All symbols exist in a symbol table, where each symbol has a scope (scope), the main script (for example: the script requested through the browser) and each function or method also have a scope.
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