Analysis of Strange PHP Reference Efficiency Issues

Analysis of Strange PHP Reference Efficiency Issues_PHP Tutorial

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Release： 2016-07-21 15:19:53

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The function is as follows:

Copy code The code is as follows:

 
function update_timelist(&$arr,$timestamp,$threshold){ 
$timequeue = &$arr['timequeue']; 
while(!empty($timequeue[0])&&($timestamp-$timequeue[0])>$threshold){ 
array_shift( $timequeue); 
} 
array_push($timequeue, $timestamp); 
if($arr['count']$arr['count' ] = count($timequeue); 
} 
} 

Do you see any problems with this function? In fact, there is a big problem in the function:

$timequeue = &$arr['timequeue'];

This line causes the program to read 22M data and generate a time node linked list It took nearly 40 seconds, but deleting the line and using $arr['timequeue'] directly shortened the time by 30 seconds. It only took about 10 seconds to process 22M.

Copy code The code is as follows:

 
function update_timelist(&$arr,$timestamp,$threshold){ 
while (!empty($arr['timequeue'][0])&&($timestamp-$arr['timequeue'][0])>$threshold){ 
array_shift($arr['timequeue']) ; 
} 
array_push($arr['timequeue'], $timestamp); 
if($arr['count']$arr['count'] = count($arr['timequeue']); 
} 

Do you see what the problem is? The problem is with the count function, I didn’t think of it. PHP marks the real content space pointed by the variable as reference type and non-reference type, like the following code:

Copy code The code is as follows:

 
$a = 'jb51.net'; 
$b = $a; 
$c = $b; 

Only one copy of the actual memory space is occupied because PHP's zend engine uses the copy on writing mechanism. It will only copy 'jb51.net' when $b and $c are modified. At this time, the content space type of 'jb51.net' is a non-reference type. If Change to the following code:

Copy the code The code is as follows:

 
$a = 'jb51.net'; 
 $b = $a; 
$c = &$a; 

What will happen to this? Is it still a memory space to store 'jb51.net'? No, because $c is a reference to $a, and the storage space pointed to by $a needs to be marked as a reference type, then a separate copy of 'jb51.net' must be made for $b, because $b points to a non-reference type. .
We can understand it this way, $c is now a reference to $a. If $b still executes the space of $a, then modifying $c will cause $b to be modified as well. Therefore, once a reference appears at this time, even if there is no write operation, it must Made a copy. It can also be understood that PHP has only two types of memory space pointed to by variables: non-reference and reference. The two types cannot be mixed or transferred. If you need to change the state of the memory space somewhere, you need to make a copy.
The following explains why adding $timequeue = &$arr['timequeue'] will cause count to slow down. Do you remember the calling process of the c function? In fact, the parameters we pass in need to be copied and passed in. The same is true for PHP. However, due to the copy on writing mechanism, count will not actually be copied when passing in non-reference types, but $timequeue = &$arr['timequeue '] specifies the memory space of $timequeue as a reference type, and count requires a non-reference type, which causes count to need to copy a copy of $arr['timequeue']. Why is there no problem if I pass $arr['timequeue'] directly? Of course count uses the copy on writing mechanism. What about array_shift and array_push? They are passed in references. Don't worry, this is not modifying the type of $arr['timequeue'] but actually passing in an alias of $arr['timequeue'].

I have just started learning PHP. The above analysis may not be correct or comprehensive. You can send emails and leave messages on my homepage to communicate with me.