Bug #72736 [Ver]: Slow performance when fetching large dataset with mysqli / PDO
Edit report at https://bugs.php.net/bug.php?id=72736&edit=1
ID: 72736
Updated by: nikic@php.net
Reported by: petr dot hrabal at gmail dot com
Summary: Slow performance when fetching large dataset with
mysqli / PDO
Status: Verified
Type: Bug
Package: *Database Functions
Operating System: Debian 8
PHP Version: 7.0.9, 7.1.0-dev
-Assigned To:
+Assigned To: dmitry
Block user comment: N
Private report: N
New Comment:
It's not really necessary to bring mysql into this. Just allocating a bunch of strings without
deallocating them in between is enough:
<?php
$n = 50000000;
$s = "aaaaaaaaaaaaaaaaaaaaa";
$a = [];
$t = microtime(true);
for ($i = 0; $i < $n; $i++) {
$s++;
$a[] = $s;
}
var_dump(microtime(true) - $t);
This gives me the following results with and without ZMM:
nikic@saturn:~/php-src-fast$ sapi/cli/php -d memory_limit=-1 t043.php
float(30.064018011093)
nikic@saturn:~/php-src-fast$ USE_ZEND_ALLOC=0 sapi/cli/php -d memory_limit=-1 t043.php
float(3.7853488922119)
Generally the ZMM is somewhat faster than the system allocator, not 20x slower.
Assigning to dmitry...
Previous Comments:
------------------------------------------------------------------------
[2016-08-06 15:32:25] nikic@php.net
Can confirm this is the case. Some further observations:
* In PHP 7 the time increases quadratically with the number of rows. In PHP 5 it increases linearly.
* PHP 7 goes back to reasonable performance with USE_ZEND_ALLOC=0
* callgrind with --cache-sim suggests that we're seeing a crazy number of LL data read misses
in zend_mm_alloc_pages (this line: https://github.com/php/php-src/blob/master/Zend/zend_alloc.c#L856)
This being callgrind, I don't know whether this is true
My wild guess: We're going through the list of chunks where the first N are all full until we
hit one which isn't. At each chunk we experience an LL miss due to aligned address conflicts.
------------------------------------------------------------------------
[2016-08-02 16:10:49] petr dot hrabal at gmail dot com
Expected resaul is that performance should be at least comparable and not dramaticaly worse
------------------------------------------------------------------------
[2016-08-02 14:55:57] petr dot hrabal at gmail dot com
Description:
------------
I have large MySQL query (1.8M rows, 25 columns - all string between 20 and 256 chars) and I need
to make 2 dimensional array from it (memory table based on primary key).
Code works as expected, but $table creation takes a long time in PHP7.0.9 or 7.1.0-dev
same proble apears when PDO is used
more extensive info/diagnostics at
http://stackoverflow.com/questions/38614982/mysqli-fetch-assoc-performance-php5-4-vs-php7-0
with tested dataset PHP5.x performs 10 times faster
Test script:
---------------
$vysledek = mysqli_query ( $conn,"SELECT * FROM table WHERE 1");
while($zaznam = mysqli_fetch_assoc ( $vysledek )){
$table[$zaznam['prikey']] = $zaznam;
}
------------------------------------------------------------------------
--
Edit this bug report at https://bugs.php.net/bug.php?id=72736&edit=1
Thread (9 messages)