WordPress Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 77 bytes) in wp-includes/wp-db.php on line 1832

Question

I noticed recently on my WordPress website I'm getting sometimes 500 Internal Server Error. I checked logs and I have many lines like:

[Mon Oct 03 01:25:24.357439 2016] [fcgid:warn] [pid 12840] [client 83.27.211.107:36968] mod_fcgid: stderr: PHP Fatal error: Allowed memory size of 536870912 bytes exhausted (tried to allocate 77 bytes) in /var/www/vhosts/mywebsite/public_html/wp-includes/wp-db.php on line 1832

I tried to increase memory limit:

define( 'WP_MAX_MEMORY_LIMIT' , '512M' );

define( 'WP_MEMORY_LIMIT' , '512M' );

And even more, but it didn't work. No matter what I set it still exceeding memory limit by some bytes. I think there's a problem with some queries to a database, but how to check which?

Content of the includes/wp-db.php:

} else {
    $num_rows = 0;
    if ( $this->use_mysqli && $this->result instanceof mysqli_result ) {
        while ( $row = mysqli_fetch_object( $this->result ) ) {
            $this->last_result[$num_rows] = $row;
            $num_rows++;
        }
    } elseif ( is_resource( $this->result ) ) {
        // server crashing at line below
        while ( $row = mysql_fetch_object( $this->result ) ) {
            $this->last_result[$num_rows] = $row;
            $num_rows++;
        }
    }

    // Log number of rows the query returned
    // and return number of rows selected
    $this->num_rows = $num_rows;
    $return_val     = $num_rows;
}

Answer 1

The problem was caused by iThemes Security plugin. I turned off it and errors have gone. I'll investigate this problem more and edit this answer if I'll know what part of this plugin caused exceeding memory limit.