About PHP memory release problem (2)_PHP tutorial

About php memory release issue (2)

I took a whole morning today to look at the code that solved the memory problem before. Relatively speaking, I am very dissatisfied with the optimization program of my code. Importing 40,000 pieces of data at a time makes the code so cumbersome. I I don't think this is the fundamental solution. Through online search, we can further analyze the problem:

In the problem of PHP memory overflow, the first two methods will not be mentioned (you can refer to "About PHP Memory Release Issues" in the expo). It is not difficult to analyze that in fact, the key to the problem lies in how to release the memory synchronously in the loop. Instead of looping hundreds of lines to release once, you will find during debugging that if we encapsulate the highly reusable code in the loop into a function, and then use the form of calling sub-functions, the execution speed of the program will be reduced. , about dozens of times, this decline will change with the amount of data.

In the form of loop nesting, when processing arrays with large amounts of data, many use unset($a) to release memory in time, but in fact it is meaningless to do so. Quote the following analysis:

"In the engine, variable names and their values ​​are actually two different concepts. The value itself is an unnamed zval* storage (in this case, a string value), which is assigned via zend_hash_add() Variable $a. What happens if both variable names point to the same value?

{

zval *helloval;
MAKE_STD_ZVAL(helloval);
ZVAL_STRING(helloval, "Hello World", 1);
zend_hash_add(EG(active_symbol_table), "a", sizeof("a"), &helloval, sizeof(zval*), NULL);
zend_hash_add(EG(active_symbol_table), "b", sizeof("b"), &helloval, sizeof(zval*), NULL);
}

At this point, you can actually look at $a or $b and see that they both contain the string "Hello World". Unfortunately, next, you continue to execute the third line of code "unset($a);". At this time, unset() does not know that the data pointed to by the $a variable is also used by another variable, so it just blindly releases the memory. Any subsequent access to variable $b will be interpreted as freed memory space and thus cause the engine to crash.

This problem can be solved with the help of the fourth member refcount of zval (which has several forms). When a variable is first created and assigned a value, its refcount is initialized to 1 because it is assumed to be used only by the corresponding variable when it was originally created. When your code snippet starts assigning helloval to $b, it needs to increase the value of refcount to 2; thus, the value is now referenced by two variables:


{

zval *helloval;
MAKE_STD_ZVAL(helloval);
ZVAL_STRING(helloval, "Hello World", 1);
zend_hash_add(EG(active_symbol_table), "a", sizeof("a"), &helloval, sizeof(zval*), NULL);
ZVAL_ADDREF(helloval);
zend_hash_add(EG(active_symbol_table), "b", sizeof("b"), &helloval, sizeof(zval*), NULL);
}

Now, when unset() deletes the corresponding copy of $a, it will be able to see from the refcount parameter that there is someone else interested in that data; therefore, it should just decrement refcount and then Leave it alone. "

To sum up, what we need to do most is actually to reduce the array in which data is initially stored. In the above example, the processed elements in the array are released in time during the loop, so that as the loop continues, the memory usage will keep fluctuating (memory Recycling mechanism problem), but it will not keep growing, which will achieve our original purpose. Of course, the maximum amount of data processed at one time still depends on the memory allocated by the server to PHP. The amount of data read into the array in a single time exceeds the limit. Even the gods have no choice, haha ​​

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