Is this normal memory behaviour? - php

Question

i have a php script that processes large XML files, and saves data from them into a database. I use several classes, to process, and store the data in the PHP script, before saving to the database, and read in the XML, node by node, to preserve memory. Basicaly, a loop in my file looks like this:
while ($Reader->read()) {
$parsed++;
if (time() >= $nexttick) {
$current=microtime(true)-$ses_start;
$eta=(($this->NumberOfAds-$parsed)*$current)/$parsed;
$nexttick=time()+3;
$mem_usage=memory_get_usage();
echo "Parsed $parsed # $current secs \t | ";
echo " (mem_usage: " . $mem_usage . " \t | ";
echo "ETA: $eta secs\n";
}
$node=$Reader->getNode();
// OMMITED PART: $node is an array, I make some processing, and check if everything exists in the array that I need in the following section
$Ad=new Ad($node); // creating an Ad object from the node
// OMMITED PART: Making some additional SQL queries, to check the integrity of the data, before uploading it to the database
if (!$Ad->update()) {
//add wasn't inserted succesfully, saving a row in a second database table, to log this information
} else {
//add succesfully inserted, saving a row in a second database table, to log this information
}
}
You notice, that the first part of the loop, is a little output tool, that outputs the progress of the file, every 3 seconds, and also outputs the memory usage of the script. I need that because I ran into a memory problem, the last time I was trying to upload a file, and wanted to figure out, what's eating away the memory.
The output of this script looked something like this when I ran it:
Parsed 15 # 2.0869598388672 secs | (mem_usage: 1569552 | ETA: 1389.2195994059 secs
Parsed 30 # 5.2812438011169 secs | (mem_usage: 1903632 | ETA: 1755.1333565712 secs
Parsed 38 # 8.4330480098724 secs | (mem_usage: 2077744 | ETA: 2210.7901124829 secs
Parsed 49 # 11.377414941788 secs | (mem_usage: 2428624 | ETA: 2310.5440017496 secs
Parsed 59 # 14.204828023911 secs | (mem_usage: 2649136 | ETA: 2393.3931421304 secs
Parsed 69 # 17.032008886337 secs | (mem_usage: 2831408 | ETA: 2451.3750760901 secs
Parsed 79 # 20.359696865082 secs | (mem_usage: 2968656 | ETA: 2556.8171214997 secs
Parsed 87 # 23.053930997849 secs | (mem_usage: 3102360 | ETA: 2626.8231951916 secs
Parsed 98 # 26.148546934128 secs | (mem_usage: 3285096 | ETA: 2642.0705279769 secs
Parsed 107 # 29.092607021332 secs | (mem_usage: 3431944 | ETA: 2689.8426286172 secs
Now, I know for certainty, that in my MySQL object, I have a runtime cache, which stores the results of some basic select queries in an array, for quick access later. This is the only variable in the script (that I know of), which increases in size throughout the whole script, so I tried turning of this option. The memory usage dropped, but only by a tiny bit, and it was still rising throughout the whole script.
My questions are the following:
Is the slow rising of the memory usage throughout a long running script a normal behaviour in php, or I should search through the whole code, and try to find out what is eating up my memory?
I know that by using unset() on a variable, I can free up the space it takes away from the memory, but do I need to use unset() even if I am overwriting the same variable throughout the whole file?
A slight rephrasing of my second question with an example:
Are the following two code blocks produce the same result regarding memory usage, or if not, which one is more optimal?
BLOCK1
$var = str_repeat("Hello", 4242);
$var = str_repeat("Good bye", 4242);
BLOCK2
$var = str_repeat("Hello", 4242);
unset($var);
$var = str_repeat("Good bye", 4242);

Answer 1

If you install the xdebug module on your development machine, you can get it to do a function trace - and that will show memory usage for each line - https://xdebug.org/docs/execution_trace
That'll probably help you identify where the space is going up - you can then try to unset($var) etc and see if it makes any difference.